Session M71: Poster Session III (11:15am - 2:15pm)

A Novel Focused Electrohydrodynamic Printing Method

The main advantages of additive manufacuring include limited waste and the ability to build complicated structures. Recently, a cost-effective, versatile method of high-resolution printing called electrohydrodynamic (EHD) printing has been introduced. This method allows for spatial resolution in the hundreds of nanometers. This process works similarly to a typical ink jetting system, except instead of the ink/polymer being pushed out of a tip, it is pulled out by an applied electric field allowing for the resultant droplet to be much smaller than the needle diameter. EHD systems are typically used in a drop-on-demand mode. Although operating in a continuous spray mode will increase throughput, its resolution is limited because of electrostatic repulsion between the drops. To overcome this limitation, we have incorporated an Einzel lens into the system to focus the droplets. To validate this approach, simulations were performed to test for different parameters, including droplet size changes and lens optimization. These parameters were then used to build a real system. This printhead system was then characterized for how well it performed. This includes comparisons of line widths of different focusing voltages.   

3D printed Liquid Crystal Elastomer mechanical devices

3D printing allows the creation of macro-sized Liquid Crystal Elastomer (LCE) devices.[1,2] Here we show 3 cm-scale 3D printed mechanical LCE devices.

The 1st project is a dynamically crystallizing LCE spinal fusion cage designed to treat degenerative disc disease (DDD). Compared to existing devices, our device conforms to the vertebral endplates and provides a large surface area and minimizes stress concentrations – thus minimizing the chance of conditions such as subsidence and adjacent level disease.

The 2nd project explores the structure property relationships of anisotropic dissipative LCE devices. Using these materials and techniques we demonstrate a prototype total disc replacement device that offers same functionality as the natural intervertebral disc.

The third project demonstrates highly dissipative and rate dependent LCE lattice structures created via digital light processing (DLP) printing. Compared to DIW, DLP printing allows us to create complex overhanging geometries with micron-resolution. The high rate dependency of our LCE resin means our devices provide effective shock absorbance over range of impact speeds.

1. R. H. Volpe et. al. in revision (2019)
2. N. A. Traugutt et. al. in preparation (2019)   

Effect of Post-Processing on Thermo-Mechanical Properties of a 3D-Printed UV-Curable Polymer

The effect of UV and thermal processing on the properties of a UV-curable polymer fabricated with 3D printing was studied. The polymers studied were proprietary methacrylate based resins (Formlabs) and samples were printed directly in test geometries. Uniaxial compression tests at room temperature were performed for Young’s modulus, Poisson’s ratio, and yield stress. Thermal tests using a Differential Scanning Calorimeter (DSC) described the glass transition temperature range, physical aging and (additional) high-temperature reactions of the samples. Post-processing consisted of either additional UV exposure time or high temperature "soak" time (or both). These indicated that post-processing has a pronounced effect on the mechanical and thermal properties. Both aspects of post-processing (UV and high-T soaks) increase the yield stress, the modulus and the glass transition. Combining high-T soaks with additional UV exposure is particularly effective.   

Competition between Phase Separating and UV Curing Kinetics during Photopolymerization-Induced Phase Separation in Confined Resin Films

Polymerization-induced phase separation is the segregation of agents in a multicomponent mixture triggered by polymerization to convert low molecular weight (MW) monomers into high MW polymers or networks. Several factors control the nature and structure of polymerization-induced phase separation, including blend composition, molecular weight of the phase separation agent, and polymerization kinetics of the monomers. When properly controlled, these factors can dictate a specific morphology of the resulting structure, allowing for tuning of characteristics of the resulting network. This study seeks to investigate these factors during photopolymerization-induced phase separation. Real-time turbidity to measure UV light transmittance through a blend sample of predetermined thickness will be employed as the main tool to monitor the phase separation process during UV irradiation. UV LED intensity and wavelength, resin reactivity, and end functionality of polymer additives will be studied to investigate the effect of each on the phase separation process and resulting network morphology within the confined space of UV-curable resin films. Scanning electron microscopy and other microscopy tools will be used for the study of morphology for the photopolymerization-induced phase separation.   

Triplet Triplet Annihilation Polymerization for High Resolution 3D Printing

Two-photon polymerization (TPP) offers by far the highest spatial resolution (< 1 μm) of current 3D printing methods, but suffers from limited volumetric write speeds compared to conventional stereolithography and requires the use of pulsed laser sources. Here, we demonstrate a new method of high-resolution 3D printing using triplet-triplet-annihilation (TTA) that achieves sub-micron resolution while maintaining high throughput and requiring only a light emitting diode (LED) source with an intensity six orders of magnitude lower than typically required for TPP. We demonstrate a robust 3D printing mechanism capable of printing a diverse array of materials with high resolution, low surface roughness, and high write speeds, without requiring high-intensity pulsed laser illumination.   

Digital Printers and Image Quality

Photoreceptor is a central device in digital printers. The outer layer, Charge Transport Layer made up of polycarbonate and Charge Transport Molecule is prone to abrasion by stresses of developing system or cleaning system when it is repeatedly used in printing process. The abrasion of the photoreceptor causes deterioration of electrical properties such as lowering sensitivity or lower charging and results in irregular image such as lower image density and image stain. Single and multiple coating were made with filled and unfilled binder polymers and their modulus and hardness were measured by nano-indenter. The effect of the nanoparticles on composite modulus is dependent on many variables particularly on the morphology of the polymer matrix as well as the interaction between the filler and matrix. The state of dispersion depends on balance of repulsive and attractive forces. on the particles.. Charge Transport Molecule reduced modulus and hardness thus increasing photoreceptor wear rate. Nano-filler silica in the polymer either polycarbonate or polyester increased modulus and hardness with higher photoreceptor life.
  

Immersion Precipitation 3D Printing (ip3DP)

We present a novel method of 3D printing to fabricate micro-to-nanoporous 3D models in one-step, which we termed immersion precipitation 3D printing (ip3DP). Methods to impart porosity to 3D printed objects have been limited to date. Addition of sacrificial materials to printing materials, followed by their removal, are the established approaches, but such approaches require post-processing to impart porosity. Solvent-cast 3D printing (SC3DP), which is direct 3D printing of polymer inks with in situ evaporation of solvents, has allowed fabricating 3D porous structures with stringent requirements of rheological properties of the printing ink (e.g., high viscosity and high vapor pressure). We developed an alternative approach to print polymeric inks directly in a bath of a nonsolvent and solidified them in situ via immersion precipitation. The porosity of the 3D printed objects was readily controlled by the concentrations of polymers and additives, and the types of solvents. This work is the first demonstration of three-dimensionally controlled immersion precipitation based on digitally controlled depositions of polymer solutions. Wide selection of printable materials, and the ability to tailor their morphologies and properties, make ip3DP a versatile method of 3D printing.
  

3D Printing with Waste High-Density Polyethylene

HDPE has widely been considered to be impossible to 3D print using fused filament fabrication (FFF). When HDPE is FFF printed, the printed object warps significantly and debonds off the print substrate. Therefore, it is difficult to preserve registry as the polymer filament is laid down layer by layer during FFF. We demonstrate a strategy for FFF of waste-derived HDPE. There are two aspects to our approach. (i) We formulate the HDPE by blending with dimethyl dibenzylidene sorbitol (DMDBS), that forms a nanofibrillar network in the HDPE melt as it cools and (≈ 10%) linear low density polyethylene (LLDPE). (ii) We use a thin “brim” around the printed object that helps it adhere to the print substrate using common paper glue. This dramatically reduces warpage, allowing FFF of HDPE objects with complex geometries. FEM simulations indicate that our approach decreases the stresses that develop due to crystallization induced shrinkage during cooling. Given the volume of HDPE that finds its way to the waste stream, our results have important implications for extending the use life of HDPE.   

Printing Resolution and Depth-Of-Cure Study For Stereolithography 3D Printing Resins

Stereolithography (SLA) is a method of 3D printing of polymer resins using photoinitiated polymerization. Polymer thermosets with crosslinked networks are being formed in SLA. We have studied how the characteristics of SLA photocurable resins affect the 3D printing resolution and mechanical properties. Characteristics include the viscosity and the depth dependent UV polymerization kinetics. Real time FTIR with attenuated total reflectance sample stage was used to thoroughly study the UV penetration depth effects on the photopolymerization kinetics in terms of the rate of polymerization, on-set time for photopolymerization and monomer conversion. Three types of commercially available SLA resins are compared for the printing resolution and depth-of-cure characteristics. Additional tuning of depth-of-cure characteristics has been performed by mixing additional photoinitiator or inhibitor for the investigation of 3d printing resolution studies. Controlled post UV-curing is also performed to further manipulate their mechanical properties with an aim to elucidate how the network structure in SLA samples affect the stress-strain behavior of thermosets under tensile elongation. The layer curing process and morphology has been also investigated by DSC and microscopy techniques.   

Geometrical and Mechanical Characterization of Interlayer Bonding Quality in Fused Filament Fabrication

To obtain a fundamental understanding of the large variation of mechanical properties and geometry of printed parts prepared by fused filament fabrication (FFF), we focused on the interlayer bonding region of polycarbonate samples prepared by FFF and performed 3D geometrical characterizations using micro-CT followed by uniaxial tensile tests. The results showed significant property variations depending on printing conditions. Specifically, the layer height impacted bonding area. In addition, there was an almost linear relation between bonding zone area and fracture strength. However, when nozzle temperature increased, the strength showed a rising trend first then reached plateau. Interestingly, we also found a trend of Young’s modulus reduction with higher layer height, which could be explained by finite element simulations based on scanned sample geometries, indicating the bonding zone geometry change is the main reason of modulus variation. We envision that our findings can guide the selection of printing parameter as well as provide benchmark data for future simulation models.   

Study of the end-to-end probability distributions of low-molecular weight, aqueous polyethylene oxide solutions using experimental DEER measurements and molecular dynamics simulations

Low molecular weight, dilute, aqueous polyethylene oxide (PEO) chain end-to-end distance (R_ee) probability distributions, P(r), were measured experimentally and calculated from simulation, filling a fundamental gap in the existing literature for one of the most widely used water soluble polymers. The distributions were measured by Double Electron Electron Resonance (DEER) spectroscopy, resolving the full P(r) distribution within the technique’s range of validity (~2-9 nm). The DEER technique uses small spin probes conjugated to the polymer ends. The probes in simulation are observed to hydrophobically aggregate below the range accessible to DEER (<1.5 nm), with the perturbation to the distributions dropping off rapidly with molecular weight. The distributions and their average R_ee indicate aqueous PEO is a semi-flexible polymer in good solvent. The average R_ee exhibits excluded volume scaling with molecular weight above the Kuhn length (~0.96 nm), which is quantitatively consistent with scattering data from high molecular weight (>10kDa) PEO.   

PEO-Sn Nanofibers

Polyethylene oxide (PEO) nanofibers loaded by various concentrations of Sn have been obtained by force spinning. Homogeneous solutions of PEO in deionized water, with concentrations ranging between 8 and 12 % wt. PEO have been prepared. Various amounts on Sn nanoparticles have been added to these solutions. The as-obtained mixtures were homogenized by stirring 4 h, at room temperature and 1,000 rotations per minute and then subjected to the force spinning, at room temperature, and various spinning rates ranging between 1,000 and 9,000 rotations per minute.
The structure and morphology of mates of PEO based nanofibers loaded by various amounts of Sn nanoparticles and of the corresponding thick films were investigated by Raman, FTIR, X-Ray, and UV-Vis spectroscopy. The effect of the concentration of the nanofiller on the position of Raman and FTIR lines was analyzed in detail and related to the elastic features of these nanocomposites. The differences between mats and macroscopic samples (thick films) are discussed in detail. Additional X-Ray data focused on the crystal structure including crystallites sizes and orientations are reported.   

Unraveling the Morphological Evolutions during Solvent Vapor Annealing for Organic Solar Cells Using in situ Resonant Soft X-Ray Scattering

Solvent vapor annealing (SVA) is one of the most effective pre-electrode deposition techniques, which has been applied in many efficient organic solar cells. In current research, we used a customized in situ multimodal system to reveal the dynamics of phase separation inside the bulk-heterojunction thin films during SVA via resonant soft x-ray scattering (RSoXS). Solvent-vapor-induced molecular crystallization is of importance to provide the driving force of the domain generation and evolution processes. However, with the introduction of solvent vapor for a long time, coarsening of the film appeared at the film surface, which led to the deteriorating of the device performances. These results indicated that in situ RSoXS is a quite useful technique to bridge the gap to understand the relationship of film growth mechanisms and device efficiency.   

The Effect of Electrostatic Interactions on the Interfacial Adsorption and Covalent Reaction of Coiled Coil Bundles

Coiled coil bundles (CCBs), known as bundlemers, are a series of computationally designed peptides that self-assemble in water into monodisperse nanoparticles with tunable surface chemistry. We have shown CCB viability as molecular building blocks by producing predesigned 2D lattices, nanocages, and nanotubes through tailoring their surface chemistry and solution assembly conditions. Recently, we have used CCBs as macromonomers to create stiff extremely high aspect ratio supramolecular polymers through covalent chemistry between CCB ends. As the CCB library continues to grow, it's important to develop processing methods to grow new nanostructures out of these versatile building blocks. By first templating a surface through the directed self-assembly of CCBs onto a flat planar substrate, it should be possible to grow CCB nanoforests of highly aligned nanorods with tunable surface chemistry and packing structure. Ongoing work on the formation of the template layer has shown a dependence between deposition kinetics and electrostatic interactions between CCBs. This presentation will overview the impact of pH, ionic strength, and CCB surface charge on deposition kinetics as well as show the stages of CCB deposition through a combination of AFM, QCM-D, and reflectivity measurements.   

Development and use of a new model for the Worm Like Chain

The behavior of polymers is determined in large part by their flexibility. Nestled between the rigid and flexible limits are semiflexible polymers, whose behavior is distinct from either of the aforementioned regimes. Whereas the rigid limit is defined by bending energetics and the flexible limit is defined by conformational entropy, the behavior of semiflexible polymers is typified by the interplay of both phenomena. This makes their behavior incredibly rich, but also incredibly difficult to model. At present, models exist to study polymers at very fine (e.g. Kratky-Porod) and very coarse (e.g. Marko-Siggia) length scales. Though efforts have been made to decrease the gap between these two modeling approaches, these still exists a length scale over which no good model currently exists. To address this modeling limitation, we utilize a different approach to develop a model for the Worm Like Chain that reproduces some of the essential behavior of the Kratky-Porod model, while being coarse grained enough to allow for study of semiflexible polymers on longer timescales and under non-equilibrium conditions.   

Enrichment and Distribution of Pb2+ Ions in Zwitterionic Poly(cysteine methacrylate) Brushes at the Solid-Liquid Interface

We prepared cysteine-based (PCysMA) polyzwitterionic brushes, and surface zeta potential investigations on pH-responsiveness of these PCysMA brushes confirm their zwitterionic character at intermediate pH range, while at pH values either below pH 3.50 or above pH 8.59, they exhibit polyelectrolyte character. Under acid (pH < 3.50) or base (pH > 8.59) conditions, they possess either cationic or anionic character, respectively. In the zwitterionic region, these PCysMA brushes show positive surface zeta potential in the presence of Pb(CH₃COO)2 solutions of various concentrations. The results are in line with microscopic investigations using anomalous X-ray reflectivity (AXRR) carried out along the absorption edge of Pb²⁺ ions. By varying the photon energies around the absorption L₃ edge of lead (13037 eV), the Pb²⁺ concentration normal to the silicon substrates, as a function of depth inside PCysMA brushes, could be revealed at the nanoscale. Both zeta potential and AXRR measurements confirm the enrichment of Pb²⁺ ions inside PCysMA brushes, indicating the potential of PCysMA to be used as a water purification material.   

Oligomeric Cellulose Based Block Copolymer

Novel water soluble block copolymer based on oligomeric cellulose and PEG was produced by coupling reaction. The oligomeric cellulose was produced by phosphoric acid assisted hydrolysis with PDI of 1.04 and a degree of polymerization of 7 (DP7). The DP7-b-PEG-b-DP7 triblock copolymer showed tunable PEG crystallization behavior by changing the molecular weight ratio between PEG and cellulose. At Mw ratio of 1:2:1, the PEG crystallization is fully suppressed. The morphologies of DP7-b-PEG-b-DP7 triblock copolymer solutions and film are also presented. The synthesis route can be extended to non-PEG based polymers.   

Resistive Pulse Sensing of Phytoglycogen Nanoparticle Translocation: Examining Structure and Brownian motion

Dendrimers are an important platform for a variety of applications such as cosmetics, lubricants, and drug delivery. Understanding the relationship between structure, mobility, and mechanical properties is crucial for designing new systems for these applications. In particular, examining the transport of nanoparticles (NPs) through nanopores and nanochannels by translocation may enable high-throughput analysis of these properties. These measurements track ionic current through the nanochannel, which is blocked when a particle occludes the channel. Here, we studied experimentally the translocation of phytoglycogen NPs through single solid-state SiN_x nanochannels with diameters between 40 and 100 nm, and lengths of 100 nm. Using Poisson-Nernst-Planck calculations, we quantitatively predicted the magnitude of the current blockade of phytogylcogen NPs by including a “hardness parameter” term to describe the degree to which the NPs occlude the nanochannel, and found good agreement with neutron scattering measurements. The NPs diffused through the nanochannels under pure three-dimensional Brownian motion, and diffusion coefficients were measured in quantitative agreement with dynamic light scattering.   

A photo-responsive protein-polymer bioconjugate for control of a model protein

Stimuli-responsive bioconjugates have been investigated as a means of controlling protein behavior through conjugation of a responsive polymer to a protein. These bioconjugates have been used in many applications including purification and recovery of proteins, biosensing, and control over protein oligomer formation. We have prepared stimuli-responsive bioconjugates to provide spatiotemporal control over enzymatic activity through a covalently linked photo-responsive copolymer. Photo-responsiveness was conferred through an azobenzene-containing monomer which undergoes a light-induced cis-trans isomerization. Controlled radical polymerization was used to create a panel of polymers of varying size and azobenzene monomer content which was assayed for its ability to allow photo-control using a model protein, alkaline phosphatase (phoA). Site-specific bioconjugation of these photo-responsive polymers was achieved through introduction of a cysteine residue to wild-type phoA which reacted with the maleimide-functionalized responsive polymer. In addition to screening the influence of polymer size and composition, several conjugation sites on the protein were tested to probe the effect of distance to the active site on photo-responsivity.   

Secondary structure drives self-assembly in weakly segregated globular protein-rod block copolymers

Protein-polymer bioconjugates combine protein functionality with polymer material properties and block copolymer self-assembly. The effect of polymer block secondary structure and chirality on solution-state self-assembly was studied using bioconjugates with a globular protein block (enhanced green fluorescent protein, or eGFP) and a poly(amino acid) (PAA) block with varying chirality. Block copolymers were synthesized by NCA polymerization, followed by native chemical ligation to eGFP. Homochiral _L- and _D- type PAAs formed α-helices, while an achiral random copolymer of _L- and _D- type monomers was structureless. All bioconjugates with an α-helical block self-assembled into lamellae with similar phase diagrams regardless of chirality type. However, bioconjugates with an achiral block remained disordered at all concentrations and temperatures measured. This was due to a non-repulsive interaction between the flexible achiral PAA and eGFP. Thus, incorporation of secondary structure into the polymer block can increase the effective segregation strength between blocks and drive self-assembly even with weak to no repulsion between blocks.   

Computationally designed coiled coil peptide bundle chains with positive charges: Self-assembly and click conjugation

Computational design has been employed to predict peptide primary structure that will intermolecularly assemble into different net positively charged coiled coils nanostructures. Stable, robust, tetrahelical anti-parallel peptide bundles with net charges varying from 0 to +32 are self-assembled from the computationally designed 29-amino-acid peptides. In this research, +4 charged peptides are synthesized via solid phase peptide synthesis (SPPS) and modified with cysteine or maleimide on the N-termini of single peptides. Those two modified peptides are respectively self-assembled into 4 x 2nm cylindrical peptide bundles through non-covalent interactions and conjugated via Thiol-Michael 'click' react to form coiled coil bundle chains with extreme rigidity. The size and morphology of coiled coil bundle chains self-assembled under various temperatures, pH and solvent conditions are investigated by techniques such as Transmission electron microscopy (TEM) and circular dichroism (CD) spectroscopy. The self-assembled peptide bundles can be further used as building blocks to construct new 1-D nanomaterials. The effect of salt and pH on the solution behavior of the positively charged coiled coil chains will be also discussed.   

Self-Assembly of Protein-based Block Copolymers – A Minimal Coarse-grained Model

Protein-based block copolymers self-assemble into various nanostructures that are instrumental in developing next-generation biocatalysts and biosensors. Phase behavior of such block copolymers depends upon several parameters: temperature, blocks volume fractions, conjugate concentration in solution, protein shape and charge density, and multiple binary interaction parameters. In this work, a highly coarse-grained dumbbell model is developed in which protein is represented as a hard sphere linked to a soft sphere denoting a flexible polymer coil. Molecular dynamics simulations are performed with both implicit and explicit solvent. It is observed that such a simple model, incorporating primarily the various binary interactions, forms all the different morphologies that are observed experimentally. Interestingly, the model also predicts a lyotropic re-entrant order-disorder transition that is peculiar to protein-based block copolymers compared to coil-coil block copolymers. Integral-equation state theories are used to compute the solvent-mediated interactions to understand the origin of re-entrant transition. This work sheds light on the key factors governing the phase behavior of protein-based block copolymer solutions.   

Tuning stoichiometry and physical interactions of peptide rigid rods through ‘click’ chemistry of computationally designed coiled coil

With computational design, peptide sequences can be customized to form specific solution self-assembly structures and to promote interactions that favor formation of unique structures such as nanotubes, needles, 2D plates. A recent publication of our collaborative group has shown that peptide rigid rods of various lengths can be formed from conjugation of N-terminal modified, anti-parallel, tetrameric coiled coil bundles by thiol-Michael reaction. Lyotropic liquid crystal behavior was observed and strong mechanical properties were expected from these peptide rods. In this work, further investigation into fine-tuning liquid crystal behavior of peptide rods and improving ‘click’ chemistry stoichiometry were carried out with computationally designed sequences. Highly charged coiled coil sequences were designed to showcase a tunable liquid crystal behavior through changing solution conditions and parallel coiled coil were designed to have inherent 1:1 stoichiometry at each end of bundle for long rods formation. Coiled coil design and preliminary results will be discussed.   

In Situ SANS on Gelatinization of Polysaccharides

It is desirable to understand, design and control the pathway of food preparation and consumption for the benefit of human health. Gelatinization occurs when starch/water mixtures are heated and native crystalline structures melt. The transport and distribution of water among polysaccharide molecules play a key role in the process of gelatinization. We have carried out in situ small angle neutron scattering measurements on rice grains in various aqueous solutions. Data imply the role of porosity and capillarity in the initial water uptake and the subsequent gelatinization kinetics. The interfacial tension between water and polysaccharides would alter the gelatinization process.
  

Directed Self-Assembly of Fluorine-Containing High-χ Block Copolymers using Top Coat and Electric Field

Directed self-assembly (DSA) of block copolymers (BCPs) have been attractive for “bottom-up” approach lithographical applications. BCPs can form nanoscale structures spontaneously in long-range alignment with small feature down to 5 nm. In this study, we report a newly designed fluorine containing BCPs (polystyrene-b-poly(2,2,2-trifluoroethyl acrylate)) which the Flory-Huggins interaction parameter (χ) was above 0.2. Film experiments were demonstrated on a neutralized substrate where a sub-10 nm perpendicular lamellar morphology was observed with simple thermal annealing using the top coat strategies. The neutral conditions of the top coats for the BCP top surface was tuned by the same monomer of the BCP and the orientation was confirmed by AFM and GISAXS. Electric field was applied for the DSA, observing a long-range alignments without neither chemoepitaxy nor graphoepitaxy.   

SCFT Study on Topological Defects of Symmetric Block Copolymers

Block copolymers, as typical soft material, have a long relaxation time and are highly susceptible to thermal fluctuations, and thus their self-assembly is often accompanied by the occurrence of a variety of defects. At present, most research focuses on some simple defects in AB diblock copolymer melts, and there are few studies on the influence of block copolymer architectures on defect stability. Herein, the stability of various defects in (AB)_n linear and A_nB_n star block copolymer systems is investigated by self-consistent field theory (SCFT) and string method. We calculate the excess free energy of different defects and barriers of defect-removal in different symmetric block copolymers with the same effective segregation. The comparison of excess free energy between different defects indicate that the creation of a defect with more disconnections cost higher free energy. Moreover, our results demonstrate that (i) whether for linear or star block copolymer systems, the relationship between n and the excess free energy of a defect of the block copolymer is not monotonic and (ii) the same defect has different minimum free-energy path in different block copolymer systems.   

Emergence of multi-stranded helices by the self-assembly of a new AB-type multiblock copolymer under cylindrical confinement

Novel nanostructures can emerge from the self-assembly of block copolymers under geometrical confinements due to the modification to the competition between interfacial energy and stretching energy. In particular, it is very appealing that helical structures can be formed by achiral AB deblock copolymers under cylindrical confinement. However, helical structures with strands more than two are rarely discovered. In previous works, most of block copolymers are hexagonal cylinder-forming in bulk. Herein, we consider a new AB-type multiblock copolymer, whose bulk cylindrical phase can be regulated by the architecture from hexagonal lattice to tetragonal one or even a graphene-like lattice. Our self-consistent field theory calculations suggest that this copolymer under cylindrical confinement can form helical structures with strands varying between 1 and 6. Our work provides a facile way for the fabrication of helical structures with a large number of strands.   

Gyroidal Thin Films from Block Copolymer Self-Assembly as Structural Directing Templates for Fabrication of Mesostructured Crystalline Inorganic Materials

Amphiphilic block copolymers swelled by carbon precursors self-assemble into bicontinuous gyroid mesophases (space group 214) compressed along <110> on Si wafers upon quenching after solvent vapor annealing. After pyrolysis in inert atmosphere, the resulting textured, mesoporous, and gyroidal carbon thin films are backfilled with amorphous Si (a-Si). The poor tolerance of carbon toward high temperature in air and the high melting points of inorganic materials present a seemingly insurmountable challenge in preparing crystalline mesostructured Si in the carbon templates. The stability of the carbon templates is significantly enhanced, however, when employing nanosecond transient laser heating. Melting and recrystallization of Si (a-Si melting point ~1250 °C) during this ultrafast, highly non-equilibrium process allow conformal template backfilling so that the templates' mesostructural order is inherited by the crystalized Si, which otherwise has little access to mesoscale ordering. Non-equilibrium transient laser technology in conjunction with block copolymer self-assembly in thin films opens up opportunities in fabricating materials for applications in catalysis, photonics, and beyond.   

Unusual Phase Behavior by Blending Star-Shaped Block Copolymer and Linear Block Copolymer

Generally the block copolymer shows the arrangement of hexagonal patterns cylinder when volume fraction of one block is larger than that of the other. However, since all processes in the semiconductor business are geared to the rectilinear structure, it is needed to rework with time and money to apply block copolymer to the semiconductor business. Therefore, creating different structure from that of general block copolymer like square array structure or asymmetric lamellae has great significance in lithography applications.
Here, we synthesized star-shaped poly polystyrene-block-poly(4-thydroxystyrene) (PS-b-PHS) and linear polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) and blended them in various proportions to observe the structures. Because the molecular weight of the PS on the core of star polymer is much smaller than that of the PS in linear polymer, the spacing between cylinders is limited and the large configurational entropy is formed, which leads block copolymer to have different structure.   

High χ-low N fluorine-based block copolymers for sub-10 nm lithography

We will discuss the attractive properties of high χ-low N fluoro based BCPs and illustrate their utility for next-generation nanolithography. The synthesis, physical characterization and thin film self-assembly of a series of lamellar and cylindrical fluorine based BCPs will be described. Total BCP molecular weights ranging from 39 kg mol^-1 to 7 kg mol^-1 were synthesized using reversible-addition-fragmentation chain-transfer (RAFT) polymerization. Tailoring the fluorine containing blocks here resulted in a similar surface free energy to the well-studied polymethylmethacrylate (~ 41 mN/m) based BCPs thereby making orientational control in thin films more attractive. Solvo-thermal vapor annealing and thermal annealing of films were evaluated with a view to standardised industry methods. Period sizes ranged from 48 nm down to 14 nm with observed feature sizes as small as 7 nm. We also demonstrate the integration feasibility of our new fluorine based BCPs using sequential infiltration synthesis to form alumina nanowire hardmasks. The favorable BCP characteristics detailed here provide a versatile material option to the current library of available BCPs for nanolithography.   

Development of Shape-Tunable Monodisperse Block Copolymer Particles through Particle Restructuring by Solvent Vapor Annealing

Uniformity and controllability of size, shape, and internal structure of block copolymer (BCP) particle are important to determine their functionality. Here, we introduce the particle restructuring by solvent engineering (PRSE) strategy to transform the size-controlled, monodisperse BCP spheres into non-spherical BCP particles with well-defined internal structure. PRSE process starts with generating monodisperse BCP spheres in a wide range of particle size using membrane emulsification. Then, successful shape transformation into non-spherical prolate and oblate was demonstrated while maintaining the monodispersity of particle size. PRSE can be applied generally to various functional BCPs including polystyrene-block-poly(1,4-butadiene) (PS-b-PB), polystyrene-block-polydimethylsiloxane (PS-b-PDMS), and polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP). Therefore, it allows an effective control of aspect ratio (AR) of the particles over a wide range, which was supported by theoretical calculation describing particle elongation. Further investigation on transformation kinetics during the PRSE showed that the morphological transformation was driven by reorientation of BCP domains, and it was strongly associated with overall molecular weight of the BCP and the annealing time.   

Assessing the Capabilities of the Sharp-Interface Gradient-Based Theoretical Framework for the Simulation of Free-Surface Block Copolymers

Recently a sharp-interface gradient-based theoretical framework for the simulation of free-surface block copolymers within the Self-Consistent Field Theory was proposed [1,2]. In this talk, we present the results for the simulation of step edges in polymer thin films using this method. First, in the case of relatively low values of the polymer-air surface tension the results are compared to those obtained using the conventional approach based on adding ``air molecules'' to the system and discuss the differences in morphologies produced by the two methods. Then, we analyze the morphologies of step edges and construct phase diagrams in the case of high (more realistic) surface tensions, which are not easily accessible by the conventional approach.

1 Ouaknin, G., Laachi, N., Bochkov, D., Delaney, K., Fredrickson, G.H. and Gibou, F., 2017. Functional level-set derivative for a polymer self consistent field theory Hamiltonian. Journal of Computational Physics, 345, pp.207-223.

2 Bochkov, D., Ouaknin, G. and Gibou, F., 2019. Simulation of Free Surface of Block Copolymers. In APS Meeting Abstracts.   

Self-Assembly in Large Molecular Weight Block Copolymers for Dual Metal Nanodot Patterning and Optical Applications

Block copolymers (BCPs) have the potential to revolutionise the manufacturing of nanotechnology primarily due to their ability to self-assemble into spatially ordered nanodomains, resulting in nanoscale patterning. High molecular weight co-polymers poly|(styrene-b-2-vinylprydine) (PS-b-P2VP) and polystyrene-b-polyethylene oxide (PS-b-PEO) are used to form self-assembled patterns that act as a polymer template. This polymer template is infiltrated with the inorganic materials gold (Au) and silver (Ag). Polymer template removal leaves behind inorganic metal nanodots that exhibits surface plasmonic (SP) resonance effects that are particularly applicable for optical applications. The shape and chemical composition of metallic nanodots can be controlled by UV ozone or plasma etching removal of the polymer. It is found that dual metal nanodot arrays of Au and Ag can be used to supress silicon reflectivity. Micelles of Au and Ag exhibit plasmonic resonance in the 400-600nm range that can be determined by the Au/Ag ratio. The transmission spectra for single metal nanodot arrays show SP dips at 430± 25nm for Au and 550 ±25nm for Ag. The resulting nanodot size, patterning and metal composition provides a low cost means of producing optical filters of tuneable plasmonic resonance effects.   

Morphological Changes in Block Copolymer Thin Films Driven by Complex Coacervation

Block copolymer thin films have been proposed as a method for immobilizing target molecules on a nanostructured scaffold. The use of a charged-neutral block copolymer (BC) enables the incorporation of oppositely charged molecules into thin films of the material. However, incorporation of oppositely charged macromolecules can trigger a phase change via coacervation. In this work, the effect of polyanion strength on the structure of BC self-assembly in thin films is examined. Complexation with the strong synthetic model polyanion, polystyrene sulfonate, leads to precipitation, while complexation with the weak synthetic model polyanion, poly(acrylic acid), leads to micellization. This difference in phenomena is likely driven by the ability of the weaker polyanion to charge modulate. The weakest polyanion tested was a model protein, amylase, which showed a rich set of phase behavior dependent on the blending ratio between the protein and the BC. The protein’s charge density is significantly lower than that of the synthetic polymers, which leads to weaker interactions between the protein and the BC by comparison. As a result, the structure of the BC is less disrupted by the protein than by the synthetic polymers at moderate loading (up to ~30 wt%).   

Self-assembly of Block Copolymers with Ionic Liquid Crystals in Thin Films

Block copolymers (BCPs) self-assembly leads to plenty of promising applications in the area of electronics and energy storage. Thus, improving the ordering of BCP is crucial to optimize its performance in different applications. Adding ionic liquids (ILs) to BCPs has been reported a facile method to realize fast ordering because of its plasticization effect and wetting characteristics tunability. With similar chemical structure to ILs, ionic liquid crystals (ILCs) are liquid-crystalline salts also consisting of cations and anions. Differently, liquid-crystalline nature and thermotropic transition provide ILC with more fascinating features than IL when it is added to BCP. However, BCP/ILC system has not been well studied as BCP/IL system does. In this study, the effect of ILC additives on the self-assembly of BCP, PS-b-PMMA, under different annealing conditions has been investigated by using atomic force microscopy (AFM), grazing-incidence small-angle X-ray scattering (GISAXS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). We observed a drastic change of ILC selective swelling behavior during its thermotropic transition. Moreover, it can be highlighted that ILC enhances the ordering in BCP without enlarging the domain size after the zone annealing.   

Influence of Charge Sequence on the Adsorption of Polydispersed and Monodispersed Polyelectrolytes onto Monodispersed Polyelectrolyte Brush

We use coarse-grained molecular dynamics to elucidate the role of charge sequence on the adsorption efficacy of oppositely charged free monodispersed and polydispersed polyelectrolytes on to a polyelectrolyte brush. We consider four different model systems wherein the free and the brush polyelectrolytes can have either brush or alternating charge sequence. Our model treats the polyelectrolytes in a bath of implicit solvent, excess salt and explicit counterions. For monodisperse systems, the adsorption efficiency is highest when both the free and the brush polyelectrolytes possess a block charge sequence, and it is lowest when both the free and the brush polyelectrolytes possess an alternating charge sequence. By computing the free energy, internal energy and entropy of adsorption using umbrella sampling methods, we find that the origin of the differences in adsorption efficiency for different charge sequences in monodispersed systems is enthalpic. Additionally, equilibrium conformations for different charge sequences reinforce the results obtained from energetic calculations. We also show the changes in adsorption efficacy as a result of polydispersity in the free polyelectrolytes.   

X-ray scattering characterization of polystyrene chains tagged with a single quaternary ammonium group per chain.

Polymers containing a moderate amount of covalently bound ionic groups are widely used as membrane materials for electrochemical energy conversion devices such as fuel cells and electrolyzers. The performance of these ion-containing polymers largely depends on the microstructures, especially the structures of ion aggregates. To investigate the effects of the local chemical environment of ionic groups to the ionic aggregate and chain structures, we synthesized and characterized polystyrene chains containing a single quaternary ammonium group at the end of chains, in the middle of the chains, and right next to the secondary-butyl initiator motif using amino-functionalized diphenylethylene monomer. X-ray scattering characterization reveals three structural features of these model polystyrene chains. First, the domain spacing between ion aggregates changes with the location of the ionic groups in chains. Second, ionic groups stretch the polystyrene chains. Third, all polystyrene chains even without an ionic group display strong upturn curves in the small-angle domain, and the features of the upturn curves vary with the location of ionic groups in chains and processing pathways.   

Transport through ionic layers in sulfonated telechelic polyethylenes

We present a set of sulfonated telechelic polyethylene ionomers that demonstrate ion transport of metal cations within ionic aggregates in a crystalline polymer matrix. These precise ionomers consist of 23 or 48 backbone carbons with sulfonic acid end groups that are fully neutralized by a counterion, Li⁺ or Na⁺. Depending on spacer length and counterion, these telechelics exhibit multiple order-order transitions at T<T_m and order-disorder transitions at T~T_m, with melting points up to 300°C, as evident in both differential scanning calorimetry and X-ray scattering. At room temperature, the most common morphology is well-defined nanoscale ionic layers with a crystalline polymer backbone. The temperature-dependent ionic conductivity is characterized using electrical impedance spectroscopy. While ion transport appears decoupled from the polymer backbone at T<T_m under certain conditions, the conductivity remains quite low.   

Current-induced morphological changes in block copolymer electrolytes

Block copolymers are attractive electrolyte materials for lithium metal batteries due to their ability to microphase separate into distinct ion-conducting and mechanically-reinforcing domains on the nanometer-length-scale. Although the formation of current-induced gradient crystals has been previously shown when the material was heated above the glass transition temperature, T_g, of the mechanically-reinforcing block, it has been previously assumed that these equilibrium morphologies remain intact throughout polarization under the T_g, e.g. at 90 °C. In-situ small angle X-ray scattering (SAXS) experiments were used to probe morphology and electrochemical impedance spectroscopy (EIS) was used to quantify the bulk resistance of the electrolyte throughout cycles of cell polarization and subsequent cell relaxation of lithium-lithium symmetric cells as a function of polarization voltage. The electrolyte is comprised of a polystyrene–b-poly(ethylene oxide), PS-b-PEO, copolymer mixed with a lithium salt that forms hexagonally packed PS cylinders in a matrix of salt-containing PEO in the absence of current. We hypothesize that the formation of salt-concentration gradients within the electrolyte from the cell polarization drive the changes seen in morphology and resistance.   

Polymer Electrolytes with Abundant Hydrogen Bonding Sites

Coulombic and/or dipolar interactions of polymers with embedded salts offer ion conduction across the polymer chains. Such polymers can be used in a wide range of electrochemical devices such as lithium batteries, fuel cells, and ionic actuators. Key challenges in developing practically viable devices based on polymer electrolytes are high conductivity, mechanical strength, and electrochemical stability. In this study, new polymer electrolytes possessing abundant hydrogen bonding sites for simultaneous achievement of high conductivity and improved mechanical properties. Owing to the high dielectric constant of such polymers, enhanced degree of ion dissociation and high ionic conductivity were obtained. In particular, with added inorganic salts, the hydrogen bonding sites in polymer structure effectively stabilize anion, thereby retarding the anion diffusion. This is connected to improved cation transference number of the resultant polymer electrolytes.   

The Effect of Host Incompatibility and Polarity Contrast on Ion Transport in Ternary Polymer-Polymer-Salt Blend Electrolytes

Conventional lithium-ion battery electrolytes are typically formed by blending high polarity and high mobility small-molecule components. These electrolytes tend to have ionic conductivities higher than those formed from either component alone. This is hypothesized to arise from a molecular-level synergy between miscible components. In short, molecular simulations have suggested that lithium ion solvation is dominated by the high polarity component. The solvated ions are then able to diffuse through a medium dominated by the high mobility component.

We hypothesize that similar effects can be seen in miscible polymeric blend electrolytes (PBEs) containing a high polarity and high mobility component. We have previously shown that increasing host polarity both improves solvation strength and slows dynamics. We thus investigate the role of changing the polarity of the nominally high polarity polymer on ionic conduction. Further, we study the influence of PBE miscibility on transport by varying the bare interaction parameter between polymer hosts. As hypothesized, we find that ionic conduction can be improved with increased miscibility and at intermediate levels of polarity contrast. We preliminarily investigate the effect of host molecular weight contrast on ionic transport.   

Ion Transport in Ether-Based Polymer Electrolytes

Polymer electrolytes, typically poly(ethylene oxide) (PEO) mixed with a lithium salt, have emerged as promising solid electrolytes for lithium ion batteries. However, polymer electrolytes still exhibit lower conductivities than commercial liquid electrolytes. We report a novel ether-based polymer, poly(diethylene oxide-alt-oxymethylene), (P(2EO-MO)), which was synthesized via cationic ring-opening polymerization of the cyclic acetal monomer, 1,3,6-trioxocane. The polymer electrolytes, which are mixtures of P(2EO-MO) and various concentrations of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, were characterized by electrochemical methods. P(2EO-MO)/LiTFSI exhibits lower ionic conductivity than PEO/LiTFSI due to increasing glass transition temperatures in the presence of lithium salts. However, the steady-state transference number of P(2EO-MO)/LiTFSI is significantly higher than that of PEO/LiTFSI at most salt concentrations. We investigate the thermodynamic factor and steady-state current of P(2EO-MO)/LiTFSI as a function of salt concentration and compare the results to PEO/LiTFSI. This work suggests that the overall efficacy of ion transport in P(2EO-MO) is greater than that in PEO, and P(2EO-MO) is a favorable electrolyte for lithium ion batteries.   

Entropic transport of interacting Brownian particles in a channel with reflecting boundaries

The entropic transport of overdamped biased Brownian particles in a symmetric channel is investigated numerically considering both the no-flow and the reflection boundary conditions at the channel boundaries. The constrained dynamics yields a scaling parameter f, which is a ratio of the work done to the particles to available thermal energy. The distinct transport features are observed with reflection boundary conditions, for example, the nonlinear mobility exhibits a nonmonotonic behavior as a function of the scaling parameter f, and the effective diffusion coefficient exhibits a rapidly increasing behavior at higher f, which are not observed with no-flow boundary conditions. We show that the transport properties can be significantly influenced by the nature of reflection, i.e., elastic or inelastic. In doing so, we identify that both the nonlinear mobility and the effective diffusion coefficient can be enhanced with inelastic reflection boundary conditions. In addition, by including the short range interaction force between the Brownian particles, the mobility decreases, and the effective diffusion coefficient increases for the optimal values of f.   

Shear-induced Counterion Redistribution of a Single Polyelectrolyte

The counterion distribution of a model polyelectrolyte, sodium polystyrene sulfonate (NaPSS), as a function of shear rate is investigated at single molecular level. The fluorescence resonance energy transfer (FRET) between the fluorescence donor attached at the charged PSS^- chain and the positively charged acceptor as the counterion probes shows the increase of average counterion-chain distance, indicating the expansion of counterion cloud by shear. Such a process is further verified by the emission spectra of the pH-responsive fluorophore labeled at PSS^- chain end, showing the local pH value is shifted to higher values by shear. The shear-induced counterion release or counterion cloud expansion is attributed to alternation of the electric filed distribution inside the solution and the overlap of electric fields of multiple PSS^- chains in the process of creating inhomogeneity in concentration inside the PSS^- solution by shear.   

Through-plane Structural Analysis of Engineered Nafion Surfaces

The thin-film structures of Nafion, a model perfluorinated ionomer, impact practical advances in proton-exchange membrane fuel cells (PEMFCs). Engineered Nafion surfaces were developed by Dowd et al. to reversibly and permanently alter the surface composition and wettability of Nafion^[a]. Using neutron reflectometry (NR), we probe the through-plane structure of surface-treated Nafion thin-films with sub-Ångstrom resolution. Humidity-related layers of phase-separated lamellae were observed in Nafion thin-films at Au, Pt, and SiO₂ interfaces by Dura et al. and DeCaluwe et al^[b,c]. We develop and apply titanium nitride (TiN) substrates to minimize the neutron scattering contrast to similar Nafion-TiN interface structures, thus optimizing the sensitivity of NR to the thickness, roughness, and scattering-length density (SLD) of the engineered Nafion surfaces. The engineered Nafion thin-films are assessed in dry (0% RH) and wet (92% RH) conditions in H₂O and D₂O vapor to independently determine water content and polymer density, before and after surface modification. Additionally, we evaluate the formation structures at the Nafion-TiN interface.

^[a]10.1149/2.1081702jes ^[b]10.1021/ma802823j ^[c]10.1039/C4SM00850B   

Single-ion polymers based on ion-conducting crystalline phases

Recently, single-ion conducting polymers have extensively been studied as future polymer electrolytes owing to the minimized device polarization at a given dc voltage. In this study, we investigate single-ion conducting polymers containing zwitterions. Based on computational calculations, design and synthesis of zwitterion were carried out to modulate intermolecular interactions in single-ion polymers via controlling dipolar orientation of ionic moieties. With balanced Coulombic and dipolar interactions, well-defined ionic crystals were formed within the ionic phases. Particularly, nanoconfinements given by the use of acid-tethred block copolymer matrix were a key to induce the crystallization of ionic moieties. Hierarchical self-assembly of the zwitterion-containing single-ion polymers was further characterized with a focus on the structure-transport relationship.   

On the Transference Numbers and Inverse Haven Ratios of Ionic Liquids and Polymeric Ionic Liquids

In this study, we used atomistic computer simulations within a non-equilibrium molecular dynamics framework to probe the transference number and inverse Haven ratio of ionic liquids and polymerized ionic liquids. In contrast to the conventional expectations, we find that the transference number in in these systems is a constant at approximately 0.5 and independent of the degree of polymerization (DP). The inverse Haven ratio increases first with increasing DP and then decreases at larger DP. We demonstrate that such results arise as a consequence of the strong cation-cation correlated motions. Together, such findings challenge the premise underlying the pursuit of single ion conductors as a means towards enhanced transference numbers.   

Comparing Ion Conductivity and Transference Number of Single-ion and Salt-doped Block Copolymer Electrolytes

Nanostructured block copolymer electrolytes with both ion conductive and mechanically robust microphases are strong candidates for solid battery electrolytes. However, their performance is limited by their low cation transference number (fractional contribution of the cation to the overall conductivity). Experimental work has showed a potential route of increasing transference number by tethering anions to the polymer backbone of the conducting segment of block copolymers. Due to the synthetic challenges of such materials, the design space has yet to be well explored. It is unclear how to optimize their conductivity and under what conditions these single-ion block copolymer electrolytes can more efficiently conduct lithium ion than the salt-doped materials. In this work, we perform coarse-grained molecular dynamics simulations with an applied electric field to calculate ion conductivity and transference number in both single-ion and salt-doped block copolymers. We aim to show how the choices of molecular weight, ion loading, and anion type impact ion transport to guide design of new materials.   

Enhanced ion transport in block polymer electrolytes through the manipulation of salt and monomer segment distributions

Solid block polymer (BP) electrolytes for lithium-ion batteries can address safety and performance concerns present in conventional liquid-state electrolytes, but the ion transport in BPs requires significant improvement to meet the demands of current and future battery applications. Transport properties in nanostructured BP electrolytes can be enhanced through the modification of salt and monomer segment distributions within the ion-conducting domain. We explored two methods to tune these distributions: the synthesis of polystyrene-block-poly(oligo-oxyethylene methacrylate) (PS-b-POEM) BPs with gradient or random copolymer regions at the chemical junction between the PS and POEM blocks (i.e., tapered block polymers) and the blending of POEM homopolymers of different molecular weights into PS-b-POEM BPs. For both methods, we connected the structural characteristics, such as the salt and monomer segment distributions, determined by X-ray and neutron reflectivity, to the segmental and ion dynamics measured through differential scanning calorimetry, ⁷Li solid-state NMR, and AC impedance spectroscopy. These results elucidated design parameters in the synthesis and fabrication of BP electrolytes that can increase ionic conductivity.   

Investigation of proton conductivity in polymer nanocomposite films

Proton-conducting membranes and solid electrolytes are critical components in many electrochemical devices, and the performance of such devices is often limited by proton transport. In this work, we study proton conductivity in polymer nanocomposite films. We experimentally observe multifold increase in proton conductivity in a composite of poly(ethylene oxide) (PEO) and cerium oxide (CeO₂) nanocrystals, in comparison to their individual counterparts. Employing kinetic Monte Carlo simulations, we model proton transport in such systems. Our results reveal the significance of percolation and connectivity of filler/polymer interface in such systems, with further scope for improving conductivity by tuning the morphology of nanocrystals. We also study the overall proton conductivity at different nanocrystal compositions, predicting an optimum composition for maximum proton conductivity.   

The effects of processing method on conductivity and dielectric relaxations in PVDF blended with a zwitterionic copolymer

The conductivity and relaxation dynamics of poly(vinylidene fluoride) (PVDF) blended with a random copolymer of methyl methacrylate and sulfobetaine-2-vinylpyridine (PMMA-r-SB2VP) were investigated. Films were prepared using two processing methods, compression molding and doctor blading from solution. Scanning electron microscopy revealed morphological differences between the films, with the doctor bladed films demonstrating a porous microstructure, while FTIR revealed the presence of different crystallographic phases for the films. Dielectric relaxation in the temperature range from 30 ^oC to 140 ^oC showed several relaxations in compression molded films due to the motion of dipoles in the PVDF crystal phase, segmental relaxations of PMMA, as well as a unique relaxation seen only in the blends. Blends demonstrated higher conductivity then the neat PVDF and copolymer. Doctor bladed films showed a large decrease in the dielectric constant and conductivity as well as different relaxation behavior compared to the compression molded films. These differences suggest that influence of the different processing techniques on the molecular environment plays a significant role on the dielectric properties of these PVDF zwitterionic copolymer blends.   

Gyroid Morphologies in Single-Ion Conducting Polymers and the Consequences for Ion Conductivity

The morphology of self-assembled ionic aggregates influences the ion transport in single-ion conducting polymers. Recently, our group reported that polymers with sulfosuccinate units containing sulfonate groups with their counterions (Li⁺, Na⁺, and Cs⁺) and precisely separated by 23 methylene units form ordered ionic aggregate morphologies (layered, gyroid, and hexagonal). In these ionomers, the gyroid morphology exhibits higher ion conductivity than layered and hexagonal ionic aggregates morphologies, demonstrating the importance of bicontinuous ion-containing channels for enhanced ion conductivity. These gyroid morphologies were reported at relatively high temperatures (120°C - 130°C) that are coincident with the melting point of the crystalline polyethylene unit. To produce the gyroid phase in a lower temperature range, polymers with shorter polyethylene units are being explored and results will be discussed using a phase diagram with volume fraction of sulfosuccinate units (f_polar) and temperature, similar to conventional block copolymer phase diagrams. This study endeavors to provide design strategies for single-ion conducting polymers with controlled ionic aggregate morphologies and enhanced ion conducting properties.   

Understanding the interactions of polyols with hexafluoroisopropanol containing polynorbornene biobutanol membranes using QCM-D

Polymer membranes offer a low-cost path to separate bio-products. However, interactions of the membrane with components in the fermentation broth can alter its performance. Here we describe how ppm levels of polyol surfactant designed to inhibit foaming of the broth can dramatically swell and plasticizing the polynorbornene copolymers in the bio-butanol separation. in-situ quartz crystal microbalance with dissipation (QCM-D) enable the quantification of both the swelling and rheological properties. We examined the molecular-weight-dependent sorption behavior of 10ppm of polyethylene glycol (PEG) and polypropylene glycol (PPG) for a series of copolymer membranes that all contain 50 mol% hexafluoroisopropanol norbornene and 50 mol% alkyl (methyl to decyl) norbornene. Changing the alkyl side length only modestly impacts the swelling behavior, which is consistent with hydrogen bonding between hydroxy groups driving the sorption. It shows that PPG, not PEG, is the primary cause for the enhanced swelling. Increasing molecular weight of the PPG leads to increased swelling (entropic effect) but also slower the sorption rate (size effect). These results illustrate that details about components added to the broth can dramatically affect the membrane properties.   

Self-assembly of Linear Block Copolymers and Bottlebrush Block Copolymers in Thin Films

Semiconductor device fabrication has been developing rapidly in recent years. It's been harder to keep up with Moore's law since top-down photolithography is approaching the diffraction limit. Self-assembly of block copolymers is a promising solution for its highly reproducible self-assembly, great diversity of morphology, good etching contrast, economic efficiency and recipes compatible for industrialization. Well-aligned self-assembled patterns with sub-10 nm feature size and good etching contrast in thin films have been achieved in recent years. However, poor solubility, microdomain orientation and high energy barrier for defect annihilation remain from their application. We developed a series of linear block copolymers and bottlebrush block copolymers containing ketal groups as side chains for low-χ to high-χ conversion in the solid state. Self-assembled lamellar and cylindrical morphologies, with a 5.4 nm full pitch have been obtained in the bulk and 9.4 nm full pitch in thin films, which is among the smallest domains obtained so far.   

Macrorheology and particle tracking to study tracer transport, viscoelasticity and network structure of mucin and mucin-like biopolymer solutions

Passive microrheology using single-particle and multi-particle techniques can be used to assess the local transport, mechanical properties and network structure of biopolymer laden biological fluids. Using single particle tracking (SPT) and multiple particle tracking (MPT) passive microrheology, we probe the dynamics of tracer particles ranging from 0.1 to 10 microns in size in mucin-like Carboxy Methylcellulose (CMC) solutions and in re-constituited mucin solutions. Tracking and linking algorithms implemented in MATLAB and Python reconstruct 2-dimensional trajectories from sequential list of particle coordinates. Corrected data is used to calculate the distributions of displacements and velocities, ensemble averaged mean square displacement, and particle velocity autocorrelations from which we extract diffusion coefficients of the particles and tracer-size dependent viscoelastic properties of the medium. Passive microrheology combined with macrorheology studies and confocal derived connectivity and morphology measurements provide a base state to understand the dynamics of living active materials such Candida albicans biofilms.   

Rheological Scaling of Imidazolium-Based Polyelectrolyte in Ionic Liquid Semidilute Solutions

Polymerized ionic liquids (PILs) are a special type of polyelectrolytes with ionic liquid moieties covalently attached to a polymer backbone. Existing studies show that electrostatic interactions play a dominant role in determining the viscoelastic properties of ordinary polyelecrolytes. Recently, we found that the charge underscreening effect, in which the screening length increases with increasing the ion concentration, resulted in expanding PIL chains at high ion concentrations in the dilute regime. In this study, we investigate the effect of charge underscreening on the rheological properties of PILs in the semidilute unentangled regime by using a model system consisting of a PIL (PC₄–TFSI) in a mixture of an ionic liquid (BmimTFSI) and a non-ionic solvent (DMF). We observe: i) both specific viscosity η_sp and relaxation time λ are initially constants at low ion concentrations c_s and then decrease with increasing c_s at an intermediate c_s. ii) both η_sp and λ increase with increasing c_s. This result indicates that the charge underscreening effect is still dominant in the semidilute unentangled regime. We capture the observed trend by proposing and validating a modified scaling law accounting for the dependence of correlation length on c_s.   

Rheological Properties of Bare and Grafted Nanoparticle Polymer Networks

This study investigates the rheological properties of crosslinked poly(methyl methacrylate) (PMMA) composites with bare and grafted nanoparticle constituents. We will present the influence of crosslinking density and particle loading on linear rheological properties of PMMA networks. These results will be compared with the PMMA-grafted particles in crosslinked PMMA matrices. Effects of including the short linear chains into crosslinked composites of bare and grafted nanoparticles will be discussed. Topological constraints of crosslinked matrix will be compared to the crosslinked grafted nanoparticles to understand the addition of free chains and entanglements in these two different system networks.
  

Effect of topological constraints in semidilute polymer solutions under planar extensional flow

Polymer solution dynamics and rheology are relevant to a wide range of processing methods. Developing an understanding of the polymer conformational dynamics and the emergent material properties is challenging because of the interplay of hydrodynamic interactions (HI), excluded volume, and topological constraints driven by concentration and polymer architecture. This is particularly true in extensional flow, which strongly deforms the polymers from their equilibrium conformations. Using a new technique for rapid Brownian dynamics (BD) simulation which we call the iterative conformational averaging (CA) method, we investigate the dynamics and rheology of linear, comb, and ring polymer solutions at concentrations increasing from the dilute limit into the semidilute regime. We apply step strain rate planar extensional flow and quantify the dynamics in startup, at steady state, and after flow cessation via conformational distributions and the polymer contribution to extensional viscosity. We show that flow enhances intermolecular HI and topological interactions, resulting in transient intermolecular entanglements. We investigate the effect of these constraints on polymer stretching and the transient solution stress.   

The effects of the structure of a confinement on the ejection rate of a polymer chain from a nanopore

Conformations of DNA inside the phage are crucial to the ejection dynamics of the DNA. The equilibrium conformation of DNA inside the viral capsid is determined by the shape of the capsid (confinement): a concentric spool DNA and a folded twist DNA are considered equilibrium conformations in a spherical capsid and an elongated capsid, respectively. Because there are a variety of viral capsids in nature, the correlation between the capsid shape and the ejection dynamics should be a topic of interest. In this study, we investigate the effects of the capsid shape on the conformation of a polymer and its ejection rate by performing Langevin Dynamics simulations. We employed two different types of capsid: 1) sphere and 2) cube capsids. We find that a polymer chain ejects out of the capsid faster in case of the sphere than the cube capsid and that the ejection rate becomes faster by up to 35% as the rigidity of the polymer increases. The segments of the polymer near the wall of the capsid travel faster than other segments, regardless of the capsid shape. However, a larger fraction of the monomers are distributed near the wall in case of the sphere capsid than the cube capsid.   

Interfacial Dynamics Governs the Mechanical Properties of Nanoconfined Glassy Polymers

Understanding the mechanical properties of nanoconfined glassy polymers is essential in design of nanostructured soft materials. Here, we investigate the mechanical properties of free-standing polymer thin films by employing an atomistically informed coarse-grained (CG) modeling approach. By examining three representative CG polymer models, i.e., polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(1-ethylcyclopentyl methacrylate) (PECPMA), we show that the elastic moduli of nanoscale thin films are substantially reduced with decreasing film thickness compared to their bulk values at their glassy state. Specifically, the PS and PMMA films exhibit similar size-dependent elastic responses and their film moduli are reduced compared to bulk values at a thickness of less than 40 nm, whereas, for PECPMA, the length scale where elastic modulus deviates from the bulk value is much larger. The local molecular stiffness within the films assessed by Debye-Waller factor further reveals a gradient a softer interfacial layer having a size of only a few nanometers. Based on our simulations, a bilayer composite model is employed to predict the elastic moduli of thin films, which uncovers the size scaling relationship that universally holds for all three polymers.   

Surface and bulk dynamics of compressed polystyrene films: A β-NMR study

Glasses gradually densify as they relax towards equilibrium, and in doing so, their spectral density of molecular fluctuations is modified. In polystyrene (PS), only a few percent increase in density is equivalent to the passage of millions of years. To access this regime, we have plastically deformed a PS thin film with nanoimprint lithography, resulting in permanent compactification without rejuventation¹. We present depth-resolved ⁸Li⁺ β-NMR measurements in PS films and discuss the dynamical effects of the mechanical processing. β-NMR is a technique sensitive to nanosecond molecular dynamics through the spin-lattice relaxation of a short-lived radioisotope. Because the isotope is implanted as a low-energy ion beam, the depth is controllable with nanometer-scale resolution. This has allowed β-NMR to directly probe the dynamics in glassy polymer thin films near both the free and buried interfaces².

¹ H. D. Rowland et al., Science 322, 720–724 (2008).
² I. McKenzie et al., Soft Matter 14, 7291–7544 (2018).   

Reduction of Dielectric Signal of the Interfacial Segmental Dynamics in Polymer Nanocomposites

In this study we focus on polymer nanocomposites (PNCs) with dispersed silica core nanoparticles. We examine the dielectric strength and relaxation behavior of segmental dynamics in the interfacial polymer layer surrounding silica nanoparticles. The presented analysis reveals the significant drop in the dielectric strength, and its anomalous temperature dependence in polymer layer adsorbed to nanoparticles. We ascribe the observed effect to the restricted amplitude of segmental relaxation in the interfacial/adsorbed layer. The theoretical model explaining the unusual temperature dependence of dielectric strength is presented. Our results provide new view on discussion of dynamics in interfacial layer in PNC that may be applied to the thin polymer films as well: Not only characteristic time scale, but also amplitude of structural relaxation can be strongly affected by the presence of an interface.   

The Influence of Polymer and Ion Solvation on Counter-ion Cloud Formation and Charge Fluctuations in Highly Charged Polyelectrolytes

We investigate the influence of solvent affinity to counter-ions and to the polyelectrolyte (PE) backbone on the charge distribution about highly charged flexible polymer chains based on coarse-grained molecular dynamics simulations that include both explicit counter-ions and solvent. Based on this framework, we find that the competitive solvation of ions and PE chains leads to an extended “cloud” of counterions about the polymers that cannot be understood without modeling the solvent explicitly. The counter-ion cloud is highly dynamic and fluctuations of the charge can be expected to great influence the polarizability of PEs in solution. After reviewing recent findings on the influence of molecular topology and interactions on the average size of the counter-ion cloud, we estimated the PE potential of the mean interaction to determine the influence of the couterion-cloud and charge fluctuation effects on inter-PE interactions. Consistent with the Kirkwood-Schumaker theory, we find evidence that charge fluctuations gives rise to a long range attractive interaction having a 1/ r² asymptotic decay at large interpolymer separation distances r in the case of strongly hydrating counterions and a PE backbone that is not strongly hydrating, a model of relevance to many synthetic PEs.   

Reflection band-gap for a transversely stretched composite cholesteric elastomer

We have studied the conduction of electromagnetic waves, impinging normally in a composite cholesteric elastomer slab, doped with metallic inclusions, aleatory distributed in the structure. We performed a theoretical and numerical model, that allow us to obtain the reflection and transmission spectra, when the system is under the action of mechanical forces in transverse direction respect to cholesteric axis. We have found that by stretching transversely the elastomer slab after the critical value of stretching in which the helical structure gets unwind, it transform from a discriminatory circular filter to a polarization independent device. Intervals of conversion from right to left circularly polarized waves in the reflection spectra, are alternated with regions of resembling transmission of both circularly polarized waves.The spectra of the pure cholesteric elastomer can be modulated specially nearby the metallic resonance where the transmittances are greatly damped and reflection is considerably increased.   

Structure Instability in Particle Filled Elastomeric Polymer Composite Under Tensile Stress

The buckling instability and crease development in elastomeric soft material under compression has been experimentally and theoretically studied over the past decades[1]. The structural instability of such material under tensile stress, however, has not been well understood. In this work, we present the structure instability of a polymer particle composite under tensile stress. It is observed that periodic undulation in sample surface develops as the tensile stress increases, manifested as linear ridges and valleys on the sample surface perpendicular to the direction of tension. The undulation is reversible at small strains. There is a memory effect in the undulation location at repeated strain applications. At large enough strain, the material eventually fractures from crack developed at one of the valley regions from the undulation structure.

References
[1] Shengqiang Cai, Dayong Chen, Zhigang Suo, Ryan C. Hayward, Creasing instability of elastomer films, Soft Matter, 2012, 8, 1301-1304   

Thermo-electro-mechanical and Rheological Properties of Rubber Composites Filled with sp2-Hybridized Different Carbon Fillers

Rubber composites (RC) with enhanced thermo-electric properties are widely used in applications such as in flexible electrodes and solid tyres. In this work, the effect of carbon black (CB), multiwall carbon nano tubes (MWCNT) and natural graphite on thermal, electrical, mechanical and rheological properties of RC was studied. In all these fillers, carbon mainly has sp² hybridization. When the MWCNT loading was increased from 5 to 60 phr, the electrical conductivity of RC increased by four orders of magnitude. At 60 phr MWCNT loading showed five orders of magnitude higher electrical conductivity than CB added sample. The graphite loaded RCs were nonconductive. Hardness, elastic modulus, tensile and tearing strengths increased gradually with the increase of filler loadings. The variation of modulus of graphite-loaded samples was subtle. Interestingly addition of graphite showed higher elongation at break than that of pristine rubber. Viscoelastic characteristics showed that MWCNT added samples have least time and graphite added samples have highest time for curing compared to CB loaded samples. Although carbon-based fillers responsible for a significant improvement in their overall properties type of filler controls the extent to which the property changed.   

Aqueous pigment dispersions: The thermodynamics of hierarchical aggregation

Many industrially important materials aggregate to form nanoscale mass-fractal structures. Unlike sintered aggregates such as fumed silica, aqueous pigment-based inks often consist of weakly bound nanoparticles stabilized by a surfactant that can break apart and re-form balancing mixing energy and the reduction in surface energy with aggregation. Rapid thermal motion of small elemental crystallites lead to dense primary particle clusters with slower thermal motion that aggregate into ramified mass fractals. It is proposed that the hierarchical structure relies on subtle and competitive equilibria between different hierarchical structural levels.¹ In the context of the removal of a subunit from a cluster, the thermodynamics of nanoparticle hierarchical equilibria was explored on surfactant-stabilized pigment dispersions using the Vogtt Theory.² Reversible nanoparticle aggregation could be described solely from the degree of aggregation and the volume fraction. In this case, the hierarchical thermodynamics at each of three structural levels is dominated by solubility of the nonionic dispersing surfactant that decreases with temperature (LCST).

¹K. Vogtt et al., Phys. Rev. Research 1 (2019)
²K. Rishi et al., Langmuir 35, 13100 (2019)   

A model nanocomposite designed to understand the interfacial behavior in a novel thermally stiffening nanocomposite

A novel polymer-adsorbed silica-reinforced nanocomposite with a heterogeneous structure was recently shown to have thermal-stiffening behavior. Although the macroscopic properties of this nanocomposite were characterized via rheological experiments, the structure and dynamics at the matrix-nanofiller interface remain to be investigated. In the current study, we use a simplified 2D system to mimic the complex 3D structure of the nanocomposite. The simplified sandwich structure consists of a flat silica substrate, a thin layer of adsorbed high glass transition temperature polymer (PMMA, P2VP, and PC), and a final layer of PEO. Modulated Differential Scanning Calorimetry (MDSC) and Thermogravimetric Analysis (TGA) experiments indicated that the conformation and the relaxation in the confined environment surrounded by the hydroxyl group-rich silica surface and flexible PEO chains were significantly altered. Besides, by carefully changing the thickness, annealing condition, and molecular weight, different trends were observed which corresponds to different possible mechanisms. The results are of great help in understanding the previous real nanocomposite results and they also provide some strategies to design nanocomposites with desired properties.


  

Compatibility/incompatibility in surface-modified, aggregated, precipitated silica nanocomposites

Industrially relevant products often display a complex multi-level hierarchical, nano- to macro- scale structure. Control over this complex multi-hierarchical structure can be achieved through manipulation of filler-polymer compatibility/incompatibility such as by varying the silanol surface density, by chemically-tailoring the surface, and by grafting low molecular weight polymers. These modifications control dispersion and the associated emergent multi-hierarchy. Kinetic dispersion in reinforced elastomers has been likened to thermal dispersion leading to a pseudo-thermodynamic model.[1,2] The dispersion of modified fillers is quantified using this approach. These interactions can be classified as weak and strong depending on the presence of correlations. Interactions and dispersion are modeled using a random-phase or a modified Born-Green approach. Surface alteration was linked to the emergence of a multi-hierarchy. From the mesh size and packing of an emergent network, the state of dispersion and the interaction potential for coarse-grain simulations were determined.

[1] Y. Jin et al., Polymer (Guildf.) 129, 32 (2017)
[2] A. McGlasson et al., submitted to Macromolecules (2019)   

New Insights into Hierarchical Structures in Polymer Nanocomposites: A Dissipative Particle Dynamics (DPD) Simulation Study

Polymeric systems such as natural rubber used in car and truck tires require the addition of suitable additives for the enhancement of numerous properties, including reinforcement and durability. The behavior of such fillers, (carbon black, silica, and metal oxides and some combination thereof), and their influence on nanocomposite effectiveness, depends on the filler structure, the interaction between filler-polymer matrix as well as the processing history. To understand this problem, we perform Dissipative Particle Dynamics (DPD) simulation of these blends, varying polymer-polymer, filler-filler, and polymer-filler interaction energy. We will discuss the effects of interaction strength, the scaling of polymer chains, and methods to quantify the filler percolation threshold and mesh size as a function of filler concentration. The simulation results are also validated against small angle x-ray scattering data. Additionally, the effect of such agglomerates on the structural and dynamical properties of the nanocomposites, measured via the radial distribution, mean square displacement, and autocorrelation function are also explored.   

Localizing Genesis in Polydomain Liquid Crystal Elastomers

Programming the local orientation of liquid crystal elastomers (LCEs) is a differentiated approach to prepare monolithic material compositions with localized deformation. Our prior efforts prepared LCEs with surface-enforced spatial variations in orientation to localize deformation when the LCEs were subjected to directional load. However, because these surface alignment methods included regions of planar orientation, the deformation of these programmed LCEs is inherently directional. The absence of macroscopic orientation in polydomain LCEs results in uniform, nonlinear deformation in all axes (omnidirectional soft elasticity). Here, we exploit the distinct mechanical response of polydomain LCEs prepared with isotropic or nematic genesis. By localizing the polydomain genesis via masked photopolymerizations conducted at different temperatures, we detail the preparation of main-chain, polydomain LCEs that are homogeneous in composition but exhibit spatially localized programmability in their mechanical response that is uniform in all directions.   

Optical reconfiguration of the blue phase in liquid crystalline elastomers

Liquid crystalline elastomers (LCEs) have been a topic of significant interest, largely motivated by their compelling performance as material actuators. Here, we detail distinctive optical reconfiguration of solid LCE compositions based on the cubic blue phases. These phases are retained in the LCE composition and exhibit selective reflection attributable to a periodic lattice superstructure. Strain-induced tuning of the selective reflection is detailed, associated with dimensional changes to lattice parameters. These materials exhibit mechanical properties that diverge from those observed within cholesteric LCEs. The influence of preparation conditions on the retention of the blue phase as well as processing conditions to increase platelet size will be discussed.
  

Predicting Stress-Strain Behavior of Thermoplastic Elastomer by Theoretical Calculation and Deep Learning

Thermoplastic elastomer (TPE) is a typical industrial product where the microphase separation of block copolymer is utilized. This elastic behavior is one of the examples where the phase separated structure affects the physical properties. However, it is not simple to find the relation between complicate phase separated structure and stress-strain behavior. To tackle the problem, we applied coarse-grained simulation and deep learning technique. Stress-strain curve of various phase separated structures of ABA type triblock copolymers, where A blocks and B blocks form glassy domain and rubbery domain respectively, are investigated by the collaborative simulation of self-consistent field theory and coarse-grained molecular dynamics. Furthermore, we applied deep learning approach to make regression between the phase separated structure and stress-strain (S-S) behavior obtained by the computational simulation. The prediction of S-S behavior using trained deep learning network showed reasonably good results, and was very fast comparing to the computationally intensive simulation.   

BigSMILES: A Digitalization Scheme for Data-Driven Macromolecules Research

In polymer research, a major hurdle preventing the adoption of data-driven approaches to modeling is the lack of a general digitalization scheme for polymeric systems. To address this issue, a digitalization scheme is proposed that consists of two components: first, a structurally based line notation that specifies how different repeating units interconnect to form polymers, and second, a data format that quantitatively specifies the distributional properties associated with the structure presented in the first part. The new line notation, BigSMILES, built on top of the popular line notation SMILES, encodes the chemical structures of polymeric fragments with “stochastic objects” that specify the constituent repeating units and the permissible set of connectivity patterns between them. Along with the accompanying data standard, BigSMILES provides a compact, machine-friendly yet versatile route to digitally encode and report polymeric materials. It is hoped that the proposed scheme can be easily utilized by both material scientist and modelling experts to enable rapid development of data-driven polymers research.   

Achieving Atomic Scale Resolution of Metastable Polymers in Solution using Machine Learning

Single-stranded DNA (ssDNA) is a metastable biopolymer that plays an important role in biological processes and has shown promise for applications in medicine and DNA nanotechnology. Understanding the structure of ssDNA in solution can provide a framework on how to control and manipulate the self-assembly and structure of ssDNA-based materials. Typically, experimental methods such as Small Angle X-ray Scattering (SAXS) are used to resolve conformational distributions of ssDNA; however, the low resolution does not provide enough information on structural details. We have developed a new method that utilizes molecular dynamic (MD) simulations in conjuncture with machine learning (ML) that can obtain converged structures of metastable polymers. This study specifically performs MD simulations on ssDNA and evaluates the conformational landscape with respect to SAXS. ML methods are first used to obtain a comprehensive collection of possible ssDNA structures through simulation and subsequently used to optimize which group of individual chains closely match experimental SAXS curves. Ultimately, this process lets us break down the ensemble average embedded in SAXS and find the most probable group of metastable conformations in solution with structures defined at the atomic scale.   

Structural Prediction and Inverse Design by a Strongly Correlated Neural Network

Macromolecules contain molecular units as the coding information for their correlated structures in physical dimensions. The relationship between these two features is governed by the interaction energies of the involved molecular units and their encoded sequences. We present a neural network algorithm that treats molecular units themselves as neural networks, which has the flexibility to allow each unit to respond to its own environment and to influence others in the system. Through a deep neural network and a self-consistent procedure, molecular units in the network establish a strong correlation to produce the desirable features in the physical world. The proposed framework is applied to the HP model. Both the forward problem of predicting folded structures from given sequences and the inverse problem of predicting required sequences for a given structure are examined.   

High-throughput study of mechanical properties of organic stable glasses by nanoindentation

Glasses with enhanced stability over ordinary glasses have been formed by the process of Physical Vapor Deposition using a sufficiently slow deposition rate and appropriate temperatures. These stable glasses have been shown to exhibit higher density, lower enthalpy, and better kinetic stability over ordinary glasses, and are typically optically birefringent. These properties depend on the temperature at which the substrate is held during deposition, with temperatures near .85T_g (glass transition temperature) producing the most stable glasses. Given such exceptional properties, it is of interest to further investigate how the properties of stable glasses vary with deposition temperature and compare to those of ordinary glasses. In particular, the mechanical properties of these glasses remain relatively under-investigated. Nanoindentation is a useful technique for determination of mechanical properties, though it can present a problem of surface detection in cases of soft surfaces. Correcting for this, and using a temperature gradient sample for high-throughput acquisition of data, mechanical properties are obtained for several organic glass-formers in order to explore the relationship between chemical structure, deposition temperature, and mechanical properties of stable glasses.   

Photovoltaic and Electrical Properties of Diketopyrrolopyrrole Based Organic Semiconductors

Methylated-diketopyrrolopyrrole (MeDPP) is a small molecule, air-stable organic semiconductor with relatively high mobility, which is capable of undergoing excitonic singlet fission. Here we study the electrical transport and optical properties of MeDPP-based field effect transistors and photovoltaic devices. Using the magnetic field dependence of photocurrent in MeDPP solar cells we present strong evidence of efficient harvesting of charge carriers from triplet excitons. These results suggest that MeDPP is a promising candidate for high efficiency, air stable photovoltaic devices.   

Low-temperature carrier transport of purely organic radicals embedded in double tunnel junctions

Purely organic radicals hold promise for molecular spintronics because the molecules have weak spin-orbit coupling and the feature is expected to achieve long spin relaxation time [1]. In this work, we evaluated the carrier transport through stable oligo(p-phenylene ethynylene)-based radical molecules (TEMPO-OPE) at cryogenic temperature (<20 K). A striking point of our device is that molecules are embedded in insulating layer of a metal–oxide–silicon (MOS) structure, where the structure acts as a double-tunnel junction [2]. Obvious stepwise currents were observed in the current-voltage measurements. Furthermore, the peak positions in differential conductance curves were consistent with the molecular orbitals of the TEMPO-OPE. These results clarify that the molecules still keep their radical characters even in solid-state devices. Our proposed device therefore paves the way to realize new molecular spintronics devices with further prospect of the integration into current silicon device.
Reference: [1] R. Hayakawa et al. Nano Lett. 16, 4960 (2016). [2] R. Hayakawa et al. Nanoscale 9, 112897 (2017).   

Glass Transition Temperature from the Chemical Structure of Conjugated Polymers

The glass transition temperature (T_g) is a key property that dictates the applicability of conjugated polymers. The T_g demarks the transition into a brittle glassy state, making its accurate prediction for conjugated polymers crucial for the design of soft, stretchable, or flexible electronics. In this work, only one adjustable parameter is used to build a relationship between the T_g and the molecular structure of 32 semiflexible (mostly conjugated) polymers that differ drastically in aromatic backbone and alkyl side chain chemistry. An effective mobility value, m, is calculated using an assigned atomic mobility value within each repeat unit. The only adjustable parameter in the calculation of m is the ratio of mobility between conjugated and non-conjugated atoms; the value for this ratio is supported by results from molecular dynamics simulations. We show that m correlates strongly to the T_g, and that this simple method predicts the T_g with a root-mean-square error of 13 K for alkylated conjugated polymers.   

Investigating vapor doping dynamics in poly(3-alkylthiophenes) using in situ technique

For conjugated polymers to be utilized in future organic electronic technology, their conductivity must be able to be controlled. Molecular doping has been used for this purpose, but details of this process are not well understood. Here, we report on a study of vapor doped poly(3-alkylthiophenes), a conjugated-polymer system that has been widely characterized. We use traditional dopant 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane and its less fluorinated cousins, along with poly(3-alkylthiophenes) with varying sidechain length, to probe how HOMO-LUMO overlap and spacing of the polymer affects the dynamics of the vapor doping process. These results are enhanced with characterization of the materials energetically and structurally at various time-points throughout the doping process. Spectroscopies such as UV-Vis, FTIR, and Raman give insight to the efficiency and mechanism of doping. This study shows how these techniques can be leveraged to probe how variables such as polymer structure, strength of dopant, and processing affect the final properties of molecularly-doped conjugated polymers.   

Mechanism of charge transfer and separation in polymer/nonfullerene acceptor organic solar cells

Recently, in the research fields on the organic solar cells (OSCs), it has been reported that it succeeded in synthesizing polymer/nonfullerene acceptor OSCs, and showed the power conversion efficiency (PCE) higher than polymer/fullerene type. Also, it was reported that nonfullerene acceptors were successfully fluorinated and showed higher PCE than non-fluorinated nonfullerene types. In this study, from the viewpoint of electronic structure, absorption spectrum, and HOMO–LUMO gap, we consider the mechanism of charge transfer and separation in polymer/nonfullerene acceptor OSCs, using time dependent density functional theory method.
We compared the absorption spectra and HOMO–LUMO gap of the PTB7/ITIC complex with those of the fullerene PTB7/PC₇₁BM complex, with those of the non-fluorinated nonfullerene PTB7/IT-4F complex. It turns out that nonfullerene type has an absorption band in the longer wavelength region compared to the fullerene type. Moreover, fluorination causes the absorption band to be slightly red shifted. In the nonfullerene type, Voc is reduced, but the PCE is increased. Therefore, the improvement in PCE is due to the Jsc improvement due to the increase in light absorption of nonfullerene acceptor.   

Towards the prediction and design of low-glass transition Donor-Acceptor semiconducting polymers

Past efforts on conjugated polymers, especially donor-acceptor (D-A) type polymers have mainly focused on the understanding of their structure-optoelectronic property relationship. However, the designing rule for fabricating soft and deformable semiconducting polymers is largely neglected. Here, we report a predictive linear model to quantitatively connect the glass transition behavior of D-A polymers to the flexibility of polymer chains, which is further verified through molecular dynamics simulation. The thermomechanical performance was characterized through pseudo-free-standing tensile test, thin film DMA and verified by AC-chip calorimetry. X-ray scattering and solution neutron scattering were applied to investigate the thin film morphology and polymer chain rigidity for further proof. This model shows its guidance role in molecular engineering of a low-glass transition pristine polymer with high mechanical deformability (~ 100%).   

Tuning side chains to affect phase behavior and charge mobilities of PCPDTBT donor-acceptor conjugated polymers

Elucidating the critical components of molecular design would accelerate the implementation of conjugated polymers for electronic and energy applications. Using a donor-acceptor alternating copolymer based on a polycyclopentadithiophene benzothiadiazole backbone (PCPDTBT), we modify side chains to perturb phase behavior and charge mobilities in organic field-effect transistors. We use both branched and linear side chains of varying length and observe changes in glass transition temperatures, melting temperatures and liquid crystalline clearing temperatures using a combination of calorimetry, rheology, and X-ray scattering. Fabricating and testing field-effect transistors with PCPDTBT as the active layer yield field-effect mobilities that depend on the side chain substituent. Optimizing side chain composition and architecture is likely crucial to achieve remarkable charge mobilities of 10 cm^2/Vs that is observed with devices based on PCPDTBT.   

The Impact of Illumination on the Photoluminescence and Depth Profile of MEH-PPV/dPS Thin Films

This study focuses on the changes in photoluminescence and film structure due to white light exposure during annealing of conjugated polymer blend thin films. Previous studies in our group have shown that annealing similar polymer blend thin films above the T_g of the polymers in a dark or illuminated environment alters the film structure. These structural changes should influence the photoluminescence (PL) activity observed for the film. The present work aims to investigate the correlation between light exposure, film morphology and photoluminescence for four film compositions, 5, 20, 35, and 50% MEH-PPV. The PL of the films are measured using Raman microscopy, which showed differences in PL activity depending on illumination during annealing at 125 °C. Samples that are exposed to light during annealing exhibited lower PL. Time-of-Flight Secondary Ion Mass Spectroscopy provides the depth profile of the films that enables the correlation of the film’s morphology and depth profile to the observed PL.   

Controlling the Backbone Flexibility of Conjugated Polymer to Achieve Superior Backbone Tensile Alignment

Recent reports have shown increased interest in the capacity of conjugation-break spacers (CBS) to soften relatively rod-like conjugated polymers (CP) while maintaining charge mobility. Here, we provide a holistic approach to understand the thermomechanical influence of CBS length on an n-type NDI-based conjugated polymer, denoted by PNDI-Cx. CBS lengths are varied from C0 (fully conjugated) to C7 with the CBS engineered into each repeat unit for systematic evaluation. Solution small angle neutron scattering and melt shear rheology were employed to provide the first quantitative evidence of CBS influence over CP chain rigidity and entanglement molecular weight (M_e), demonstrating an increase in chai flexibility, as well as a M_e independent of CBS length. Thermomechanical property, including: modulus, glass transition temperature, and melting temperature were all shown to decrease with increasing CBS length, while a high ductility was observed for PNDI-C4 which we attribute to a large number of entanglements. Furthermore, a high degree of backbone alignment was observed upon strain through GIXD, polarized UV-vis, and AFM. We will also discuss coalignment in a blend system of PNDI-C0 (tie-chain) in a ductile PNDI-C4 (matrix) to yield enhanced charge mobilities.   

Photoabsorption of acceptor molecules in non-fullerene type organic thin film solar cells

The π-conjugated system is important as a semiconductor material in organic electronics such as organic light emitting diodes, field effect transistors, and organic solar cells (OSCs). Organic semiconductors can adjust the energy levels of HOMO and LUMO, so it is also possible to adjust the relative energy between the molecular orbital of the organic semiconductor and the Fermi level of the metal electrode. Therefore, selective insertion/transport of holes/electrons is essential to the realization of organic devices. Recently, it has been reported that the bulk heterojunction OSCs, which contain fluorinated naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole-based non-fullerene acceptors has a better photovoltaic performance. In this study, we theoretically investigate the effects of fluorine atoms on the properties and photovoltaic performance from the perspective of molecular structure, HOMO-LUMO energy gap, and absorption spectrum. FNTz-T_eh-FA was found to have a lower LUMO energy. Therefore, by fluorine substitution, it is expected that J_SC will be increased, and charge separation will be enhanced. These results provide the key to develop new organic semiconductors with the application of donor-acceptor type donors and non-fullerene acceptor materials in OSCs.   

Controlling mixed Li/electronic conduction in conjugated polymeric ionic liquids through the addition of ionic and electronic dopants

Conjugated polymeric ionic liquids (conjugated PILs) may be useful in electrochemical applications, owing to their potential for simultaneous ionic and electronic conductivity. The addition of ionic and electronic dopants is essential to control the mixed conductivity, although the doping design rules remain largely unknown. Here, we investigate the roles of LiBF₄ as an ionic dopant and NOBF₄ as an electronic dopant on the overall mixed conduction of a model conjugated PIL, P3HT(Im⁺)BF₄^-. The thiophene backbone is selected to allow hole conduction while the ionic (Im⁺)BF₄^- sidechain solvates mobile ions. The addition of either the ionic dopant LiBF₄ or the electronic dopant NOBF₄ leads to a simultaneous increase in the mixed conductivity. Improvements in ionic conduction by LiBF₄ and NOBF₄ can be interpreted as a result of plasticization and increased mobile ion density respectively. The enhanced electronic conductivity is thought to proceed from oxidative or Lewis acid doping of the polymer backbone by NOBF₄ or LiBF₄ respectively. Molecular dynamics simulation will reveal the role of LiBF₄ to enhanced electronic conductivity. This study offers a new insight on the impact of ionic and electronic dopants on mixed conduction and future optimization of conjugated PILs.   

Effect of pH on the phase behavior of multiple proteins in oppositely charged weak polyelectrolyte solution

We discuss the effect of pH on the phase behavior of weakly dissociating proteins in the presence of oppositely charged polyelectrolytes. We have used a hybrid methodology of coarse-grained single chain mean field simulation with constant pH method in a semi-grand canonical framework to include the fluctuating charge of the proteins and polyelectrolytes. Using the characteristics of the resulting phase behavior pertaining to multiple proteins and several polyelectrolytes, we compare the results due to charge regulation and equivalent fixed charge distribution on proteins.   

Effect of divalent ions on mixtures of like-charged polyelectrolytes

We study the rheology of mixtures of carboxymethyl cellulose and polyacrylate, two anionic polyelectrolytes in the presence of monovalent and divalent ions of the alkaline earth group. M²⁺ are shown to promote interactions between the two systems, the nature of which depends on the specific interactions between the carboxyate groups and the metal cations. Addition of NaPA to M²⁺CMC in salt-free solution leads to a non-monotonic dependence of the viscosity, entanglement modulus and longest relaxation time of the mixtures, presumably arising from an interplay of single chain collpase due to electrostatic screening and network strengthening resulting from associative interactions.
  

Interpolymer Hydrogen Bonding in the Presence of a Low-Molecular Competitor

We examine the effect of dimethyl sulfoxide (DMSO) on binding enthalpy, strength of association between hydrogen-bonding polymers (poly(methacrylic) acid, PMAA), and polyvinylpyrrolidone, PVP) and their deposition within layer-by-layer (LbL) films, In solution, isothermal titration calorimetry (ITC) showed that the addition of DMSO to aqueous solutions resulted in a switching in enthalpy of binding from endothermic to exothermic. In good agreement with the ITC data, the growth mode of PVP/PMAA LbL films changed from linear to exponential with an increase in DMSO content. Neutron reflectometry (NR) studies showed that while LbL films constructed from DMSO-free solutions were stratified, significant layer intermixing occurred in the films constructed from polymer solutions with high DMSO content. This study demonstrates a facile means to control binding of polymer components and structure of assembled films in hydrogen-bonded systems.   

Physical property scaling relationships for polyelectrolyte complex micelles

Polyelectrolyte complex micelles (PCMs) are widely used in the delivery of hydrophilic payloads. As oppositely charged polyelectrolytes assemble, counterions are liberated, however, when counterion concentrations are sufficiently high they prevent, or disrupt, polyelectrolyte complexation. Measuring complex stability versus salt provides a metric for the strength of ion pairing between polymers. PCM attributes are also strongly dependent on the size and chemical structure of each polymer block. Neutral blocks drive nanoscale phase separation while charged blocks control micelle core size and stability. An understanding of physical property behavior controlled by block size, chemistry, and salt conditions is crucial when designing for use in dynamic or biological environments and provide a greater understanding of the physics of polyelectrolyte assembly. In this work, we use small angle x-ray scattering, light scattering, and electron microscopy to determine scaling behaviors of micelle shape, size, and stability for many commonly used polyelectrolytes.   

Polymer Infiltrated Nanoporous Metals to Create Bicontinuous Composite Materials

Most research on polymer composites has focused on the addition of discrete nanofiller to a polymer matrix to enhance properties. However many applications would benefit from a percolated network of the organic or inorganic phase, such as ion conductivity. This work focuses on the development of bicontinuous materials created by infiltrating polymer into nanoporous gold (NPG) thin films. The optical properties of the NPG, characterized via ellipsometry, are reminiscent to those of gold nanorods exhibiting a plasmon peak, which is controllable through the ligament size. Polymer films of amorphous Poly(Styrene) and Poly(2-Vinyl Pyridine) which have different affinities for the gold scaffold, are infiltrated into the NPG through thermal annealing, which provides further control over the optical response. A range of chain molecular weights are studied to probe the relation between chain radius of gyration and average NPG pore size and its effect on film properties. The polymer chains in the confined pores exhibit a slowdown in segmental dynamics measured through the glass transition temperature. The broad tunability of these hybrids represents a unique template for designing functional network composite structures from flexible electronics to fuel cell membranes.   

Morphological Effects on Ionic Conductivity in Solid Polymer Nanocomposite Electrolytes

Perfluorosulfonic acid (PFSA) polymers, such as the benchmark Nafion®, are consistently used as proton exchange membranes (PEMs) in fuel cells and batteries. However, PFSA polymers’ high price, environmental safety issues and reduced lifetime have motivated the search for adequate alternatives. This work investigates two systems: a neat sulfonated polystyrene random copolymer (PS-S_x) and a blend of PS-Sx mixed with sulfonated PS grafted iron oxide nanoparticles (PS-S_x NP). Random copolymer films were characterized with electrochemical impedance spectroscopy (EIS). Conductivity values were determined for PS-S_x with sulfonation levels ranging from 0.7mol% to 47.9mol% as a function of relative humidity (RH, 30%, 60%, 90%) at 40°C. The highest sulfonation level of PS showed a conductivity value of 0.04 S/cm at 90% RH, half that of Nafion®. The conductivity of the binary composite is investigated as a function of film morphology (i.e. discrete or percolated NP domains) and characterized through small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). These morphologies aim to provide increased mechanical strength and ionic conductivity compared to pure PS-S_x systems, and offer insights into ion mobility through percolated domains.
  

Spectroscopic Investigations of PEO Based Polymers and Nanofibers

Polyethylene oxide (PEO) is a special polymer, soluble in both water and some organic solvents (such as chloroform). Fullerenes are not soluble in water but are soluble in chloroform. These features have been exploited in the study of the effect of solvent and nanofiller nature on the morphology and crystalline structure of PEO. Both “bulk” samples and mats of PEO nanofibers obtained by force spinning from solutions of PEO in water or chloroform have been investigated by spectroscopic techniques (Raman, UV-VIS, X-Ray diffraction). X-Ray investigations were focused on the crystallites’ size, the unit crystal parameters, and potential stress effects as revealed by the dependence of the X-Ray line position and width on the spinning rate, PEO concentration in the solvent, and Sn load. Additional information regarding crystalline phases and the mechanical stresses/strains in PEO polymers and mats of nanofibers have been obtained by Raman spectroscopy, using as excitation both a green (583 nm) and a red (785 nm) laser. The dependence of the position of Raman lines on spinning rate and nanofiller concentratios investigated in detail.   

Deformation Mechanisms of Polyolefin Hard-Elastic Films during Uniaxial Stretching

Polyolefin hard-elastic films composed of highly oriented lamellar stacks can serve as one ideal sample to study the intrinsic deformation mechanisms of lamellar stacks, the basic structural and deformation unit in semi-crystalline polymers. With the quick development of synchrotron radiation X-ray scattering techniques, we were able to track the structural evolutions of hard-elastic polyolefin films during uniaxial stretching with the aid of a home-made tensile device. The effects of strain, strain rate and temperature on the nonlinear mechanical behavior and structural evolution of oriented lamellar stacks were systematically studied. And the different dominant deformation mechanisms, including microphase separation within interlamellar amorphous, microbuckling behaviors, crystal slipping and melting-recrystallization, dominate in different external tensile fields. The roadmap of microstructural evolutions of oriented lamellar stacks was constructed in temperature-strain space, which might aid to guiding the real processing of high-performance polymer films.
References:
Macromolecules 2018, 51, (7), 2690-2705.
Polymer 2019, 178, 121579.
Polymer 2018, 148, 79-92
Polymer 2018, 154, 48-54.   

The Rheology of Crystallizing Polymers: Towards a Universal Description

A longstanding goal in polymer rheology is to develop a physical picture that relates the growth of mechanical moduli during polymer crystallization to that of structure. We have recently shown that the rheology is dominated by the formation and growth of the spherulitic superstructures. Here, we aim to develop a universal description of the process by exploring the roles of temperature, surface and bulk nucleation densities, and gap thickness. We study the model system of isotactic polypropylene (iPP) through simultaneous mechanical rheology and optical microscopy, with augmentation by deterministic reconstruction and stochastic simulations. We collapse the variable space by considering two scaled parameters; one related to bulk nucleation density and the other to surface.   

Suppression of crystallization in thin films of cellulose acetates and its effect on gas transport characteristics

This study elucidates the discrepancy in gas permeability between bulk films and asymmetric membranes of semi-crystalline cellulose acetates (CAs) from perspectives of thickness-confinement and crystallization suppression. CAs are the workhorse membrane materials for industrial CO₂/CH₄ separation. Bulk films of CAs often exhibit CO₂ permeability values of 1.8-6.6 Barrers at 35 ^oC, which correspond to permeance values of 36-132 GPU for asymmetric membranes with assumed selective layers of 50 nm. However, commercial CA membranes can have CO₂ permeance values as high as 200 GPU with a CO₂/CH₄ selectivity comparable to the bulk polymers. We hypothesize that as the CA films become thinner, the thickness confinement inhibits crystallization and thus increases gas permeability while retaining gas selectivity. To validate this hypothesis, freestanding cellulose diacetate (CDA) films with thicknesses ranging from 218 nm to 20 µm were prepared, and their crystallinity was determined using Differential Scanning Calorimetry and Wide-angle X-ray Diffraction. Crystallinity can be suppressed by the decrease in the film thickness. Gas solubility and permeability can be satisfactorily correlated with the crystallinity using the empirical equations available in the literature.   

Prefreezing of Different Folding States of Linear Polyethylene on Graphite

Prefreezing is the interface induced crystallization of a melt on a solid substrate. In contrast to heterogeneous nucleation, prefreezing is an equilibrium phenomenon that refers to the reversible and abrupt formation of a crystalline layer at a temperature T_max above the bulk melting point T_m. Recent experimental results evidenced that thin films of oligomeric linear polyethylene prefrozen on graphite have a complex structure consisting of a thin layer of extended chain (EC) crystals directly at the graphite and folded chain (FC) crystals on top of this layer. Temperature dependent AFM experiments showed that EC crystals prefreeze at a higher T_max than that of FC crystals: T_max,EC>T_max,FC. To explain this behavior, we extend the recently developed phenomenological theory of prefreezing to the case when a melt can crystallize in phases of different thermal stability with melting temperatures T_m1>T_m2. Our analytical results indicate that while the more stable crystal phase prefreezes at a higher temperature T_max1, prefreezing of the less stable phase is thermodynamically preferred at lower temperatures, T_max2<T_max1. This behavior is in contrast to bulk crystallization, where crystallization of the less stable FC crystals is often kinetically preferred.   

Epitaxial Crystallization of PE Atop Graphene by Vapor Deposition

Epitaxial crystallization of polymers is a fascinating phenomenon that drives crystal structures under lattice matching conditions as a result of the interfacial interactions between specific pairs of a polymer and an underlayer material. However, fundamental investigations of epitaxial crystallization are impeded by the difficulties in preparing uniform ultrathin polymer layers and fine tuning of the film thickness, especially for polymers with low solvent solubility or fast crystallization kinetics such as polyethylene (PE). In this study, exploiting a physical vapor deposition technique, an additive bottom-up approach that is attracting more interest in processing polymers, we demonstrate the ability to achieve better film quality and control of layer thicknesses. By applying the flexible control of substrate temperature and deposition time to the model system of PE epitaxial growth atop graphene substrates, we determine how temperature and film thickness influence crystal structure and lamellar orientation. Characterization include grazing incidence x-ray diffraction (GIXD),TEM and AFM.   

Chiral Recognition of Poly(Lactic Acid) Stereocomplex

Chiral recognition is a very important concept for structural formation of sterocomplex (SC) of right and left handed helices. A well known system is Poly(Lactic acid) SC. Recently, re-investigation of X-ray fiber diffraction proposed that there is no chiral recognition in PLA SC. In this work, solid-state NMR spectroscopy can be used to study both chain-packing and folding structure of PLA SC by ¹³C isotope labeling and Double quantum (DQ) spectroscopy. ¹³C CPMAS NMR spectra supported by isotope labeling technique provides stoichiometry of L:D in the SC phase in varied mixing ratio in the blends. DQ spectroscopy provides detailed packing structure as well as chain-folding structure. Uniqueness of interemolecular packing (chiral recognition) in SC will be highlighted.   

Chain-Level Structure of Semi-crystalline Polymer in Thermodynamically Stable Crystal and Quenched Glass

Crystallization of long polymer chains changes random coil state to folded structure in thin crystal lamellae. In this work, we will study chain-level structure of ¹³C CH₃ labeled Poly (Lactic Acid) with diverse molecular weights in thermodynamically stable a crystal and quenched glass by using Solid-state NMR spectroscopy. Molecular weight effect on folding event and crystallization will be discussed.
  

Nanoindentation of nanocomposites

The spray coatings were dried and the dried coatings were subjected to nano-indentation using a nano-indenter made by Hysitron. Contact area was calibrated by using silica and polycarbonate of known modulus and hardness. This work on nano-indentation of polycarbonate containing nano-particles of silica shows that there is an increase in modulus and hardness up to a certain concentration threshold in concentration. As the level of silica increases beyond a threshold level, aggregates form which results in weakening of the structure. Polymer silica interface is found to be weak as silica is non-interacting promoting interfacial slip at silica -matrix junctions. At critical stress level for silica –polymer interfacial slip is reached and filler-matrix debonding (sliding) is activated resulting in a decrease in the composite’s stiffness.
Departure from the analytical composite models is analyzed in terms of interface adhesion between nanofiller and polymer matrix. Results on polycarbonates are compared with those of nanocomposites of polyesters where the effect of composite hardness and modulus depended on dispersion of nano-filler in polycarbonate or polyester.. Experimental results are compared against analytical models   

Physical Aging in Anhydride-Cured DGEBA Epoxy

In amorphous polymers, there is a temperature (T_g) reached upon cooling at which the thermal energy is insufficient to allow for molecular rearrangements on the time scale of the temperature change. At this point, the material becomes kinetically trapped in a glassy, non-equilibrium state characterized by excess free volume and enthalpy. Physical aging occurs as a material is held below T_g and begins to relax towards equilibrium. This relaxation can change the properties of the material and is important to understand when considering the long-term reliability in applications. In this work, the physical aging of an anhydride (Aradur 917, Huntsman) cured DGEBA resin (EPON 828, Hexion) was investigated. Four sets of samples were held between 8°C and 35°C below T_g (132°C) for 0 to 4000 hours. The properties of these samples were tested through uniaxial compression and differential scanning calorimetry to observe the changes in yield stress and heat capacity.   

Accelerated aging of an epoxy glass under elevated temperatures and compressive stresses

The aging of the diglycidyl ether of bisphenol-A (DGEBA) cured with diethanolamine was studied both in its "neat" state and filled (at 50 vol%) with glass microballoons (D32-4500, 3M). Cylindrical test samples were compressed uniaxially and the stress-strain curves were analyzed for materials properties (principally the maximum,or yield, stress). Samples were oven-aged at five sub-Tg temperatures for extended times before testing at their aging temperatures and a range of strain rates. As the glass densifies during aging, the yield stress increases. The effect of applying uniaxial stress to the samples during aging is also studied. The samples are held at ~70% of the yield stress over periods of time (up to 24 hrs.), unloaded and then reloaded through yield. The recovery strain and yield stress are found. The yield stress is found to increase and the recovery strain to decrease as aging time increases. The rate of increase of the yield stress is compared to the oven-aged tests.   

Metallization of Chiral and Grooved Polymers at Nanoscale

Different types of polymers with chiral or grooved pattern on the surface were covered (decorated) with metal nanoparticles (silver, gold, nickel, etc.) or thin metal films (silver) in order to create surfaces with increased light reflection and to study possible plasmonic effects arising from ordered arrangement of nanoparticles and metal films. Decorated surfaces were inspected under the microscope and studied by atomic force microscopy (AFM). It was shown that for chiral surfaces the nanoparticles tend to arrange themselves along the spirals not only on the surfaces of original chiral polymers with focal conic domains but also on their replicas made from different polymers. Changes in light scattering and transmission were attributed to the presence of metal inclusions on the surface and are discussed in detail.   

Apparent effect of crosslinker concentration on structure and dynamics of polymeric microgels

The effect of the amount of crosslinker on the structure and dynamics of polysaccharide microgels synthesized in a surfactant solution was studied below and above volume phase transition. When the relative amount of crosslinker was varied by a factor of a hundred, three apparent behavioral regimes emerged from static and dynamic light scattering measurements. At low crosslinker concentrations, microgel behavior was found to be consistent with homogenously crosslinked microgels that displayed uniform reversible deswelling above the transition temperature. These microgels became more diffusive with temperature increase. At high crosslinker concentrations, microgels showed an unusual temperature dependence and signs of inhomogeneous crosslinking. In this regime, microgels grew in size and became less diffusive with increase in temperature. At intermediate crosslinker concentrations, microgels didn’t show significant dependence of their size on temperature except for a reproducible jump in size at the transition temperature. The apparent regimes are likely due to nonuniform crosslinker distribution in the polymer microgel, which leads to nonuniform density of microgel particles, especially at large concentrations of the crosslinker.   

Guided Design of Composite Graphene-Polymer Foams: From Graphene Stabilized Emulsion to Electrically Conductive Foams

The surface activity of graphene enabled synthesis of composite polymer/graphene foams showing a strong coupling between electrical and mechanical foam properties. To develop a general framework for computationally driven design of composite polymer/graphene foams we use large scale coarse-grained molecular dynamics simulations. In particular, we study the affinity of the 2D elastic graphene-like sheets (G-sheets) to the interface between two immiscible solvents. The established envelop of interaction parameters was used to model emulsion polymerization resulting in polymeric foams which cells are coated with elastic G-sheets (G-shells). Upon uniaxial deformation or under foam swelling conditions, the percolating network of the G-sheets coating foam cells breaks down. This break down is manifested as an increase of the foam's electrical resistance. The disruption of the graphene networks occurs through crack formation of the G-shells covering the surfaces of the polymeric foam cells. The results of the computer simulations are compared with corresponding experimental studies of the graphene stabilized emulsions and of mechanical and electrical properties of composite graphene/polymer foams.   

Designing Polymer Nanocomposites for Membrane Gas Separation: an Integrated Experimental and Modeling Approach

Membrane technology is an energy-efficient approach for pre-combustion CO₂ capture and H₂ purification. Conventional membranes are based on rigid polymers with strong size sieving ability, such as poly[2,2’-(m-phenylene)-5,5’-bisbenzimidazole] (PBI) that provides high H₂/CO₂ diffusion selectivity. In this study, we demonstrate enhanced H₂ sorption and diffusion in PBI films with embedded palladium (Pd) nanoparticles, which have strong affinity towards H₂. Pd nanoparticles with uniform diameters of 6 - 8 nm are prepared via a hot-injection colloidal synthesis. The loading of Pd nanoparticles in PBI increases H₂ sorption by almost 1,000 times, and at high Pd loadings, the Pd nanoparticles may form fast channels allowing the H₂ molecules to jump from one particle to another and thus increasing the effective H₂ diffusivity. For example, adding 70 wt.% Pd in PBI increases H₂ permeability from 25 to 70 Barrers, and H₂/CO₂ selectivity from 13 to 29 at 150 °C. Such performance is above the Robeson’s upper bound for H₂/CO₂ separation, demonstrating the potential of these new materials for industrial H₂/CO₂ separation. The gas transport in these PBI-Pd nanocomposites is being modeled using computational fluid dynamics (CFD) to elucidate the mechanisms for the facilitated H₂ transport.   

Fabrication of Carbonized Block Copolymer Particles for Cathode Catalyst of Proton Exchange Membrane Fuel Cell

A porous carbon particle attracted great attention as supporting material for electrocatalyst due to its high conductivity and large surface area. Herein, we demonstrate a robust strategy to carbonize block copolymer (BCP) particles with maintained their size and morphology using etchable BCP (i.e., polystyrene-block-polydimethylsiloxane (PS-b-PDMS)). Key of generating porous carbon material is to introduce cross-linking, which can enhance the thermal stability of linear BCP during carbonization. We produced monodisperse cylinder-forming BCP particles using membrane emulsification, followed by cross-linking and carbonization. We confirmed that its morphology and size distribution is well maintained after carbonization. Moreover, we demonstrate carbonized BCP (cBCP) particles as a supporting material of electrocatalyst with depositing low amount (1 wt%) of platinum (Pt) catalyst on the cBCP particles (Pt@cBCP). Pt@cBCP showed 3.6 times-higher mass activity than commercial Pt/C catalyst in oxygen reduction reaction (ORR). Moreover, we confirmed the advantages of the porous structure by comparing them with a nonporous carbon sphere.   

Effects of Heterogeneous Segmental Friction on the Decoupling of Segmental and Chain Dynamics

Segmental and chain dynamics often deviate from Rouse model predictions due to dynamic heterogeneity introduced by interfaces or temperature reduction. However, the effect of heterogeneous segmental friction on such multi-scale dynamics of polymers remains poorly understood. This work aims to elucidate a possible mechanism for the decoupling of segmental and entire chain motion by systematically altering segmental friction in statistical copolymers. A model statistical copolymer of styrene and 2-vinyl pyridine (2VP) is loaded with amine functionalized silsesquioxane nanoparticles. These nanoparticles form hydrogen bonds with 2VP, while not interacting strongly with styrene monomers. Thus, the heterogeneous polymer architecture leads to heterogeneous segment-nanoparticle friction. Segmental relaxation is studied by differential scanning calorimetry, while the chain relaxation is studied by rheology. These measurements suggest that segmental and entire chain dynamics are slowed due to increasing copolymer-nanoparticle friction by increasing nanoparticle loading and/or increasing the 2VP content of the polymer. The relationship between segmental and entire chain dynamics of these copolymer/nanoparticle composites will be discussed in this presentation.   

Effect of Softness of Polymer Grafted Nanoparticles on the Co-assembly Behavior in 3D Confined Nanoparticle/Block Copolymer Hybrid System

Controlling the spatial alignment of inorganic nanoparticles (NPs) within polymer matrix is of great importance due to its great potential for building novel hybrid materials. Herein, we demonstrate the precise spatial distribution tuning of softness-controlled polystyrene-grafted Au NPs (Au@PS) within polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) particles. The softness of Au NPs was controlled by changing the ratio of the PS ligand length to the inorganic core size. The hybrid particles produced via solvent-evaporative emulsions led to the formation of spherical PS-b-P4VP particles with radially-stacked lamellar morphology, while the spatial distributions of Au@PS NPs showed strong dependence on the softness of Au@PS NPs within PS-b-P4VP particles. Au@PS showing the characteristics of hard spheres were excluded from the block copolymer domains and formed well-ordered hexagonal packing on the particle surface. In stark contrast, Au@PS with the characteristics of soft spheres hierarchically stacked within the inner favorable domains and showed stronger selectivity to the domains which close to the center of onion particles. Finally, the phenomena will be explained by considering the entropic interactions between the BCP chains and PS ligands on Au surface.   

Interconnected Nanoporous Polysulfone Membranes by Microphase Separation of Randomly End-linked Copolymer Networks

Co-continuous nanostructures have been widely studied in blends, block copolymers, interpenetrating networks (IPN), and other polymer architectures. In contrast to block copolymers where equilibrium self-assembly yields ordered co-continuous phases over only small regions of composition (~5 vol. %), microphase separation in randomly end-linked copolymer networks (RECNs) has been shown to provide a kinetically insensitive route to co-continuous nanostructures and interconnected nanoporous materials across broad ranges of composition. Previously, interconnected nanoporous polystyrene (PS) materials were realized by selective etching of polylactic acid (PLA) in co-continuous PS/PLA RECNs. However, the resulting nanoporous PS structures have proven too brittle to be applied as membranes. To overcome this, we extend the concept to an engineering polymer, polysulfone (PSU), and generate co-continuous PSU/PLA RECNs and interconnected nanoporous PSU. We demonstrate that co-continuous PSU/PLA displays better mechanical properties than co-continuous PS/PLA, and that the resulting interconnected nanoporous PSU structures offer potential for applications as nanoporous templates and membranes.   

Brush Structure of Polymer Grafted SiO2 Nanoparticles Measured with Neutron Scattering

Polymer chains are grafted to nanoparticles (NPs) surfaces for a variety of purposes, including altering their solubility and dispersion within a polymer matrix. Here, we have grafted either poly(methyl acrylate) (PMA) or poly(N-isopropylacrylamide) to d ≈ 10 nm SiO₂ nanoparticles, and measured the structure of the brush with small-angle neutron scattering (SANS). For moderate grafting densities (σ ≈ 0.3 nm^-2), high molecular weight polymers adopt two primary conformations. Polymer chains near the NP core are stretched in the concentrated polymer brush region (CPB). Farther away from the core, polymer chains are less confined and the conformation becomes more ideal in the semi-dilute polymer brush region (SDPB). To highlight scattering from these two regions, we took advantage of selective deuteration and contrast matching. SANS measurements are analyzed using the “core-chain-chain” form factor that we recently developed. We compare and contrast the conformation of poly(methyl acrylate) in a good solvent to that of PNIPAM in both D₂O and ethanol-d_6, in which PNIPAM has LCST and UCST behavior, respectively.   

Quantum metamaterials from block copolymers: synthetic pathways to and emergent properties of superconducting gyroids from triblock terpolymer nanocomposites

The last three decades have established block copolymer self-assembly as a scalable route to complex morphologies with exquisite control over the resulting mesostructures. The incorporation of inorganic nanoparticles into these materials has produced record-setting solar cells, batteries, and other devices. However, most of these applications make little use of the crystallographically ordered and highly topologically complex nature of the resulting domains. This unique strength of block copolymers has been shown in simulations to result in emergent properties including negative refractive indices in the visible and circularly polarized light propagation. The primary barrier to realization of these properties has been a lack of synthetic approaches to produce electronic-grade materials, such as superconductors, using block copolymer self-assembly. Using thermal treatments of PI-b-PS-b-PEO triblock terpolymer-niobium oxide nanocomposites, we develop synthetic routes to niobium carbonitride superconductors whose transition temperature varies with morphology and confinement, a first example of the plethora of emergent phenomena we expect from this exciting new class of quantum metamaterials.   

Deformation-Structure Correlations in Glassy Polymer-Grafted Nanoparticle Assemblies

Composites made from polymer-grafted nanoparticles (PGNs) with entangled canopies can avoid the nanoparticle aggregation common in polymer-nanoparticle blends, and have allowed mechanically robust designs for structural, separation, electronic, and optical applications. Recent studies have elucidated the relationship between PGN architecture (graft density Σ, graft degree of polymerization N, and nanoparticle core radius r₀) and the retention of mechanical strength and polymer-like plasticity (i.e. crazing) below T_g. In parallel, the ARGET-ATRP emulsion-polymerization method has expanded the available composition and quantity of PGNs. Herein, we discuss the correlations between the structure of PGN assemblies derived from ARGET-ATRP and their elasticity, plasticity, and failure mechanisms, with a focus on elucidating the impact of secondary interactions within and between canopies, and the impact of core shape and composition.   

Controlling Morphology of Self Assembling Nanocrystalline Reinforcing Domains by Grafting Density Design

Thermoplastic elastomers (TPEs) attain good mechanical performance by virtue of hard, reinforcing domains resulting from microphase separation of two immiscible, covalently connected parts of the chains. The use of monodisperse hard segments with a strong tendency to self-assemble into β-sheet secondary structures via cooperative multiple hydrogen bonds has received attention as a strategy for forming reinforcing domains. The β-alanine trimer grafted polyisobutylenes (βA₃-g-PIB) studied here are TPEs. The β-sheet crystals provide physical crosslinks and reinforcement, and the PIB chains provide elasticity. In molecular designs f-PIB-g-βA₃ and p-PIB-g-βA₃ the chain tethering density on the surfaces of the β-sheet crystals was varied to change the long dimension of the reinforcing domains. Small Angle X-ray Scattering (SAXS) and Small Angle Neutron Scattering (SANS) measurements reveal that indeed when the chain tethering at the crystal surface is more crowded the domain dimension in the direction of β sheet stacking is reduced from over 200 nm to less than 10 nm, so all three domain dimensions are truly “nano”. SANS of samples swollen with deuterated cyclohexane show that portions of the backbone near the grafting points are stretched   

Influence of Graft Chain Properties on Polymer Grafted Nanocomposites

The viscoelastic behavior of polymer grafted nanocomposites (PGNs) with significantly different glass transition temperatures (T_gs) between the graft and matrix polymers is investigated with molecular dynamics simulations. The effect of the dynamic coupling of the grafted and matrix polymer chains is studied by molecular dynamics simulations. These types of PGNs have been shown to have reversible and repeatable stiffening behavior upon heating (Senses, E.; Isherwood, A.; Akcora, P. ACS Appl. Mater. Interfaces 2015, 7, 14682). This unique thermal stiffening behavior was attributed to the dynamic coupling of the high-T_g adsorbed chains and low-T_g matrix chains. The PGN studied in the current work consists of a nanoparticle with grafted high-T_g polymer chains in a low T_g polymer matrix. The influence of the matrix density on viscoelastic properties is investigated to identify the mechanism of the observed stiffening in these types of PGNs. The influence of the graft properties (density and length) is also investigated to identify the mechanism of the aforementioned stiffening behavior.   

Development of a CVD assisted PLD system for growing atomic monolayers

The fundamental step for development of semiconductors involves stacking of layers of thin films of materials with desired properties on a particular substrate. PLD is a technique employed for growing thin films using laser ablation of a target material. CVD is an alternate method used to deposit solid materials from a gaseous phase. However, combining these two techniques can enhance plume, gas and laser interaction to facilitate the growth of novel materials with new properties. Conformity and purity play pivotal roles in the thin film growth process. While working under Ultra-high vaccum can eliminate impurities, on the other hand, proper screening and thermal activation of the plasma of ablated materials ensures a smooth registration of the film with the substrate. In order to control the ratio of CVD to PLD action, we use an optical chopper. We also aim at studying the effect of substrate, temperature and carrier gas on the resulting film. For analysis and characterization of the developed film, we employ standard techniques like SEM, XRD, Raman spectroscopy, fluorescence and profilometry. For initial proof of concept, we demonstrate growing a metal nitride.   

Nonlinear Elasticity and Swelling of Comb and Bottlebrush Networks

We use a combination of analytical calculations, coarse-grained molecular dynamics simulations and experiments to elucidate the effect of branched architecture on swelling of comb-like and bottlebrush networks. The equilibrium swelling ratio of such networks is shown to be larger than that of conventional linear chain networks as a result of two effects: architectural disentanglement of network strands and amplification of polymer-solvent interactions by side chains. For networks of brush-like strands with poly(dimethyl siloxane) side chains in toluene, we achieve a swelling ratio of Q = 30, which is larger than that of linear chain networks with the same strand length. All of the studied systems, including linear chain, comb, and bottlebrush networks, follow a universal scaling relation, G(Q) ∝ Q^-δ, between the shear modulus G(Q) and swelling ratio Q with scaling exponents δ = 2.6±0.08 (simulations) and δ = 2.6±0.12 (experiments). These values agree with the theoretically predicted exponent δ = 8/3, confirming dominant contribution of three body interactions to the osmotic pressure which drives network swelling.   

Gelation of DGEBA epoxy in the presence of a tertiary amine for temperatures above and below the ceiling temperature

DGEBA epoxy is reacted with the curative diethanolamine (DEA). DEA has a single secondary amine that reacts rapidly with epoxide forming a non-crosslinked adduct in about 30 minutes under normal, 70°C, cure conditions. The subsequent crosslinking reaction is much slower, taking 24 hrs. at 70°C to near completion for the standard DGEBA/DEA mix. Cures at elevated temperature (above ~80°C) display a slowing of the overall rate of reaction in a "ceiling temperature" effect which results from a series of reactions associated with the zwitterion driven addition reaction. Here we measure the gel point across a wide range of temperature. At low temperatures, a simple bubble viscometer is used, while traditional rheology is employed for high temperature tests. We find that the extent of cure at gelation is 42±3% for all cases. We show that our measurements are in good agreement with predictions of Carothers and Flory-Stockmayer Theories of gelation.   

The effect of water sorption, high temperature aging, and cooling rate on the calorimetric signature of the aging of an epoxy glass

Stephan Comeau, Brandon McReynolds, Taylor Le
John D. McCoy, Jamie M. Kropka 2
American Physical Society March Meeting 2020
New Mexico Institute of Mining and Technology
2) Sandia National Laboratories

The cure of DGEBA with diethanolamine results in an epoxy that is susceptible to complex aging signatures. Aging at room temperature (about 50°C below Tg) and ambient humidity results in three distinct features: a pre-Tg peak starting 30°C below Tg, a broad post-Tg Shoulder from Tg (75°C) to 175°C, and a decrease in Cp between 175°C and 200°C (chemical aging). Unlike the other features the high-T decrease in Cp persists after the first heat. Extended isothermal holds at 200°C remove the high-T decrease in Cp and slightly increase the Tg (indicating additional cure). Water soaked samples and samples stored in sub ambient contiditons, show an evolving pre-Tg peak and an evolving post Tg-shoulder. The pre-Tg peak also occurs in Samples rapidly cooled (10°C/min) from above Tg and then aged.   

Introducing imide-based functional groups for enhanced self-healing properties of polyurethane

A functional polyurethane with imide moiety (PUI) is synthesized and the self-healing properties are compared with conventional polyurethane (PU). As the imide-based molecules with either mono-hydroxyl or di-hydroxyl groups are added to the mixtures of reactants including polyols and cross-linkers, the imide groups are successfully incorporated to the polyurethane chains. Therefore, while the mono-hydroxyl imide-based molecules give only interacting sites to PU chains, the di-hydroxyl imide-based molecules provide both interacting sites and chemical crosslinks. Interestingly, despite a small amount of the imide moiety in polyurethane, it is observed that the self-healing properties are drastically enhanced as compared to those of PU. Especially, PUI based on imide derivative with di-hydroxyl groups exhibits more excellent self-healing performances compared with that with mono-hydroxyl groups because of the unique intermolecular networks of reversible interactions and chemical crosslinks.   

Relationship of Local Strain-Field and shape of Crack-tip in the Dynamic Crack of Filled Elastomers

The correlation between the crack-tip shape and local strain-field in the dynamic crack with both subsonic (V < C_s) and supersonic (V < C_s) motion of filled elastomers is investigated, where V and C_s are crack-growth rates and shear wave speed. The shape of the crack-tip is characterized by the deviation from the parabolic shape (δ) and the opening displacement (CTOD). The strain-field around the crack-tip is calculated by the two-dimensional digital image correlation technique (DIC) on the basis of the speckle images captured by a high-speed camera. The vertical strain field (e_yy) near the crack-tip can be characterized by the relation e_yy ~ (1/r)^α, known as a crack-tip singularity. We discuss the correlation between the features of the local strain field (α and strain field distribution) and those of the crack-tip shape (δ and CTOD).   

From quantum mechanics to viscoelasticity: A multiscale modeling and characterization of radical initiated modification of polyolefin in molten state

Multiscale approaches for peroxide-initiated grafting of vinyl silane monomers to polyolefins have been investigated with the intention of the development of a mechanistic model for the synthesis of functional copolymers by melt phase processing. A comprehensive mechanistic view of the complex radical mediated reactions is achieved by determining the reaction kinetics through both quantum theory and model compound study. Our results clearly show that the overall mechanism is dominated by grafting single monomer of vinyl silane to hydrocarbon substrates, rather than forming localized or homopolymer grafts, and this occurs at the expense of polymer crosslinking due to the termination of radicals via combination. A fundamental kinetic model is therefore established to depict the general chemistry involving all the critical reactions in modification of molten polymer and their relationships to processing conditions. Combined Fourier transformation infrared spectroscopy with gel permeation chromatography, the further implement of this kinetic model to our recently-developed topology-based viscoelastic model allows, for the first time, to estimate both the yield of graft content and the change of rheological properties during the synthesis of polyolefin graft copolymer in molten state.   

Electro-mechanical transduction of ionoelastomer junctions

Ionoelastomers are an emerging class of ion conducting materials wherein one of the ionic moieties of an ionic liquid pair is covalently attached to a polymer backbone, leading to solid-state electrolytes that selectively conduct ions of one charge. Herein, two oppositely charged ionoelastomers were prepared based on 1-ethyl-3-methyl imidazolium (3-sulfopropyl) acrylate (ES) and (1–(2–acryloyloxy–ethyl)–3–buthyl–imidazolium bis(trifluoromethane) sulfonimides (AT). At the interface of ES/AT, an ‘ionic double layer’ (IDL) is formed due to diffusion of mobile ions away from the interfacial region, resulting in a build-up of excess fixed charges with a capacitance of ~ 1 mF/cm². Thanks to the highly elastic properties of ILEs, along with stretchable electrodes based on embedded carbon nanotubes or graphene, ILE diodes can be repeatedly deformed to large strains (~ 100%) without failure. Remarkably, we have found that uniaxial stretching of an ILE diode produces a spike in the open circuit voltage, or correspondingly a current when the system is connected to an electrical load. This effect can be used for strain sensing or energy harvesting, especially for ambient mechanical energy sources with relatively low frequency and large strain.   

Comparison of the mesh size for semi-dilute worm-like micelles obtained through rheology, neutron scattering, cryo-TEM, and theory.

Worm-like micelles (WLMs) are amphiphilic structures that display viscoelastic properties due to entanglement of robust thread or hair-like morphology. In the semi-dilute regime, the properties of wormlike micelle solutions are no longer governed by the features of individual micelles but rather by the emergent inter-micellar structure reflected in a transient network characterize by the mesh size. A new approach was developed to obtain the mesh size information from small-angle neutron scattering (SANS) data. This observed mesh size was compared with those obtained through rheology, cryo-TEM and by theory. The different physical origins of these values are discussed. The corresponding osmotic pressure was also calculated based on the mesh size from SANS at various salt concentrations and temperatures. It was used to predict concentration and thermal critical points which correspond with observed phase separation. Reasonable agreement was found for the dynamic, structural, and theoretical mesh size, sufficient to support structural interpretations of rheological data which form a basis for theoretical predictions of the complex rheology displayed by WLM systems.   

Nonlinear shear flow experiments suggest no missing physics in slip-link models of entangled polymer melts

The idea that the dynamics of concentrated, high-molecular weight polymers are largely governed by entanglements is now widely accepted and typically interpreted through the tube model. However, recent work has shown that tube models can not predict the maximum strain, of polymer melts under start-up of shear flow at high Rouse-Weissenberg numbers. Based on these observations it has been suggested that there are physics missing in the tube theory. Here, we show that the slip-link model can predict all the features observed in start-up of shear experiments for melts and solutions of various chemistries (PS, PI, PBD, SBR) and for various numbers of entanglements. Specifically
we use the the open-source GPU implementation of the clustered fixed slip-link model, and perform simulations for the startup of shear flow for melts with 14 and 30 entanglements. We find that the maximum strain depends weakly on the number of entanglements and that it does not necessarily follow a single power law, in agreement with data. Moreover, a master curve is obtained for the shear stress normalized by its peak value vs. strain normalized by its value at the maximum stress, also in agreement with experiments.   

Electrical Properties of Hafnium Dioxide

Since the discovery of graphene in 2004, there has been a renewed interest in 2D materials -which have the potential to increase efficiency and speed, and decrease cost, size, and power consumption within electronic devices. Current insulating material, such as silicon dioxide, limits our ability to comply with Moore’s Law -which predicts that the number of transistors within an integrated circuit doubles about every 2 years. To scale devices even further, hafnium dioxide (hafnia) has shown to be a suitable replacement in the gate oxide insulating layer in complementary metal oxide semiconductor devices due to its comprehensive performance. Atomic Layer Deposition of hafnia was grown and annealed at various temperatures between 1-2 hours and its electrical characteristics were examined. Results show that with an annealing temperature of 700C for 1 hour, the crystallinity of the bulk material improved significantly compared to lower temperatures regardless of the annealing time. Results also show that the capacitance remains nearly constant with increasing voltage frequency. Also, the dielectric loss remains the lowest in the same sample. However, the decrease in impedance shows no significant improvement among other samples annealed at different temperatures.   

Biaxial Stretching of Nearly Critical Gels with Extremely Sparse Network Structures

Nearly critical gels, which are obtained slightly beyond the gel point, have very low modulus due to their extremely sparse network structures. The nonlinear elasticity of such sparse polymer networks is intriguing, but their very low modulus has prevented the characterization of the large deformation behavior. The present study investigates the biaxial stress-strain behavior of nearly critical gels by means of a custom-built tensile tester optimized for soft gels. The biaxial data show that the cross-effect of strains becomes smaller as the gels approach the gelation point, and that the effect vanishes in the nearly critical gels.
  

Capturing change in microstructure of physically assembled gels as a function of temperature and strain using in-situ RheoSAXS technique

The mechanical properties of physically assembled gels depend on their microstructure. During large deformation, gel microstructure changes leading to a change in their mechanical properties. Here, we have considered two gels that consist of 10% and 20%(w/w) of poly(styrene)-poly(isoprene)-poly(styrene) [PS-PI-PS] in mineral oil, a midblock selective solvent. At room temperature, collapsed PS-blocks form aggregates, which are bridged by the PI-chains resulting in a three-dimensional gel. We capture the microstructural transformation as a function of temperature in a RheoSAXS setup. The scattering data capturing the change in gel microstructure subjected to oscillatory shear-strain was collected in the 1-2 plane. Strain amplitude values of 10, 100, and 300% were applied on the gels, therefore we have been able to collect data both at small and large strain amplitude. At high strain amplitude, an elliptical two-dimensional scattering pattern has been observed indicating structural orientation along the applied strain direction. To quantify such orientation, we have estimated the anisotropic factor as a function of strain from these patterns.   

The Geometric State of a Solid-Solid Interface

The evolution of a static frictional interface is surprisingly elaborate. It is well known that the frictional strength of a solid-solid interface changes with normal load, but it also evolves in another way: slowly over time, in a process known as ‘aging.’ The effects of aging and a change in normal load on an interface are traditionally seen as unimportant, if they are assumed to exist at all. In a glass-silicone rubber interface, we hold the real area of contact constant over time and demonstrate the existence of differences between these two types of evolution as well as their dependence on contact geometry. This suggests that a frictional interface cannot be fully described by the real area of contact, or any such instantaneous variable, as different configurations of the same interface could all have the same total real area of contact. As the real area of contact is often used as a proxy for frictional strength, these results have important implications for contemporary models of friction, such as the widely used Rate and State Laws.   

Surface Micro Replicas of Self-Assembled Chiral Polymers and Grooves

Transparent polymer replicas of different surface patterns appearing on chiral surfaces and gratings are created and studied at nanoscale by atomic force microscopy (AFM), infra-red (IR) spectroscopy and optical methods. Replicated polymers were represented by chiral cholesteric glassy materials with focal conic domains [1] and isotropic networks with grooved surfaces. Polymer replicas were successfully prepared from different types of glassy polymers by direct deposition from either solution or melt, creating novel opportunities for designing novel nanostructures. Different methods of replica preparation employed and optimal strategies leading to the most effective transfer of chiral surface patterns and grooves were developed and discussed. Studies of light scattering and reflection from polymer replicas were conducted in order to evaluate their suitability for subsequent metallization and design of optical metamaterials.

[1] Surface reconstruction of chiral glassy oligomers under the action of volatile organic compounds (VOCs)
P Shibaev, D Carrozzi, L Vigilia, A Panariti, GA Schwartz
Liquid Crystals 46 (1), 102-107   

Interactions and Competitive Adsorption at Solid/Liquid Interface

Broadness in vibrational spectrum is usually associated with heterogeneity of surrounding environment. At solid/liquid interface, surface-sensitive sum-frequency generation spectroscopy (SFG) has shown wide distributions of frequency shifts of sapphire surface hydroxyl groups with several liquids in contact. Even though the shifts were associated with the interfacial interactions, the origin of variation and a physical picture of interfacial interactions are elusive in experiments. To better understand the interfacial interactions, we perform molecular dynamics (MD) simulations of liquid molecules (acetone, chloroform, and dimethylformamide) in contact with sapphire. The energy distribution profiles from MD correlate well with the experimental SFG spectra, highlighting the ability to interpret spectroscopic features with the physical insights gained from MD simulations. Further, we use MD simulations to gain insights into the preferential adsorption of acetone from acetone-chloroform binary mixtures on sapphire. The current study paves the way for the theoretical understanding of adsorption, which can benefit the development of new technologies in adhesion, coatings, and medicine.   

Nanoparticles as Universal Adhesives

Nanoparticles are shown to be able to act as effective adhesives capable of binding two soft materials together. We performed coarse-grained molecular dynamics simulations to study contact mechanics of hard and soft nanoparticles at the interfaces between two elastic surfaces. Our simulations have shown that a nanoparticle at the interface between two elastic substrates could be in a bridging or Pickering state. The degree of penetration of a nanoparticle into a substrate is shown to be determined by nanoparticle size, strength of nanoparticle-substrate interactions and nanoparticle and substrate elastic properties. Using the Weighted Histogram Analysis Method, we calculated the potential of mean force for separation of two substrates which interface was reinforced by deformable nanoparticles. These simulations show that interface reinforcement is a function of nanoparticle size and elastic modulus. In particular we have shown that the softest nanoparticles are most effective in interface reinforcement demonstrating about eight times increase in the work of adhesion.   

Does Flexoelectricity Drive Triboelectricity?

Since the first reports of friction-induced static electricity in 600 B.C., the phenomenon of triboelectricity has fascinated, and perplexed, generations of scientists. While much progress has been made in the ensuing centuries regarding the nature and identification of charged species transferred during tribocharging, a universal thermodynamic driver for charge transfer has not been found. We identify flexoelectric potential differences induced by inhomogeneous strains at nanoscale asperities as the thermodynamic driver for tribocharge separation [PRL 123, 116103 (2019)]. Using single asperity elastic contact models, we show that nanoscale flexoelectric potential differences of 1–10 V or larger arise during indentation and pull-off. Importantly, we also demonstrate our model agrees with several experimental observations including bipolar charging during stick slip, inhomogeneous tribocharge patterns, charging between similar materials, and surface charge density measurements.   

Unravelling the Behaviour of Brush Block Nanocomposites at Ultrahigh Strain Rates

Poly(tert-butyl acrylate)-block-polyethylene oxide (PtBA-b-PEO) brush block (BB) copolymers offer rapid self-assembly kinetics and ready access to long-range order. Taking advantage of cooperative self-assembly using hydrogen bonding additives, we incorporate phenol formaldehyde (Resol) resins into the PEO block to create a crosslinked network, and thus enable formation of alternating soft (PtBA) and hard (Resol crosslinked) multi-layered films. We then explore ultrahigh strain rate behaviour (>10⁶ s^-1) of these BB nanocomposites films using Laser Induced Projectile Impact Test (LIPIT). Rigid spherical alumina particles of 15-25 µm in diameter are accelerated towards these composites with velocities ranging from 50 to 450 m/s and the corresponding impact and residual flight paths are captured by ultrafast stroboscopic imaging with exposure times of less than 1 ps. We analyse the change in kinetic energies, the deformation depth as measured by profilometry, and scanning electron microscopy images to interpret real-time deformation propagation across the BB nanocomposite.   

Guided Design of Strain-Adaptive Polymer Networks

Mimicking the mechanical behavior of biological tissues is crucial for medical implants and wearable electronic devices. Many biological tissues are supersoft at small deformations (Young’s Modulus E₀<10 kPa) and stiffens as deformation increases. This combination of softness and nonlinear elasticity cannot be duplicated by synthetic elastomers composed of linear polymers. Graft polymers (combs or bottlebrushes), consisting of linear backbones with grafted side chains, endow polymeric materials with diluted backbone entanglements and backbones pre-stretching due to steric repulsions between side chains. These distinct features pave the way for the design of supersoft materials with controllable nonlinear elasticity. Such materials can be prepared by chemical crosslinking of graft polymers or by self-assembly of linear-bottlebrush-linear triblock copolymers. We have developed a design strategy to encode the nonlinear mechanical response of soft materials through the architecture of graft polymer networks by changing the strand length, side chain length, side chains grafting density and block lengths. Materials replicas of jellyfish, artery and skin tissues based on poly(dimethylsiloxane) bottlebrushes were synthesized to verify the design approach.   

A Comparative Study of Hydrogen Bond Organization between Hyperbranched Polymers and Dendrimers Based on bis-MPA

Physical properties hyperbranched polymers (HBPs) based on 2,2-bis(hydroxymethyl) propionic acid (bis-MPA), which have been widely studied, are largely determined by the hydrogen bonds (H-bonds). However, similar studies have not yet been extended to bis-MPA based dendrimers which require considerable synthetic efforts. In this study, hydrogen bond organization and interrelated structural order formation of a bis-MPA based second-generation dendrimer (D2) were investigated in comparison with its HBP analogue (HBP2). WAXS spectra of both polymers showed similar structural ordering originated from clustering of multiple hydroxyl groups mediated via H-bonds. MD simulations well predicted the WAXS spectra and revealed ‘chain-like’ clusters of different lengths from single H-bond associations to clusters containing tens of hydroxyls. D2 showed higher propensity of forming long clusters than HBP2 indicating terminal hydroxyls are more capable of forming long associations than linear ones. Meanwhile, an additional narrow peak which superimposed on the amorphous halo was only observed on the WAXS spectra of HBP2. The origin of this narrow peak was assigned to a pseudo-hexagonal mesophase formed by linear chain segments.   

Unusual Protein Adsorption Phenomena on Ultrathin Homopolymer Films

Recently, we designed a new anti-fouling polymer nanolayer of a few nanometer-thick composed of non-charged homopolymer chains physically adsorbed onto a solid [1]. Interestingly, the anti-fouling property of this polymer nanolayer emerged regardless of the degree of hydrophilicity of the polymers against a model protein (bovine serum albumin (BSA). However, it was observed that 50 nm-thick spin-cast thin films composed of the same homopolymers showed BSA adsorption. To shed light on the anti-fouling/fouling switching between the nanolayer and thin film, BSA adsorption was studied on a series of ultrathin films of different thicknesses (2-200 nm in thickness) using polystyrene, poly(2-vinyl pyridine), polybutadiene, poly(methyl methacrylate), and polypropylene. Additionally, we examined the adsorption behavior of another protein, Fibrinogen, to see generality/differences in the anti-fouling/fouling switching. To quantify the protein adsorption, photon counting spectrofluorometry along with the fluorescence-labeled proteins was utilized. We will discuss the universal anti-fouling/fouling switching as a function of film thickness regardless of the polymer and protein choice.
[1] Endoh et al., ACS Macro Lett., 2019, 8, 1153.   

Unravelling the mechanism behind adhesion failure events at the polymer-solid interfaces

Polymer thin films on solid substrates play a crucial role in nanocomposite materials as well as protective industrial coatings. In particular, polymethylpentene (PMP) has been reported to have a wide array of very promising properties such as high heat tolerance (melting point of 220°C) and resistance to a variety of both inorganic and organic chemicals and materials. In this presentation, we chose PMP thin films on a weakly attractive silicon substrates as a rational model to understand the mechanism of adhesion failure at the solid interface. We will discuss the critical role of a physically adsorbed polymer layer (“adsorbed nanolayer”) at the polymer-substrate interface in the adhesion failure event.   

Phase behavior of disk-coil block copolymers under cylindrical confinement: Curvature-induced structural frustrations

We explore the self-assembly of disk-coil block copolymers (BCPs) confined within a cylinder using molecular dynamics simulations. We obtained concentric lamellar structures with a different number of alternating disk-rich and coil-rich bilayers as a function of the cylinder diameter and coil length. Our study focuses on the curvature-induced structural behavior in the disk-rich domain after self-assembly, investigated by local density distribution P(r) and orientational distribution G(r, θ). In the inner layers of the cylinder-confined system, both P(r) and G(r, θ) show characteristic asymmetry within a bilayer, directly contrasted to its bulk and slab-confined counterparts. We successfully attribute the curvature-induced structural behavior of disks to (1) packing frustration of disks, and (2) asymmetric stretching of coils to regions with different curvatures of a bilayer. Our results are important to understand the self-assembly of BCPs containing rigid motif in confinement, such as the self-assembly of bacteriochlorophyll confined by lipid layer to form a chlorosome, the photosynthetic antennae complex found in nature.   

Disordered Assemblies of Rubber Bands as a Model of Polymer Rings

A comprehensive understanding of the packing structure of dense assemblies of semiflexible rings is not only fundamental for the dynamical description of polymer rings but also key to understand biopackaging, such as observed in the circular DNA of viruses or genome folding.
In this work, we use X-ray tomography to study disordered packings of rubber bands in a cylindrical container as a simple model of semiflexible polymer rings.
Using advanced computational tools, we study the geometrical and topological features of disordered packings of rubber bands at a single ring-level. The degree of entanglement in the structures is quantified through minimal surfaces and generalized Voronoi tessellations. We found that assemblies of short bands assume a liquid-like disordered structure, with short-range orientational order, revealing a minor influence of the container. In the case of longer bands, the confinement causes folded configurations, and the bands interpenetrate and entangle. Most of the systems are found to display a threading network that percolates the system. Surprisingly, for long bands whose diameter doubles the diameter of the container, we observe that all bands interpenetrate each other in a complex fully-entangled structure.   

Multiple arms star polymer translocation from a cylindrical cavity subject to a pulling force

A Langevin dynamics computer simulation is used to investigate the dynamics of star polymer translocation from a cylindrical cavity (tube), which is connected to a plane wall having a circular nanopore along the tube axis. Star polymers of different masses and number of arms or functionalities are considered in the present study. The translocation is carried out by applying an external pulling force which is exerted only on the end monomer of one arm. For a strong pulling force regime and for a given functionality, the translocation time exhibits a power law dependence on the polymer mass where the exponent is found to be close to 2. We have also found that the translocation dynamics of chains of constant mass but varying functionality is significantly affected by the pore radius. For wider pores, the translocation time decreases continuously as the number of arms increases, while in the case of smaller pores, the average exit time shows a non-monotonic behavior with a minimum around a critical functionality f_c. In addition, the translocation dynamics was also investigated in terms of tube length and tube aspect ratio.
  

The Cononsolvency Effects: A Coarse Grained Simulation Study

One puzzling phenomenon about polymer conformations in a mixture of solvents is the cononsolvency effect -- polymers in a good solvent undergo reentrant coil-to-globule-to-coil transition upon addition of a good cosolvent. Recent studies have suggested the generic nature of the cononsolvency effect independent of chemically specific details of the polymer and solvents. In this study, we first reinforce such claim by showing that a coarse-grained soft model frequently used in studying polymers is able to reproduce the reentrant transition at the single-chain level, provided that solvents are treated explicitly. In our Monte Carlo simulations, the coil-to-globule-to-coil transition occurs as the chemical potential of polymers in solution decreases upon increasing the cosolvent fraction, ruling out increasingly worse solvent quality as the cause of collapse. To further our microscopic understanding, we apply the soft model to studying conformational responses of surface-grafted polymers under the cononsolvency effect. By analyzing the inter-polymer density correlations, we illustrate the highly correlated nature of polymer clustering mediated by the cosolvents. Conformational responses of high grafting density brushes will also be discussed in the context of smart sensor surfaces.   

Detection of Hip Infections Using an Injectable Hydrogel Based Synovial Fluid pH Sensor

A hydrogel-based sensor was developed which could be attached to prosthetic hips prior to implantation, to measure pH in the joint fluid in order to detect, monitor, and study infection. A common complication of hip surgeries are post-surgery infections. Delayed diagnosis would lead to reduced function, increased morbidity and may require more complex surgeries. Therefore, early detection of infections is important for successful management of hip infections. In order to detect infection biomarkers, the joint is aspirated and the synovial fluid is analyzed. However, joint aspiration performed by a radiologist under fluoroscopy or ultrasound guidance is painful and is impractical for routine screening or serial monitoring during treatment. The developed synovial fluid sensor provides a method of early detection and monitoring of hip infections using plain radiography. The sensor is made of a pH responsive polyacrylic acid-based hydrogel and the response was determined from the radiograph by measuring the position of a radio-dense tantalum bead embedded in the hydrogel. Thus, the developed sensor could be used as a potential X-ray imaging functional chemical sensor.   

Shape-Switchable Block Copolymer Particles Exhibiting Light-Responsive Surfactants

Particles capable of changing their shape in response to external light irradiation may serve as promising bio-inspired, smart materials for clinical and biomedical applications due to their remarkable spatial/temporal resolution. Herein, we report polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer (BCP) particles exhibiting shape and morphological transition induced by light irradiation. Key of forming light-dependent BCP particles is to design surfactants containing light-active groups (i.e., nitrobenzyl ester and coumarin ester), which can light-dependently modulate interfacial activity. With irradiating light, a sphere with onion-like inner morphology changes to prolate or oblate ellipsoids with axially stacked nanostructures. In addition, wavelength-selective shape transformation of BCP particles is achieved by a mixture of two light-active surfactants, which respond to the light with different wavelengths (i.e., 254 and 420 nm). Moreover, we demonstrate the color- and shape-changing particles in response to the light, which can be achieved by the use of light-emitting, photo-responsive surfactants. Finally, we integrate shape-switchable BCP particles into a hydrogel film to demonstrate the modulation of the particle shape in sub-micrometer resolution.
  

Configurational contribution to the Soret effect of a protein ligand system

Many of the biological functions of proteins are closely associated with their ability to bind ligands. Since binding state and conformation of a protein affect its response to a temperature gradient, they may be probed with thermophoresis. In recent years, thermophoretic techniques to investigate biomolecular interactions, quantify ligand binding, and probe conformational changes have become established. To develop a better understanding of the mechanisms underlying the thermophoretic behavior of proteins and ligands, we employ a simple, off-lattice model for a protein and ligand in explicit solvent. To investigate the partitioning of the particles in a temperature gradient, we perform Wang-Landau type simulations in a divided simulation box and construct the density of states over a two-dimensional state space. This method gives us access to the entropy and energy of the divided system and allows us to estimate the configurational contribution to the Soret coefficient. For dilute solutions of hydrophobic proteins, we find that a hard-sphere solvent model captures important aspects of protein-ligand interactions and allows us to relate the binding energy to the change in Soret coefficient upon ligand binding.   

Packing density, homogeneity, and regularity: quantitative correlations between topology and thermoresponsive morphology of PNIPAM-co-PAA microgel coatings

We investigated the formation of monolayers of microgel particles comprising poly[(N-isopropylacrylamide)-co-(acrylic acid)] on solid substrates, their surface morphology, and stimuli-responsiveness. Crosslinked microgels with different composition were produced
showing a range of response in swelling with temperature and pH. Microgels were deposited on silicon wafers primed with a bilayer of poly(octadecene-alt-maleic anhydride) and polyethyleneimine. The characterization of the microgel-coated wafers led to the identification of three metrics describing microgel arrangement: density (ρ); heterogeneity (H), which correlates strongly with ρ and depends on deposition temperature and pH; and packing efficiency (PE), which portrays the regularity of microgel arrangement and exhibits no correlation with ρ nor H. The values of ρ, H, and PE calculated for in silico models of microgel coatings confirmed that these three metrics portray distinct characteristics of surface topology. Finally, laser profilometry showed that microgel coatings respond to thermal stimuli with sensible variations in surface roughness; which correlate strongly with ρ and H, and to a lesser extent with PE.   

Magnetically Induced Self–Healing in Iron Oxide–Poly(ethylene oxide) Nanocomposites

Current research aims to quantify self-healing capabilities of iron oxide (Fe₃O₄) nanoparticle (NP) infused poly(ethylene oxide) (PEO). Iron oxide NPs of varying surface chemistries (bare, aminopropyl triethoxysilane coated, and polyethylene glycol α–, ω–diphosphate coated) are used to prepare nanocomposites of varying concentrations less than 1% by weight. Each sample, in the form of a cylindrical disc, is indented using a LECO M400 Microindenter at five different locations between the center and edge. The indentation site is examined before and after being placed in an alternate magnetic field (AMF) to induce healing. The micrographs of each indent were collected with an Olympus PMEG microscope at the same imaging parameters. Healing efficiency is quantified using visual and software-based image analysis, identifying the percentage of healing as a function nanoparticle concentration and surface chemistry. Multiple methods of software-based image analysis were developed to perform this analysis. Nanoindentation experiments were also carried out to evaluate impact of surface coating and concentration on mechanical properties and viscoelastic behavior of these nanocomposites.   

Tuning Diblock Copolymer Morphologies by Stimuli-Responsive Supramolecular Interactions

Ability to tune the microstructures formed by block copolymers via easy-to-use physical approaches offers additional handles to the materials for practical applications. One common approach is through adding homopolymers, which induces morphological changes due to preferential partitioning of homopolymers into specific micro-domains. Recently, supramolecular forces that are chemistry-specific and stimuli-responsive have been exploited to enable stimuli-switchable morphologies. To offer microscopic insights into this process, here we present a simulation study of diblock copolymers blended with homopolymers that are associative to one of the blocks through supramolecular forces. By manipulating the manner of associations, we investigate structural changes induced by supramolecular complexations, and to elucidate the differences from the counterpart van der Waals (VDW) force-driven systems. It is found that the homopolymer-receiving microdomain exhibits non-monotonic size changes accompanied by cluster formation as preferential partitioning occurs. Dynamics analysis suggests that both morphologies and supramolecular binding kinetics exert significant influence on the diffusion of homopolymers across the microdomains, implicative of the extent of “responsiveness” of such materials.
  

Water Dynamics and Poly(N-isopropyl acrylamide) Co-nonsolvency in Water-Methanol Solutions at Variable Temperature and Pressure

The phase behavior of 25 % poly(N-isopropyl acrylamide) (PNIPAM) in a 80:20 v/v water methanol mixture is investigated by quasi-elastic neutron scattering (QENS) and Raman spectroscopy at variable temperature and pressure with focus on the co-nonsolvency effect. The susceptibility spectra span the frequency range from 2 GHz to 2 THz at momentum transfers between 0.2 to 1.7 A^-1 and reveal the relaxation peak of the hydration water near 10 GHz, in addition to the known processes of bulk water. The solvent phase is enriched with methanol at high pressure, implying that water is preferentially adsorbed at the chains. The exchange of methanol with water takes place mainly at the hydrophilic Amide groups of PNIPAM. The hydrophobic hydration of alkyl groups in the side chain and the backbone of the polymer in the presence of methanol is reduced at ambient and high pressure. In the two-phase region the preferential adsorption of water at the chains is diminished.   

Phase behavior and self-assembly of liquid-crystalline block copolymers in nematic solvents

Block copolymers (BCP) are an attractive class of materials that can be utilized for active/stimuli-responsive/adaptive materials. The performance of BCPs in various target applications largely relies on their controlled, long-range assembly with high fidelity. In this context, liquid-crystalline BCP (LC BCP) is a compelling material platform because of its ability to control the directional and positional arrangement by leveraging the nature of the LC block. While ranging successful examples have been showcased using LC BCPs in recent years, one may envision that incorporating LC BCPs into other material systems can be the next step forward to open up new possibilities for formulating multi-functional materials. However, neither the fundamental studies on these blend systems nor their useful functionality has been rarely reported thus far. Here, we present the phase behavior and self-assembly of LC BCPs in the blends with other LC materials such as labile mesogens and lamellae-forming block molecules. We demonstrate that LC BCPs are co-assembled with other LC materials with preserving or shifting their original phases as a function of the composition. It is also demonstrated that the blended LC materials can be aligned uniaxially under the magnetic-field.   

Toward molecular modeling of ductility and drawablity of semi-crystalline polymers

We contrast the mechanical responses of different crystallizable polymers such as PLA, PET, PA and PS in their respective amorphous and semicrystalline states using uniaxial extension and compression at varying temperatures below and above T_g. Coherent phenomenology will be presented in terms of the stress response to ongoing deformation, stress relaxation behavior from both preyield and postyield, creep and real-time POM images, aiming to develop a detailed understanding of the mechanical/structural interactions between amorphous and crystalline regions. Through these experiments, we expect to gain more insights regarding the molecular mechanisms for ductility (T < T_g)/drawability (T > T_g) and effective strategies to avoid brittleness/non-drawability.
  

Probing the Impact of Polymer Hydrophobicity on Solution and Hydrated Surface Conformation

Sequence controlled polypeptoids provide a valueable platform for systematic study of molecular level changes in polymer patterning and chemistry; however limitations in our understanidng of sequence effects on polymer conformation and challenges in modeling polypeptoids persist. In this work, distributions of end-to-end distances calculated with molecular dynamics (MD) simulations are compared to those obtained experimentallly via Double Electron Electron Resonance (DEER) spectroscopy. This pulsed electron paramagnetic resonance technique determines a distribution of distances between spin labels placed at each end of a polymer chain, providing insight on sequence-conformation relationships and validation of exisiting simulation force fields. Together, MD and DEER provide a methodology for understanding the impact of polymer sequence and chemistry on polymer conformation as well as confidence in MD predicition of other properties, such as local hydration dynamics at polymer surfaces.   

Unfolding of Polymer Thin Films on Liquid Surfaces

Ultrathin polymer films are difficult to handle. Liquid support layers are often used to ease the manipulation of these films. However, removing the film from the liquid support leads to the fluid draining from the interface, and without the liquid, the polymer interface adheres to itself, forming a crumpled film. The polymer thin film will remain crumpled or tear due to the adhesion forces being higher than the strength of the film. Here, we report a processing method to allow for reversible folding and unfolding of ultrathin polymer films from liquid surfaces. We explain the folding and unfolding mechanism through tuning the surface interactions at the polymer-liquid interface. We demonstrate the ability to fold and unfold several types of polymer thin films on different liquid surfaces.   

Polymers in confinement: Free energy scaling and folding transitions

Geometric confinement of a polymer chain results in loss of conformational entropy. For an athermal polymer the associated free energy increase is expected to exhibit power law scaling with an exponent that depends on confinement dimensionality. For a chain that can fold into a compact native state, confinement primarily reduces the number of possible unfolded states, thereby providing entropic stabilization of the folded state and allowing for the possibility of confinement driven folding [1]. Here we investigate these confinement effects for flexible hard-sphere (HS) and square-well (SW) sphere chains (where the latter exhibit all-or-none folding characteristic of many small proteins [2]). We use a Wang-Landau simulation approach to construct the partition function for a polymer confined within a hard-wall slit, a cylindrical pore, and both a cylindrical and spherical cavity. Scaling analysis of the HS-chains shows significant finite size effects. For the confined SW-chain, isothermal reduction of the confinement dimension can induce folding, unfolding, or crystallite restructuring. Scaling results and phase diagrams will be presented. [1] Taylor, Macromolecules 50, 6967 (2017); [2] Taylor, Paul, and Binder, J. Chem. Phys. 145, 174903 (2016).   

Tunable Assembling of Soft Polymer Janus Nanoparticle at Liquid Interface

We present a study on the assembly of polymeric Janus nanoparticles (pJNPs) at the toluene/water interface. The soft spherical polymeric Janus nanoparticles pJNPs, made by cross-linking polystyrene-block-polybutadiene-block-poly(methyl methacrylate) (PS-PB-PMMA), show a high interfacial activity where the preferential affinity of the PMMA to the aqueous phase causes a spreading of the PMMA block at the interface, even though neither component is soluble in water. The Janus character is later tailored by substituting the PMMA domain to poly-tert-butylmethacrylate (PtBMA) or poly(methacrylic acid)(PMAA) to probe the influence of Janus balance (solvophilic-to-solvophobic balance) to areal density of assemblies of JNPs at the liquid interface and the response of the assemblies to a compression. We investigated the influence of structural asymmetry of JNPs on the assembly, packing, stability and responsiveness of pJNP assemblies by varying the PS-to-PB-to-PMMA ratio. We demonstrate the potential of structure one liquid structure in another by using JNPs complex with salt at liquid interface to obtain bi-continuous liquids.
  

Mussel inspired polymers for flexible electronics applications

Electroless plating of solid metals from a solution onto a catalytically active surface has been widely used in the printed circuit board industry for production of wiring layers and inter-layer (via) connections. Smooth substrate surfaces, like polyimide (PI) for flexible electronics applications, are particularly challenging as the electrolessly plated metal tends not to adhere to the surface. It is therefore desirable to develop new chemistries that can be used for adhesion promotion between the polyimide substrate and the deposited copper. Herein, we introduce a mussel-inspired universal adhesive moiety, dopamine, as a side group to a water soluble polymer backbone and demonstrate its application as an adhesion promotion layer for electroless plating of flexible substrates. When the polymer is deposited on a substrate surface, the DOPA moiety reacts with the substrate and adheres to it while the polymer chain extends and folds to generate a smoother outer surface through minimizing the surface energy, creating a smooth and uniform coat of deposited copper.   

Study of a passive enhancement architecture for FRET-enabled molecular communication.

Nanomachine technology has advanced by improving complexity and function yet, their future value relies upon superior control and communication; this requires reliable and highly efficient networks. Data transfer techniques of global networks are not scalable for nanomachines, and thus a different approach is needed. Non-radiative energy transfer offers high efficiency, localized, and rapid signal transfer: Förster Resonant Energy Transfer is one of such promising paradigms. Application of light-induced FRET, by combining donor/acceptor pairs with a nanostructure architecture, creates nanosecond signaling data transfer to highly specified locations. Developed through novel, arene-based, donor/acceptor moieties - integrated into a thin-film device designed for signal enhancement - a theoretical and experimental approach has been developed and implemented for tuning scattered excitation energy for increased absorption by the donor for higher energy transfer to the acceptor. This architecture results in increased energy transfer and overall higher output of acceptor emission passively increasing the signaling performance and feasibility of application to molecular communication.   

Charge density gradients of polyelectrolyte thin films generated by diffusion and reaction in the vapor phase

We present fabrication and characterization of charge density gradients on polymeric thin films. Tertiary amines in poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) thin films were quaternized with methyl iodide (MI) through vapor phase diffusion and reaction. The quaternized PDMAEMA (qPDMAEMA), a strong polyelectrolyte, bears permanent charges. The degree of quaternization (DQ) was characterized by FT-IR spectra as well as a refractive index from ellipsometry at elevated temperature to minimize moisture uptake. The resulting films have position-dependent gradients of charge density. The gradient location on the sample depends on process time and concentration of MI. We discovered that the diffusion of MI through air is the rate-limiting step for the entire process. The vapor phase reaction allows for creating charge density gradients in both grafted PDMAEMA brushes and non-grafted PDMAEMA films. In addition, we also evaluated the coefficient of thermal expansion (CTE) and thermo-optic coefficient (TOC) as a function of DQ and showed their linear relationship.   

Spontaneous degrafting of weak and strong polycationic brushes in aqueous buffer solutions

Polymers grafted to substrates have usually been considered to be stable because of the covalent bonds that anchor the macromolecules to the substrate. Several recent reports have reported on degrafting polymers from substrates under specific conditions. We have conducted a systematic study of degrafting polycationic brushes with different degrees of quaternization (DQ), molecular weight, grafting density, which have been incubated in buffer solutions (pH 4, 7.4, and 9) with the same ionic strength (0.05 M). We also explored the effect of the bonding environment at the anchoring point of the polymer graft on the overall stability of polymer assemblies on the substrate. The major findings in this study are: 1) Degrafting of polycationic grafts from flat silica substrate increases with increasing DQ of the polymer, 2) Polymer degrafting likely occurs both in the initiator ester group and the silane head-group at the silicon substrate, 3) The degrafting kinetics can be described by time-dependent rate constant modeled by stretched exponential function.   

Structure of Irreversibly Adsorbed Star Polymer Layers

Viscosity, glass-transition temperature, and diffusivity of polymer chains in thin films deviate from bulk values due to the presence of adsorbed polymer chains on the substrate. The structure of irreversibly adsorbed layers (IALs) formed by linear chains has been extensively investigated and found to be composed of two layers: high density inner layer in which chains are flattened and bulk-like density outer layer in which chains adopted more tail and loop conformations. However, the role of chain branching on the structure of adsorbed layer is not clear.
Well-defined linear polystyrene (PS), 4-arm and 8-arm star PS chains with the same total molecular weight, and 8-arm star PS with different molecular weights are used to elucidate the effect of polymer architecture on the structure of IAL by ellipsometry, X-ray reflectivity (XR) and atomic force microscopy measurements. Our results indicated that the normalized layer thickness increases as the number of arms increases and arm length decreases on both SiO_x and Si-H surfaces. Adsorbed star polymers formed thicker layers on Si-H surfaces compared to SiO_x surfaces. Adsorbed layer thickness is found to be a function of initial film thickness up to 100 nm.

  

Visualization of interface effect on the molecular morphology in block copolymer thin films by SVSEM tomography

Applications of block copolymer thin films include their use in lithography, photonics and biomedical devices, and is in large part, due to their internal nanostructures. Thin films are particularly sensitive to surface confinement effects, which on one hand enriches the molecular assembly while on the other hand can produce distortions of the structure from thermodynamically stable symmetries. Here, we study polystyrene-b-poly(2-vinylpyridine) thin films with different microdomain morphologies (double gyroid, lamella, disordered network). By applying Slice-and-View scanning electron microscopy (SVSEM) tomography, the domain structures, distortions, and defects within the polymer films can be measured over large areas/volumes and the influence of the field of the interface (substrate/film and film/air) on the molecular assembly morphologies examined.
  

Universal cohesive law governing interaction between nanoparticles in hairy nanoparticle assemblies

Polymer-grafted nanoparticle assemblies have significant advantages over traditional nanocomposites as they overcome the dispersion issues, attain high structural order and allow accurate tailoring of mechanical properties. Modeling these assemblies using atomistic or even coarse-grained molecular dynamics simulations is quite challenging due to spatiotemporal limitations. To overcome this challenge, in this work, we develop a universal cohesive law that governs the interaction between nanoparticles in these assemblies. This effective interatomic potential is based on the strain energy density between two nanoparticles modeled as plates. We find that the potential consists of empirical constants dependent on polymer fragility, molecular weight and grafting density. Using these design parameters, we were able to collapse all the strain energy curves into a universal curve governing the interaction between nanoparticles. By eliminating the need to explicitly model polymer beads, we can simulate micron scale systems of these hairy nanoparticle assemblies without the loss of underlying physics.   

Recyclable Bio-based Thermoset Furan-Epoxy Networks via Diels-Alder Crosslinks

Strong and lightweight thermoset polymeric can be used for many structural applications. However, thermoset materials may cause significant environmental problems since many of such materials cannot be recycled and have to be stored in landfills. Thus, thermoset that can be recycled, or even renewed will have a substantial impact on reducing the adverse environmental impact of such materials. Renewable polymer composite is a promising path for reducing environmental pollution while improving the sustainability of such materials for the long future. By introducing the thermo-reversible covalent bonds in the molecular network, the thermoset material can be reprocessed upon a suitable thermal trigger. One of the most effective reversible covalent bonds is the Diels Alder (DA) bond between furan and maleimide because of its relatively fast kinetics and mild reaction conditions. In this work, the reversible DA networks were prepared from bio-based furan grafting on epoxy and maleimides crosslinkers of different structures. The kinetics and thermodynamics properties of the dynamic network were studied following the evolution of structure formation and dissociation.
  

Ordering kinetics and steady state in Active Nematic with quenched disorder

A finite disorder in an active liquid crystal has a significant effect on the ordering dynamics and the steady-state. System of self-propelled apolar particles with nematic interaction is one of the way to understand the defect topology and their dynamics in an active liquid crystal and hence, the ordering kinetics in the system. We construct a two-dimensional monolayer of self-propelled apolar particles with the quenched inhomogeneous field. We write hydrodynamic equations of motion for the hydrodynamics fields, viz. density ρ and orientation Q in a coarse-grained description with the addition of finite quenched disorder. We found that, although the presence of disorder in the system slows down the ordering dynamics, static and dynamic scaling properties remains the same. Our study will motivate experimentalists to verify results and may find applications in living and artificial systems in the presence of inhomogeneous quenched disorder.   

Multicellular Magnetotactic Bacteria under an Applied Magnetic Field Form Active Crystals

Multicellular Magnetotactic Bacteria (MMB) of the species Magnetoglobus multicellularis live in spherical colonies composed of 10-50 individual bacteria. These bacteria are the known known obligately multicellular bacteria. The colony swims as a single unit parallel to the Earth’s magnetic field. When a magnetic field is oriented normal to a glass surface, aggregates accumulate into a monolayer on the glass surface. As the magnitude of magnetic field increases, the density of the colonies increases. At a critical field strength, the mean free path of the colonies shrinks to the radius of a single colony. The colonies display a crystalline packing. Unlike previous examples of active crystals (e.g., with colloids and fast swimming bacteria), these bacteria spontaneously detach and reincorporate into the structure at rates dependent on the strength of the applied field. As a result, active crystals composed of MMB display numerous vacancies. We describe the dynamics is this new state of active matter and compare them to active crystals and active super-critical fluids.   

Polar flock with bond disorder

Understanding the collective behaviour of self-propelled particles (SPP’s) is an active area of
research. Although collection of SPP’s are made of similar particles. But each individual can differ
deu to their much details of biological complexity. We focus on difference in interaction ability
with neighbours. Hence in this study we introduced a collection of SPP’s interecting through a
short range Vicsek type alignment interaction, but strength of alignment is different for different
particle and obtained from random distribution. Equalibrium analog of one model is XY-model with
bond disorder. we find that presence of such bond disorder, long range ordering (L.R.O) remains
unchanged as for clean system. Also ordering kinetics remain uneffected but large disorder.   

Measuring Force Fluctuations and Diffusion in Active Baths

Actively driven systems such as suspensions of active colloids, bacteria, and enzymes have been observed to enhance diffusion at the microscopic scale. Active suspensions also exhibit anomalous mechanical properties that are not observed in typical equilibrium fluids. To investigate the interplay between enhanced diffusion and fluid response we measure fluctuations of forces, motion, and viscosity in active baths composed of actively swimming bacteria. To do so, we use two approaches: (1) Differential Dynamic Microscopy, to analyze motion in the form of image intensity fluctuations to quantify decorrelations of the active suspension. (2) Optical tweezers, to trap suspended colloids and measure the force fluctuation spectrum due to non-equilibrium fluctuations of the active bath. We quantify the space-time correlations and calculate the energy dissipated by non-equilibrium processes to extract information about the system.

  

role of annulus confinement on 2D active nematic behaviour

Defects play an important role in active-matter systems. They are nucleated because of the bend instability and in steady state are continuously created and annihilated. In this study we investigate a 2D active nematic confined in an annulus, which exhibits a rich dynamical behavior of the plus and minus half defects. The confinement effectively transforms the turbulent dynamics of the active nematic into coherent flow. We measure the positional-orientational distribution of defects in different confinements. We define three different states for the defects based on their positions and orientations, and calculate their transition rates.   

Physical Basis for Coordination among Bacterial Cells in a Proto-Multicellular Colony

Multi-cellular Magnetotactic Bacteria are the only known obligately multi-cellular bacteria. Cells live within a colony composed of 10-50 cells. Each cell precipitates a magnetic crystal. The average magnetic moment of the colony aligns with the Earth’s magnetic field. The outer surface of each cell is covered in about 30 flagella. Colonies move as a single unit as individual cells in the colony rotate their flagella. It is not understood how cells in a colony align their magnetosomes, how this order is maintained during division, or how cells coordinate their flagella to exert a force parallel to their average magnetic moment. Here, we propose a physical mechanism to show this coordination may arise without direct cell-cell communication. We use published data to show that the organization of the cells in a colony is consistent with a Fibonacci packing. Next, we show that coupling chemotaxis to magnetotaxis allows cells to exert a net force parallel to the average magnetic moment of the colony. These results give insight into the evolution of complex life by showing that cells in a protomulticellular organism may coordinate their behaviors through purely physical mechanisms before the evolution of shared chemical signaling pathways.   

Near-surface Motion of Bacteria in Polymer Solutions

Many bacteria live on solid-liquid interface and they are surrounded by polymers that naturally exist or excreted by themselves. Polymers may facilitate the formation of biofilm by protecting bacteria from external stress. In addition, the interactions between swimming bacteria, polymer and surface are crucial for dynamic processes of bacteria such as surface attachment and dispersal. Here we will present our preliminary work on how polymers may affect the single cell motion pattern near solid surfaces and assist the spreading of bacterial colony.   

Evolutionary game theory of sticky motile bacteria

Bacteria typically reside in heterogeneous environments with varying nutrition and toxin profiles. Motile cells can gain an advantage over non-motile cells by migrating to more favorable environments. Since motility is energetically costly, cells must optimize their swimming speed and behavior to maximize their fitness. Here we look at how cheating strategies might evolve where slow or non-motile microbes exploit faster ones by sticking together and "hitching a ride."' We theoretically and computationally study the effects of sticking on the evolution of run speed in a controlled chemostat environment. We find stickiness allows slow cheaters to dominate only at intermediate distance between nutrition sources. In contrast, for long run durations slow microbes do gain a small advantage from sticking, but only get so far before falling behind; and for short run durations it is best for no one to swim.   

Failure propagation in multicellular tissues as mediated by advective flow

Aging is not just due to the death of cells but due to systemic failures due to the accumulation of malfunctions in a network of interdependent components. Here we experimentally and theoretically study how failure propagates in a synthetic multicellular tissue, as mediated by cooperative factors transported by diffusion as well as advective flow. We first experimentally show that fluid flow induces a reduced death rate upstream. Based on this observation, we then develop a model where cells secrete cooperative factors that enhance their survival which diffuse, decay, and advect in space according to the laws of fluid dynamics. From this model, we derive further predictions for the conditions for which a propagating front of failure should form, and its velocity.   

Aggregation Dynamics of Active Spinning Superparamagnetic Particles in Dense Passive Media

Active matter systems exhibit emergent non-equilibrium dynamical phenomenon which is driven by the activity-induced effective interactions between active particles or units. Here we study aggregation dynamics of many active spinning superparamagnetic particles, spinners, embedded in a dense complex 2D colloidal monolayer of passive particles. Utilizing coarse grained Lattice-Boltzmann simulations and experiments we observed that the aggregation of dynamics of active spinning particles resemble classical 2D Cahn-Hilliard coarsening. The spinners will aggregate and display Cahn-Hilliard coarsening when the passive monolayer is dense enough so that it behaves elastically and when the spinner activity exceeds a minimum activity threshold. For the concentrations investigated here the cluster size scaling is independent of the number of active units. We also observe a critical cluster size which maximizes spinner aggregation by minimizing viscous drag through the dense passive monolayer while maximizing the stress applied on the passive medium. In simulations, we can create ternary mixtures of co-rotating, counter-rotating, and passive particles. The aggregation behavior of such mixtures show distinct aggregates of co and counter-rotating spinners.   

Confinement effect on active turbulent dynamics

Dense bacterial suspensions organize into mesoscale active turbulent vortices when confined in quasi-2D or 3D chambers. The spatial and temporal length scale of the turbulent vortices is dependent not only on the active fluid itself but also on the rheological properties of the confinement interface. Moreover, the turbulent vortices transform into a single spiral vortex state when the confinement size reaches the turbulent correlation length. Here we discuss the confinement interface and dimension effects on active turbulent dynamics and hence the macroscale behavior. This study is aiming to provide physical insights for designing environments of controllable collective motion and active assembly.
  

Statistical mechanical sum rules for active colloids at surfaces - a touch of equilibrium

We study the mechanical properties of active particles in the presence of curved walls by computer simulation of Active Brownian Particles (ABPs), Active Ornstein-Uhlenbeck Particles (AOUPs) and a passive system with effective interactions. The effective theory admits analytic results for pressure, surface tension and adsorption of an active ideal gas at a two-dimensional circular wall. It further predicts that an equilibrium sum rule also holds for active fluids, which we confirm numerically for both ABPs and AOUPs in the limit of small curvature.

More precisely, we find within each model that the slope of the pressure as a function of the curvature equals the surface tension and adsorption (up to an effective temperature scale) on a planar wall. Intriguingly, the explicit value of these coefficients is model-dependent, which can be explained by the different velocity distributions.

We also discuss the influence of interactions and find that the effect of curvature on the wall pressure is reduced when increasing the density. Within numerical accuracy, the equality of the slope of the pressure and the planar surface tension appears to hold at finite density.

Publication: R. Wittmann, F. Smallenburg and J. M. Brader, J. Chem. Phys. 150, 174908 (2019).   

Capillary Forces on a Janus Sphere Straddling a Liquid-Gas Interface

Particles with a patterned surface can exhibit interesting packing and self-assembly behavior. One class of such particles is Janus particles whose surfaces are divided into two halves with distinct physical properties. We compute the capillary forces on a Janus sphere, one side of which is solvophobic while the other is solvophilic, straddling a liquid-gas interface via molecular dynamics simulations. In equilibrium, the liquid-gas interface is flat in the horizontal plane and intersects with the equator of the Janus sphere, with its solvophilic side immersed in the liquid. When the Janus sphere is pulled or pushed out of its equilibrium position along the vertical direction, a capillary rise or fall occurs but the contact line is first pinned at the equator. The contact line only starts to slide when the apparent contact angle becomes equal to the acute (obtuse) contact angle on the solvophilic (solvophobic) side for the Janus sphere pulled upward (pushed downward). An analytical model is developed to explain the observation. The capillary force on a Janus sphere oriented upside-down with the solvophobic side submerged in the liquid is also computed with simulations and possible metastable configurations are found.   

Acheiving highly ordered nanoparticle structures in polymer solution without chemical grafting

Nanoparticle arrays with highly ordered structures have drawn great attention based on their potentials to improve the physical properties compared to the randomly oriented structures. However, it is often difficult to disperse nanoparticles in order in a neat polymer matrix keeping the liquid-like processability, thus requiring complicated and time expensive laborious procedures. In this work, we report that nanoparticles can be highly ordered by simply dispersing them in poly(ethylene glycol) derivative solution. Without chemical grafting, the strong hydrogen bonding between nanoparticles and the PEG complex could retain the superstructures in the liquid-phase with excess amount of water. The systematic investigation is carried out by varying molecular weight/concentration of polymers/nanoparticles and temperature with small-angle X-ray scattering and oscillatory rheometry experiments.   

Sticky diffusion: How to achieve complex motion with random sticky feet ?

Beating equilibrium diffusion is a paradigm challenge that biological or artificial systems of small particles have to face to achieve complex functions. Some cells (like leucocytes) use ligand-receptor contacts (sticky feet) to crawl and roll along vessels. Sticky DNA (another type of sticky feet) is coated on colloids to design programmable interactions and long-range assembly features. The dynamics of such sticky motion are complex as sticky events (attaching/detaching) often occur on very short time scales that affect the overall motion of the particle on much longer time scales, and makes predictions challenging. Here we present analytical predictions in several cases (with different geometries of sticky feet). We rationalize what parameters control diffusion and how they can be compared to existing systems. We investigate furthermore how complex motion like rolling may be favored compared to lateral motion.   

Droplets of Colloidal Ferromagnetic Nanoplates

Disk-shaped, ferromagnetic barium hexaferrite nanoplates in isotropic solvents exhibit a first-order transition from a paramagnetic isotropic (I) phase to a ferromagnetic nematic (N_F) phase for sufficiently high volume fractions [Nat Comm, 7: 10394, 2016]. In samples prepared at a volume fraction within the I – N_F coexistence range, it is possible to create metastable dispersions of oblate spheroidal N_F droplets in an isotropic background. Magnetostatic interactions strongly favor tangential alignment of the N_F magnetization field and the corresponding nematic director field along lines of latitude. However, such a circumferential director configuration requires the formation of a +1 disclination line along the symmetry axis of the droplet, with a correspondingly large elastic free energy cost. The formation of a disclination line is avoided through the escape of the polar director field into the third dimension, giving rise to spontaneously chiral droplets whose handedness is determined by the sign of twist of the magnetization. Coalescence of droplets leads to highly complex morphologies that depend on the polar and chiral structure of the constituent droplets.   

Elucidation of Structural Information of Colloidal Assemblies from Binary Particles using Scattering Techniques

Self-assembly plays an important role in materials and life sciences, for example, in production of structural colors in various taxa, folding and assembly of proteins in living cells, and creation of metamaterials by self-assembly of complex nanostructures. Previously we have reported a simple one-pot emulsion-based process to form photonic colloidal assemblies called supraballs. In this work, we have measured the small angle neutron scattering from supraball assemblies constructed using binary mixtures of melanin and silica particles. The neutron scattering results can be used to determine the various partial structure factors of the nanoparticles, which is important in understanding scattering of visible light. Designing tunable structural colors is of interest for many applications including cosmetics, paints, and food colorings.   

Dynamical processes of interstitial diffusion in a two-dimensional colloidal crystal

We report the first study of the dynamical processes of interstitials in a 2D colloidal crystal. The diffusion constants of both mono- and di-interstitials are measured, and found to be significantly larger than those of vacancies. Di-interstitials are clearly slower than mono-interstitials. We found that, by plotting the accumulative positions of 5- and 7-fold disclinations relative to the center-of-mass position of the defect, a 6-fold symmetric pattern emerges for mono-interstitials. This is indicative of an equilibrium behavior that satisfies local detailed balance that the lattice remains elastic and can be thermally excited between lattice configurations reversibly. However, for di-interstitials the 6-fold symmetry is not observed in the same time window, the local lattice distortions are too severe to recover quickly. This observation suggests a possible route to creating local melting of a lattice (similarly one can create local melting by creating di-vacancies). This work opens up a new avenue for microscopic studies of the dynamics of melting in colloidal model systems.   

Glass Transition in PEO-C60 Nanocomposites

Nanocomposites of polyethylene oxide (PEO) loaded by various amounts of fullerenes have been prepared by dissolving PEO into a given solvent and homogenizing the PEO-solvent solution by stirring. Fullerenes nanoparticles have been added to the PEO solution, then the mixture was stirred for about 1 h at 1,000 rotations per minute followed by a 30 minutes high power sonication. The homogenized mixture/solution was poured on microscope slides and the solvent was removed in an oven at 90 ^oC, for 12 hours. The complete removal of the solvent was confirmed by TGA. Two solvents: deionized water and chloroform were used. The nanofiller is not soluble in water but is soluble in chloroform, opening thus the door towards a refined understanding of the differences between a nanocomposite and a molecular dispersion. The as-obtained nanocomposites were investigated by using a TA Instruments Q 50 DSC at various heating and cooling cycles with rates ranging between 5 ^oC/min to 30 ^oC/min. The research is focused on the effect of the heating/cooling rates, solvent type, and concentration of fullerenes on the glass transition temperature of PEO-C₆₀ nanocomposites.   

Hierarchically Organized Chiral Supraparticles of Twisted Gold-Cysteine Sheets

The structural complexity of composite biomaterials and mineralized particles arises from hierarchical ordering of inorganic building blocks over multiple scales. While empirical observations of complex structures from nanoparticles are abundant, assembly mechanisms leading to their geometrical complexity are still puzzling especially for non-uniformly sized components. Here, we report the assembly of hierarchically organized particles (HOPs) with twisted spikes and other morphologies from polydispersed Au-Cys nanoplatelets. Complexity of HOPs is comparable to biological counterparts as enumerated by graph theory methods. Their intricate hierarchical organization emerges from competing electrostatic and elastic restrictions that render assembly pathways primarily dependent on nanoparticle symmetry rather than size. Upon varying the enantiomeric excess and synthesis temperature, a large family of colloids with complex architectures and unusual chiroptical and chemical properties are explored.   

Terahertz time-domain spectroscopic study on an oxide glass with entropic elasticity

For disordered systems, universal excitation called boson peak appears in the terahertz range. In addition, for polymeric glasses, fractal dynamics, so-called fracton, is expected to appear above boson peak frequency as a result of self-similarity of monomer unit. In this study, we performed terahertz time-domain spectroscopy on an oxide glass with entropic elasticity: mixed alkali metal metaphosphate glass with the chemical composition Li_0.25Na_0.25K_0.25Cs_0.25PO₃, to detect the boson peak and fracton. Obtained spectra are compared with results of low-frequency Raman scattering and low-temperature specific heat measurement.   

Proton Conductivity in Protic Ionic Lquids (PILs)

Protic ionic liquids (PILs) have attracted significant attention due to their promising properties and potential use in various applications. Recent studies revealed strong decoupling of proton conductivity from structure relaxation in a mixture of lidocaine with phosphoric acid, ¹ in which the proton conductivity of this mixture even exceeds that of phosphoric acid at the same viscosity. In this study, we selected bases that exhibit similar properties to lidocaine to facilitate the formation of hydrogen bonded networks to promote high proton diffusivity. Using broadband dielectric spectroscopy (BDS), the conductivity relaxation of several different mixtures with phosphoric acid can be compared and used to determine the best structures for increased conductivity.

1. Z. Wojnarowska, Y. Wang, K. J. Paluch, A. P. Sokolov, M. Paluch, Observation of highly decoupled conductivity in protic ionic conductors. Phys.Chem.Chem.Phys 2014, 16, 9123-9127.
  

Self-limiting electrospray deposition on polymer masks

Electrospray deposition (ESD) can produce monodisperse generations of droplets down to hundreds of nanometers in diameter by applying high voltage to liquids flowing through capillaries. This deposition method has been combined with insulated stencil masks for fabricating micro patterns by spraying nanoparticles, polymers, or biomaterials. To optimize the fabrication process for micro coatings, a self-limiting electrospray deposition (SLED) method has recently been developed. Here we combine SLED with a pre-existing polymer film to study the fundamental behavior of this process in a bilayer geometry. SLED has been observed when insulating materials are sprayed onto conductive substrates. A thickness-limited film will occur when charge accumulates and repels the arrival of additional charged droplets. In this study, polystyrene (PS) and Parylene C thin films of varying thicknesses are utilized as insulated spraying substrates. Polyvinypyrrolidone (PVP), a thermoplastic polymer is sprayed to investigate the SLED behavior on the pre-deposited insulating films. Moreover, to examine the effects of in-plane confinement on the spray, a microhole array patterned onto the PS thin film by focused laser spike (FLaSk) dewetting was sprayed with PVP in the SLED mode.   

Contact singularity between curved dielectric surfaces

The induced polarization charges density on two dielectric surfaces in proximity appear to diverge as separation decrease. This diverging charge density cannot be resolved by the conventional numerical methods or series expansions because the surface charges demand ever-increasing spatial resolution that becomes intractable in the near-contact regime. We analyze the asymptotic behavior of this contact singularity by adopting the 'lubrication' approximation, and demonstrate explicitly the singular dependence on the gap distance and the curvature of two approaching surfaces. The result agrees with the known logarithmic singularity for conducting surfaces and, for dielectric surfaces, provides a means for isolating the singularity and calculating the cohesive energy for ensembles of dielectric particles in close contact.   

Programmable Multistable Mechanisms for Locomotion

We characterized the stability behavior of a multistable mechanism and applied the results to create a new locomotion mechanism based on serially connected bistable beams. The beams of our mechanisms have axial loads that can tune their stiffness and thereby modify the number and location of the stable states. This mechanism is an instance of programmable multistable mechanisms introduced in our previous work.

Using analytical modeling, numerical simulation, and experimental measurements, we demonstrated how the transition sequence between the stable states depends on the beams' axial loads. We exploited this property to produce locomotion with a highly simplified control system. We applied our concept to construct a skating robot, where velocity can be tuned by varying beam stiffness.   

Jamming in a bubble raft: order, disorder and glassy behavior

Soap bubbles floating at an air-liquid interface form stable aggregates resulting from capillary attraction between bubbles. Under external forcing the aggregates unjam and experience plastic events. By using bubble shape deformations as a proxy for the stress, we correlate bubble rearrangements with the stress field as the raft is subject to uniaxial oscillatory compression between parallel plates. We find that most rearrangement events occur immediately after a turning point in the cycle when the stress field changes direction and is most heterogeneous. We also compute metrics from tessellations of polygonal regions set by idealized contact points as well as particle bond orientations and analyze their statistics at different stages of the cycle.   

Systematic control of anisotropy and percolation in patchy particle gels

Patchy particle interactions enable the design of so-called ‘equilibrium gels’, a system where arrest is achieved without an underlying phase separation, resulting in structurally equilibrated gels which do not undergo coarsening-induced aging. We show that nanoparticle-incorporated supramolecular hydrogels - consisting of reversibly polymer-grafted metallic nanoparticles which are cross-linked with end-functionalized polymers – exhibit behaviour consistent with systems undergoing equilibrium arrest. We show that the interaction patchiness of this system can be controlled through the ratio of polymeric linkers to nanoparticles, thus resulting in a canonical system with tunable self-assembly, local structural anisotropy, and mechanical percolation thresholds. Moreover, we show that the addition of metal ions as a second competitive reversible cross-linking species results in the stabilization of locally anisotropic nanoparticle structures, thus resulting in a globally anisotropic structure and a dramatic reduction in the mechanical percolation threshold of the nanoparticle network in the hydrogel. These findings allow the systematic design of stable particle gels with tunable morphology and rigidity.   

Effect of chain architecture on self-diffusion in a model associative network.

Associative networks are ubiquitous both in natural and synthetic materials, and self-diffusion within these networks dictates many of their desirable properties such as self-healing and stress relaxation. Self-diffusion studies of various associative networks have shown that over length scales of several times the radius of gyration, the dynamics of the network lead to the observation of an apparent super-diffusive regime prior to transitioning to the Fickian regime at larger length scales. In this work, the effect of chain architecture was investigated by comparing the self-diffusion of a random copolymer with one where the stickers are clustered at the end of the chain. Since the chemical composition of the model associative networks is kept constant, this approach allows for a more direct comparison of the role of chain architecture. The insights gained from this study will improve our understanding of the effect of sticker distribution on the transport properties of natural and synthetic associative networks and how it could impact the macroscopic properties that rely on these processes.   

Chiral Elastic Waveguide

In this work, we investigate the propagation of elastic transverse and longitudinal waves in a helical medium through a cylindrical waveguide. By solving the Navier-Cauchy equations together with the constitutive equations for a helical medium. By assuming quasi-planar waves, we reduce our system to a set of ordinary differential equations. We have obtained the band structure of the system and the propagation parameters by imposing the corresponding boundary conditions for vacuum outside the waveguide. We also calculate the corresponding strain and stress distributions within the waveguide. We discuss our result and address our conclusions.   

Binary Clay – Graphene Oxide Liquid Crystals

Colloidal suspensions (CS) of electrically charged nanosheets (NSTs) form liquid crystalline phases in polar solvents, such as graphene oxide¹ or clays^2,3. Self organization of NSTs due to competing van der Waals attraction and electrostatic repulsion forces can form several coexisting phases such as isotropic (I), nematic (N) or lamellar (L). Several single-component CS^1,2 exhibit transition from I to N liquid crystalline phase; moreover, existence of L phase for CS of electrically charged NSTs has been observed recently⁴.
In contrary, complexity of binary CS (BCS) of charged NSTs can result in multi-phase systems. This study is focused on BCS of negatively charged synthetic clay⁵ and graphene oxide, serving as promising precursor for fabrication of clay-graphene oxide mixed ionic-electronic conductors. Results obtained using X-ray diffraction (SAXS, WAXS) and imaging (optical, MRI) techniques will be discussed, emphasizing formation of N and mixed-domain states.
1.Kim, J. E. et al. Angew. Chem. Int. Ed. 50, 3043–3047 (2011).
2.Fonseca, D. M. et al. Phys. Rev. E 79, 021402 (2009).
3.Hemmen, H. et al. Langmuir 25, 12507–12515 (2009).
4.Davidson, P. et al. PNAS 115, 1–6 (2018).
5.Breu, J. et al. Chem. Mater. 13, 4213–4220 (2001).   

Chiral Composite Networks for Sensing Volatile Organic Compounds Towards Liquid Crystal Nose (LC-nose).

Composite chiral networks were prepared by co- polymerization of cholesteric siloxane monomers with low molar mass monomers bearing hydrogen bonding, polar and non-polar side groups ( acrylic acid, vinylpyridine, benzyl acrylate, and aliphatic acids ). Polymerization was conducted between two plastic plates at elevated temperatures when compounds formed chiral liquid crystalline state. After polymerization thin polymer films were formed, the upper plastic film was peeled off and the surface of polymer composite was exposed to a particular VOC. The response of polymer films to VOCs (spectral shift of the selective reflection band) was studied by spectroscopic methods and morphological surface changes were studied by atomic force microscopy (AFM). It was shown that by varying the composition of materials it is possible to change their sensitivity to polar and non-polar VOCs. The mechanism of this sensitivity was also studied by modeling both gas diffusion inside the polymer and polymer optical properties. The selectivity of the whole system can be further improved by comparing relative spectral and color changes of a few polymer films simultaneously. Thus, the prototype of LC-nose based on polymer networks could be created.   

Environmentally Sensitive Optical Fibers and Waveguides Based on Hydrogen-Bonding Compounds

Novel optical fibers and waveguides were designed and created from blends of hydrogen bonding compounds ( polyvinyl alcohol, polyvinylpyridine, acrylic acid ) with glassy polymers and low molar mass liquid crystals. Optical fibers display significant birefringence in their cores since the drawing procedure of the fibers lead to the higher concentration and orientation of liquid crystal inside the core of the fiber. The fibers were studied by differential scanning calorimetry and optical methods. It was shown that fibers gain additional stability if they are physically crosslinked with diacidic low molar mass compounds ( sebacic acid ). Optical response of the fibers (changes in propagating light intensity or additional leakage of light) to the action of volatile organic compounds (VOCs) was studied and discussed for different types of polar and non-polar VOCs. It was found that optical response depends on structural reorganization of the fibers that starts in the outer layer and then propagates towards the core in a way similar to the response of liquid crystals [1]
[1] Rebirth of liquid crystals for sensoric applications: environmental and gas sensors
PV Shibaev, M Wenzlick, J Murray, A Tantillo, J Howard-Jennings
Advances in Condensed Matter Physics 2015   

Optics of multilayered cholesteric liquid crystals with disorder: towards LC-nose.

Optical properties of multilayered disordered cholesteric liquid crystals (CLC) with different types of disorders were performed by employing 4x4 matrix method in order to simulate a diffusion of volatile organic compound (VOC) molecules inside the cholesteric material. A simulated CLC consisted of three layers, the central layer being unaffected by VOCs and two outer layers being subjected to diffusion of VOC molecules. The diffusion of VOCs inside the CLC was simulated by changing either the order parameter or helical pitch of the two outer layers of the CLC. The results were compared with experiments of VOC diffusion in a number of CLC systems. It was determined that the change of order parameter plays an important role in liquid CLCs and change of helical pitch mostly affects CLC polymers. The applicability of these calculations for designing artificial LC-nose [1] is discussed.

[1] Liquid crystal nose, chiral case: towards increased selectivity and low detection limits

PV Shibaev, D Carrozzi, L Vigilia, H DeWeese
Liquid Crystals, 1-9   

Verifying XY-Model Predictions for Topological Defects in Films of Smectic Liquid Crystals

We describe experiments exploring the applicability of the XY model to defect-rich smectic C liquid crystal films. Topological defects in the orientation field are created by rapidly deflating a hemispherical `bubble’ of a molecularly thin film, resulting in a flat film with a high density of defects. The subsequent pair-wise annihilation of these defects is captured with sub-millisecond resolution using high-speed, polarized light video microscopy. The simplest theoretical description of the orientation field, the XY model, makes specific predictions about the annihilation dynamics, predicting sub-logarithmic scaling of the number of defects present at short times and logarithmic scaling at longer times. Progress towards verifying the applicability of the XY model to these systems at early times has been stymied by experimentally difficulties in tracking the defects. Through improvements in illumination and by leveraging recent advances in machine-learning, we can now identify and track individual defects at early times, allowing us to confirm the predictions of the XY model.   

Study of a cylindrical fiber cored by a double twisted chiral nematic.

We study a region of the space where an electromagnetic field is propagating within a waveguide consisting of a material having double helix as found in some chiral nematic materials such as blue phases. We establish the electromagnetic equation governing the dynamics of the propagating modes. We found the band structure of the mentioned modes, the profiles field amplitudes and the Poynting vector distributions versus the radius of the fiber. We discuss the composition and conditions for the modes to propagate.   

Deformation of structurally chiral polymer stabilized networks in electrically tunable filters

The cholesteric liquid crystalline phase self-organizes into a helical structure. In the planar orientation, the periodicity naturally exhibits a selective interference reflection. Dynamic optical responses observable as red-shifting tuning, blue-shifting tuning, or bandwidth broadening can be achieved under an electrical field with the inclusion of a polymer network. Our recent research activities have focused on further elucidating the fundamental electrochemical response of the ion-mediated deformation of the polymer stabilizing network in the cholesteric phase. A variety of liquid crystalline and non-liquid crystalline monomers are selected in order to determine how intermolecular forces and liquid crystalline interactions affect the performance of the material. Multiphoton fluorescent imaging is used to visualize pitch and polymer network deformation. The correlated influence of photopolymerization conditions and ionic impurities will be discussed.   

Electro-optical switching and order parameter measurements of dual-frequency nematic liquid crystals: regimes of thin and thick cells.

Liquid crystals utilized in conventional display applications typically have a layer thickness of less than 10 µm. However, emerging non-display applications of liquid crystals require thicker material layer with faster response. Electro-optical performance of relatively thin liquid crystal cells is well-documented, butt little is known about the properties of thicker layers. Also to address the need for the fast electro-optical switching a dual frequency liquid crystals are investigated. We are presenting the electro-optical response of dual-frequency nematic liquid crystals using a broad range of the cell thickness (2–200 µm). Two regimes of electro-optical switching of dual-frequency nematics are observed and analyzed. To uncover the origin of the observed two regimes of electro-optical switching, measurements of the orientational order parameter of dual frequency nematic liquid crystals were carried out over the same range of thicknesses. The non-monotonous dependence of the order parameter S on the cell thickness obtained for relatively thin layers (< 60 µm), and is followed by a rapid monotonous decay for thicker (60-200 µm) samples. This provides a basis for understanding of two regimes of electro-optical switching of thin and thick layers of dual frequency nematics.
  

Assembly of microparticles in disclination of LCLCs confined to cylindrical capillaries.

We investigate the assembly of microparticles in nematic lyotropic chromonic liquid crystals (LCLCs) confined to cylindrical capillaries. Two line defects with double helical configuration is observed for Sunset Yellow FCF with the homeotropic anchoring being obtained by parylene-N coating. Here, we show that disclinations in Sunset Yellow FCF can act as templates for particle dispersions. Additionally, we use magnetic micropaticles to study effect of external magnetic fields on the lclcs. The observed assemblies extend our understanding of the structure of disclinations and ability to design the architectures of soft materials.   

Predicting the Steady Flow of a Fluid with Particles by Deep Learning

Computational Fluid Dynamics (CFD) simulation has the potential for application in material science. In these applications, a common study object is a highly viscous fluid that passes structures in the microscale. However, its computational cost is a barrier to application. Meanwhile, some recent studies successfully reduced the computational cost of CFD by utilizing machine learning and deep learning techniques [1, 2].
Using deep learning, the authors predicted the steady flow around a large number of particles and examined the effectiveness of the prediction for the study of materials development. The particle-fluid interaction is computed using the Smoothed Profile Method [3]. After learning the flow that passes the particles obtained by iterative calculation, the deep learning quickly and accurately predicted the flow of the system with unknown particle concentration and arrangement. The fluid force applied to each particle was also accurately predicted.

References
[1] X. Guo, W. Li, and F. Iorio, KDD ’16 (2016).
[2] O. Hennigh, arXiv, arXiv:1710.10352 (2017).
[3] Y. Nakayama and R. Yamamoto, Phys. Rev. E, 71, 036707 (2005).   

Deep Convolutional Neural Network for Tomographic Reconstruction of Strain

The neutron transmission Bragg edge method has gained interest over the past decade as a technique for providing a fast, high spatial-resolution and high sample penetration mapping of strain. Since its advent, there has been a strong desire to extend the transmission Bragg edge method to provide a full 3D tomographic reconstruction of the full strain tensor. However, it has been shown that traditional tomographic reconstruction algorithms are, in general, unable to provide a unique solution as the mathematical problem of inversion of Bragg edge data is ill-posed. The major complication of strain tomography compared to traditional scalar tomography is the directional dependence of the strain tensor. Deep Convolutional Neural Networks (DCNNs) have shown success in scalar tomographic reconstruction, in particular for cases in which scalar tomography is ill-posed (e.g. when sample exposure or rotation is limited). We show that DCNNs can be extended to perform tomographic reconstruction of strain tensors based on neutron transmission Bragg edge data. We show three training strategies and a DCNN architecture with success in reconstructing 2D strain fields. Additionally, we explore the advancements necessary to make a fully generalizable model for tomographic reconstruction of strain.   

Machine learning for detecting microscopic parameters characterizing mechanical properties of liquid crystal elastomers

Liquid crystal elastomers (LCE) are of great scientific and technical interest because of the potential of sensors and soft actuators. Molecular dynamics (MD) simulation is a promising means to clarify macroscopic deformation of LCE from a microscopic viewpoint, however, a detection of major parameters characterizing the deformation is difficult because there are many microscopic parameters. Therefore, a systematic analysis should be done for detecting the relation between microscopic parameters and mechanical properties. In this study, a machine learning (ML) approach is used to explore the relation between microscopic characteristics and mechanical parameters. With these models, we perform MD simulations and compute stress-strain curves. Then a regression analysis with random forest method to explain the difference of stress-strain curves is performed with 20 types of microscopic parameters. The ML results reveal the effective set of data descriptors that predict well the stress-strain curves. Therefore, ML technique has a capability to overcome the difficulty to manually explain the complex relation between microscopic parameters and macroscopic properties.   

Predicting Soft Matter Evolution Using Machine Learning

Soft matter including colloids, polymers and granular material display behavior and self-organization that are difficult and sometimes impossible to predict due to complex interactions with the environment, which itself can change and self-organize. The traditional approach to understand these problems is to study structural evolution by theory and experiment. We are taking the different approach of machine learning. We build convolutional neural networks (CNNs) to process the large amounts of data. We are applying these new tools to direct imaging in rheo-optics, to fitness evaluation of cell growth when they pass near obstacles, and to ecological microsystems with soft matter flavor. The common element is to predict properties varying with time.   

Confined filaments in soft vesicles - case of sickle red blood cells

Abnormal shapes of red blood cells (RBC) have been associated with various diseases. Diverse RBC shapes have also been intriguing for membrane biophysics. In our work, we focus on sickle-shaped RBC which form due to abnormal growth of semi-rigid Hemoglobin (HbS) fibers confined in RBC. Using the area difference elasticity (ADE) model for RBC and worm-like chain model for the confined HbS fibers, we explore shape deformations at equilibrium using Monte-Carlo simulation. We show while a single HbS fiber is not rigid enough to produce sickle-like deformation, a fiber bundle can do so. We also consider multiple disjoint filaments and find that confinement can generate multipolar RBC shapes and can even promote helical filament conformations which have not been discussed before. We show that the same model, when applied to microtubules confined in phospholipid vesicles, predicts vesicle tubulation. In addition, we reproduce tube collapse transition and tennis racket type vesicle shapes, as reported in experiments. We conclude that with a decrease in the surface area to volume ratio, and membrane rigidity, the vesicle prefers tubulation over sickling.   

AZO-modified lipids bilayer and their dynamics associated with optical stimulation.

In this work we have studied the dynamics of membranes with azo-modified lipids from molecular scales (atomistic molecular dynamics simulations) to vesicle scale (experimental work), with the aim of increasing understanding regarding steady states, elastic constants and membrane permeability. We have found that vesicles dilate and become flexible and much more permeable as a result of trans-to-cis conversion. Therefore, to the best of our knowledge, these are the first studies to look at the membrane dynamics associated with optical stimulation. In addition, the relative energies of cis and trans states were theoretically determined. Finally, it is worth to highlight that this combined studies (computational and experimental work) open new avenues regarding rational molecular design in order to tune response.
  

Enhanced diffusion of tracer microspheres in a temporally fluctuating porous structure

Transport in porous materials is at the center of a wide range of natural and industrial processes. While many studies have focused on the properties of porosity with spatial fluctuations, much less is known about the effects of temporal fluctuations in the medium’s structure. We have designed a simple experimental system aimed at investigating the transport properties of a crystalline porous material made of a hexagonal lattice of 2um spheres, which are either static, or fluctuating in time about an average position (dynamic). Using complementary illumination, we can image and track both diffusive tracers (100-400nm) and lattice spheres. We compare the transport properties of the static and dynamic lattices with similar average densities and find that diffusion is significantly faster within the dynamic lattice. We hypothesize that this might be due to a combination of different effects. First, the motion of lattice spheres could reduce the hydrodynamic hindrance of the tracers, and increase diffusivity within a cavity. Second, fluctuations in pore sizes, induced by the fluctuations in distances between adjacent spheres, could permit a higher transition rate between cavities than a fixed pore size of the same mean value. We present results relating to these two effects.   

Fibers on the surface of thermo-responsive gels induce controllable formation of helical structure

We use computational modeling and analytical calculation to investigate the behavior of gel-fiber long thin films; we specifically show that the arrangement of fibers localization on the outer surface of the sample provides a powerful means of tailoring the overall shape of the sample, as to form helical structure with controlled chirality. We focus on thermo-responsive gels, which exhibit a lower critical solubility temperature (LCST) and thus shrink when heated above a certain temperature. The stiff fibers are attached to this gel and inhibit the nearby network from undergoing the heat-induced collapse. Away from the fibers, however, the network can readily shrink in response to the increased temperature. This competition between the constrained regions and the unconstrained regions of the heated gel regulates the structural evolution and final geometry of the sample. Our simulations use the gel lattice spring model (gLSM) to determine how the temperature, arrangement and number of the fibers control the bending and twisting of thin ribbons. Our comprehensive 3D analytical calculation incorporates elasticity, differential geometry, and variational principles.   

Curved Geometries from Planar Director Fields: Solving the Two-Dimensional Inverse Problem

Thin nematic elastomers, composite hydrogels, and plant tissues are among many systems that display uniform anisotropic deformation upon external actuation. In these materials, the spatial orientation variation of a local director field induces intricate global shape changes. Despite extensive efforts, until recently there was no general solution to the inverse design problem: How to design a director field that deforms exactly into a desired surface geometry upon actuation, or whether such a field exists. In this work, we phrase this inverse problem as a hyperbolic system of differential equations. We prove that the inverse problem is locally integrable, provide an algorithm for its integration, and derive bounds on global solutions. We classify the set of director fields that deform into a given surface, thus paving the way to finding optimized fields.   

Elastic-Viscoplastic transition in a modified Soft Glassy Rheology model

The elastic-viscoplastic transition of glassy materials is studied with a modified Soft Glassy Rheology (SGR) model that is newly suggested for more accurate description of rheological behavior of glassy materials. The modified SGR model incorporates sophisticated features of glassy materials, such as collective dynamics. The modified SGR model demonstrates different elastic-viscoplastic transitions depending on conditions such as methods of glassy dynamics description, flow conditions, and shear history. The rheological transition of SGR model under startup shear is interpreted in terms of local strain distribution change that represents physical state change. Based on the findings, we interrogate how various features of glassy dynamics affect the elastic-viscoplastic transition in glassy materials and discuss effective ways to describe glassy materials. Our results provide some clue to understand the physical origin of the elastic-viscoplastic transition in glassy materials.   

Bifurcated yielding response of aging fibrillar networks

Yielding in disordered materials occurs with a remarkably broad variety of characteristics and remains a poorly understood phenomenon. Here, we investigate the yielding and aging behavior of disordered fibrillar networks of cellulose nanofibrils (CNF), micron-sized semiflexible fibers with aspect ratios >100. At high concentrations, CNF form dense physical networks that support finite stresses and in which terminal relaxations are suppressed. We observe a strong decrease in the yield strain of the networks with increasing volume fraction (γ_y ∼ φ^-2). Further, we observe a strong aging response of the system after an initial yielding event. Subsequent yielding occurs at a time t_y that depends sensitively on the magnitude of applied stress and the aging time t_w between initial yielding and reapplication of stress. The yield time diverges at finite waiting times for small stresses and asymptotes towards a finite yield time at large stresses, with bifurcation around a critical stress that is similar to the yield stress of the system. The waiting time at which the yield time diverges increases with applied stress below the critical stress, indicating that the bifurcation is rooted in the rate of thermally driven network restructuring relative to stress-driven de-structuring.   

Yielding of FCC crystals at zero deformation rate: evidence for hidden transition

Despite much effort, no theory, consistent with all rigorous thermodynamic constraints, has been derived that can predict the yield point of a real solid. An exact result states that the free energy of any material, made up of entities interacting with short ranged forces, cannot depend on the shape of the boundary. This implies that crystalline solids are guaranteed to yield at infinitesimal stresses when deformed at vanishing rates. Here we present our work on yielding of an ideal FCC solid in the strictly zero strain rate limit. In this limit, the yield point vanishes for infinitely large systems. Earlier, we showed, for an ideal 2d triangular crystal that the yielding phenomena is a dynamic consequence of a hidden first order phase transition. Our calculations of the free energy of such solids also show that the solid phase is always metastable at any finite temperature and infinitesimal deformation. Our prediction for the dynamical yield points agree with MD simulations for a wide range of deformation rates. We describe, here, our extension of this previous work to the initially defect-free FCC crystal. We discuss about commensuration, finite-size effects and consequences of systematically introduced disorders as random interactions of varying strength in such solids.   

Interfacial Thermodynamics of Surfactants at Elevated Pressure

While surfactant use under elevated pressures is not a novel field, experimental measurements of surfactants in extreme environments is lacking in the literature. Thermodynamically, pressure impacts the concentration of surfactants at a pressurized interface, thus affecting the interfacial tension. The relationship between the concentration, pressure, and interfacial tension for a given surfactant has not been measured sufficiently for nearly any surfactant. In this work we use a novel high pressure microtensiometer to probe surfactant transport and equilibrium thermodynamics at the water-CO₂ vapor interface. We show that surfactant isotherms are highly dependent on pressure illustrating that key thermodynamic and kinetic parameters are strong functions of pressure. Furthermore, we show that surfactant performance is significantly decreased at elevated pressures. We outline the importance of experimental measurements of surfactant behavior on predicting high pressure performance of a variety of surfactant chemistries.   

Role of solvent identity in impact-activated solidification of dense suspensions

Applied stress can drive flowing suspensions of solid particles in a Newtonian liquid into a shear-jammed solid state. Beyond steady state rheology, recent work demonstrated that high-speed ultrasound imaging could be used to visualize transient flow fields generated by impact and track the propagation of “jamming fronts” in optically opaque cornstarch suspensions [1][2]. We here use this ultrasound imaging technique to reconstruct the flow fields caused by impact of dense suspensions containing silica nanoparticles in polymeric liquids. We find that the solvent molecular weight plays an important role in mediating frictional contacts between particles required for shear jamming.
[1] E. Han, I. R. Peters and H. M. Jaeger. High-speed ultrasound imaging in dense suspensions reveals impact-activated solidification due to dynamic shear jamming. Nat Commun, 7:12243, 2016.
[2] E. Han, L. Zhao, N. Van Ha, S. T. Hsieh, D. B. Szyld. and H. M. Jaeger. Dynamic jamming of dense suspensions under tilted impact. Phys. Rev. Fluids 4, 063304, June 2019.   

Colloidal Assembly at Curved Liquid-Liquid Interface

Colloidal particles can readily assemble into crystalline structure under confinement. When confined on a flat surface, spherical particles can pack efficiently with an arrangement of hexagons. However, the regular hexagonal packing configuration could be disrupted on a curved surface with resulting formation of defects. In this study, we investigated the self-assembled crystallography of fluorescence labeled polystyrene (PS) microspheres at lutidine-water interfaces, where liquid-liquid separation occurs as increasing the temperature to exceed the critical solution temperature of the mixture around 27-28 ^oC, We determined the assembled packing microstructure of PS microspheres at the liquid-liquid interface of varied curvature by direct confocal microscopic particle tracking. We observed disordered-to-crystalline structural transition in self-assembled PS colloidal monolayer at curved liquid-liquid interfaces when a critical curvature is exceeded.
  

Unique assembly structures of amphiphilic Janus particles in ionic liquid solutions

Janus particles assemble into remarkable superstructures due to different chemistry on the two sides of a single particle. The directional interactions present new structures and the ensuing physics are essential to the formation of intriguing clusters, chains and crystals. Our recent experiments demonstrate unique assemblies of Janus particles in ionic liquid solutions that have never been observed before. Despite extensive studies focused on purified Janus particles, their self-organization in the presence of surface-active molecules has not been systematically studied. The intriguing orientation correlation and crystal structures formed in this multicomponent system cannot be explained by the current theory. Our central hypothesis is that the association of surface active molecules with Janus spheres, especially the boundary on the particle surface dividing the two sides, can effectively modulate the interactions among Janus particles and alter their assembly behaviors. The results provide critical information regarding new phenomena observed through experimentation and offer guidance for future applications of Janus particles.   

REU: Synthesis, Assembly and Characterization of Soft Matter Systems

Researchers at Cleveland State University’s Department of Physics and Department of Chemical & Biomedical Engineering collaboratively study the unique properties and applications of soft matter materials. Together, we are in our third year of an NSF-sponsored Research Experiences for Undergraduates (REU) site on Synthesis, Assembly and Characterization of Soft Matter Systems. The objective of our site is to involve undergraduate physics and engineering majors in meaningful interdisciplinary research projects within soft matter science and engineering. A primary focus of our site is to encourage students to continue in STEM fields as either graduate students or workforce members. CSU’s focus on Engaged Learning has cultivated a strong culture of support for undergraduate research, and REU participants benefit from this culture. Students receive one-on-one mentoring from experienced faculty and participate in a variety of professional development opportunities. This poster will give an overview of the student research accomplishments over the last three years. It will also discuss the benefits of the experience to both students and faculty mentors.   

Introducing Mutations in an artificial self-replication system

We have started a study of Darwinian evolution using an artificial self-replication system of DNA origami dimer rafts. By introducing a small error (3 bases) in the sticky-ends recognition strands, there is a small chance for the system to mutate templating a different dimer raft which can replicate itself effectively starting a new species. The mutation rate is small, but the mutated nanostructure shares the same replication ability as the original dimers. Although the original dimers are dominant at the beginning of self-replication, after many replication cycles we should have an equal mixture of mutated and original dimers. In addition, we can modify the mutated structures with the capability of growing faster, then after many replication cycles, the mutated species will take over the system. We can use the functionality of the different species to affect this takeover. Mutation and population domination by the fittest species would amount to natural selection in this artificial system   

Flow-induced fracture in freestanding wet colloidal pillars

Self-assembly of colloidal particles is an attractive means to create materials with engineered properties by controlling the hierarchy of particle composition, size, ordering and macroscopic form. We recently demonstrated a direct-write method [1] that enables self-assembly of colloidal particles from liquid suspension into centimeter-scale freestanding crystalline structures by utilizing a combination of evaporation-driven liquid flow and capillary attraction. However, in a subset of the structures we constructed, cracks formed during drying, throughout the height and at the free end. We detail our observations and explain how the Darcy flow of liquid through the structure drives fracture, despite the structure being compressed by a uniform capillary pressure at its surface. We derive a criterion for the presence and spacing between cracks, based on an energy scaling argument, that compares favorably with our experiments. This work ultimately provides a guideline for constructing crack-free 3D colloidal structures, which can potentially achieve complex freeform macroscale geometries.
[1] Tan*, Beroz*, Kolle, Hart, Adv. Mater, (2018).   

Block copolymer assemblies under 3D confinement: from spherical boundary to continuous network template

Depending on the interplay between polymer chain stretching and interfacial energy of the intermaterial dividing surface (IMDS), different self-assembled block copolymer nanostructures can be achieved. Directed self assembly in 2D employing confinement via walls, posts and various surface treatments can improve long range order and induce new microdomain patterns. By applying templates with spatially varying 3D geometric boundary conditions, block copolymers have the possibility to form a host of new morphologies and complex, hierarchical block polymer/template 3D ordered composites. The self-assembly behaviors of a bulk phase lamella-forming polystyrene-b-polydimethylsiloxane diblock copolymer within different 3D confinement boundary conditions are presented. The 3D confinement varies from that of within a simple sphere to occupation of the connected void space of a complex 3D network template. By taking advantage of Slice-and-View scanning electron microscope tomography, large volume 3D information of the polymer/template assembles can be produced at high resolution. The domain morphology, IMDS curvature, polymer chain stretching, and morphological defects are discussed.
  

Hydrodynamically-driven assembly of nanoparticles in an anisotropic media

Nanoparticle (NP) self-assembly in liquid crystals (LCs) depends on the elasticity of the material to form arrays with crystalline symmetry. These arrays can be tuned via the anchoring of the NP, and the orientation of the director field, effectively using the defects around the NP as sites for assembly. Additionally, confinement and hydrodynamic fields can also be used to control the assembly. Scenarios often consider one, two, or three NPs under various flow regimes and in moderate confinement. The simulations presented here use the Stark-Lubensky formalism, where the Landau-de Gennes free energy functional is coupled with the momentum balance through a Poisson-bracket formulation. To describe NP-LC suspensions, a transient three-dimensional Galerkin finite element framework was implemented to achieve a numerical solution. We show that, independent of NP anchoring, defects are displaced in the up-stream direction, ultimately forming a hedgehog defect. The assembly mechanism for a pair of NPs is modified in the large Ericksen regime, where the NPs show an unexpected non-monotonic tendency to aggregate. The modifications to the defect structure and the free energy landscape open a new avenue to the directed assembly of NPs immersed in LC under conditions far from equilibrium.   

Knitting Patterns of Pentasil Chains reveal MFI-MEL Heterostructures in Two-Dimensional Zeolite Nanosheets

The zeolite MFI is a widely used catalyst and adsorbent, which also holds promise as a thin film membrane for the separation of hydrocarbon isomers and other difficult to separate mixtures. The discovery of nm-thick 2-dimensional (2D) MFI nanosheets has enabled methods for thin film zeolite fabrication that open new horizons for membrane science and engineering. However, the crystal structure of 2D-MFI nanosheets and its relationship to separation performance remain elusive. Using transmission electron microscopy, we find that one- to few-unit-cells wide intergrowths of the zeolite MEL exist within 2D-MFI. We identify the planar distribution of these 1-dimensional (1D) or near-1D-MEL domains, and show that a fraction of individual nanosheets have high MEL content while the majority of nanosheets are MEL-free. Atomistic simulations suggest that commensurate knitting of 1D-MEL within 2D-MFI creates more rigid and highly selective pores as compared to those in pristine MFI nanosheets and permeation experiments show an unprecedented separation factor of 60 using an industrially relevant (undiluted 1 bar xylene mixture) feed.   

Structural and mechanical properties of nucleic acid nanotubes: A combined all-atom and coarse-grained molecular dynamics study

In this work, we introduce a computational framework to model nucleic acid nanotubes and estimate their mechanical properties using various levels of theory. Using atomistic molecular dynamics (MD) simulations, we report the enhancement of the structural and mechanical stability of DNA nanotube (DNT) by changing the salt concentrations. The calculated persistence length (L_p) of the DNTs is ~1-2 μm which is an order of magnitude higher than that of a single dsDNA. DNTs have stretch modulus (γ) value in the range of ~6-8 nN. We find that, with the gradual increment of salt concentration, an increase in L_p and γ which reaffirms the structural and mechanical stability of the DNT at higher salt concentrations. We also model DNT using two widely used coarse-grain (CG) models – namely Martini and oxDNA. We compare and contrast the all-atom MD and experimental results with the results obtained using these CG models. We also propose a model of hexagonal nanotubes made of dsRNA connected by double crossover at different positions. The calculated γ and L_p of the in silico modeled RNTs are in the same range of values as in the case of DNTs. Using helicoidal parameters of individual base pairs, we explain the relative flexibility and rigidity of the RNTs having different sequences.   

Role of Chirality in Self-Assembly of Amino Acid Tryptophan on Au (111)

Proteins only use single L-enantiomer of amino acids owing to the homochirality in a living system. Interestingly, a small amount of tryptophan (Trp), which is one of the essential amino acids, is required in the proteins. We studied the self-assembly of Trp on Au (111) using scanning tunneling microscopy and density functional theory. When single Trp enantiomers were deposited on Au (111), we confirmed 1D chain structures. The deposition of racemic mixture of two opposite Trp enantiomers also built chain structures, but these structures differed from those formed by single Trp enantiomers. In the self-assembly of two opposite Trp enantiomers, the absence of the single Trp enantiomers-formed chain structures is because the heterochiral configurations were more favorable energetically than the homochiral ones. In the heterochiral chain structures, an enantiomer obstructed the access between amino and carboxyl groups in the opposite enantiomers, resulting in the hindrance of formation of peptide bonding between the opposite enantiomers. From these results, we can estimate that proteins utilize a small amount of Trp to avoid the disturbance of peptide bonding formation. In addition, our study also will contribute to the understanding for origin of homochirality in a living system.   

Biomimetic Tunable Disordered Photonic Structures via Programmable Polymer Blend Phase-Separation

Biopolymers such as proteins undergo self-assembly resulting in complex hierarchical structures with well-defined structure and fascinating multi-functional properties. For example, disordered photonic structures in avian birds are responsible for white and blue non-iridescent colors. These structures are thought to arise from phase-separation between β-keratin and cytoplasm during cell growth and the degree of disorder determines coloration. We utilized temperature-induced phase-separation in polymer blends as an approach to attain multi-functional non-iridescent structural colors similar to nature. We further examined the dynamics of phase-separation to gain insights into the molecular self-assembly process and understand the temporal-evolution of structural and optical disorder. Our results indicate that structural disorder scales with domain-size as well as phase-separation time and a strong direct correlation between structural and optical disorder is observed. Tunable disordered photonics can also be obtained simply by tuning the molecular weight of the polymer blend components. In future photonic designs, it is important to exercise good control on structural disorder at multiple length scales in order to obtain tunable visible colors with multi-functionality.   

Oscillatory Microdroplet Constituted Urea Biosensor Stimulated by Acoustic Waves

A proof of concept microdroplet based urea biosensor has been presented that employs a facile but robust detection technique. To substantiate, variation in electrical resistance across a 10 µL salt-laden water microdroplet placed on a glass substrate was observed when it was subjected to sinusoidal acoustic waves at its natural frequency, ~ 320 Hz. For the biosensing application, the conducting water microdroplet was loaded with a suspension of urease-linked gold/cadmium sulfide nanocomposite. Upon adding urea solution in the droplet under the influence of acoustic waves, the changing electrical resistance could be effectively correlated with the changing urea concentration. Higher reaction rate was observed owing to the presence of nanocomposites which provided enzymatic stability and higher interfacial area. Also, the confined droplet geometry resulted in formation of vortices that improved the mixing characteristics. A calibration chart was prepared across a wide range of urea concentrations that was employed for working with unknown human serum samples.
Reference
S. Thakur, M. Bhattacharjee, A. K. Dasmahapatra, D. Bandyopadhyay, Acoustic wave catalyzed urea detection utilizing a pulsatile microdroplet sensor. ACS Sustainable Chem. Eng. 7, 12069 (2019).   

Proton Dynamics of CsH2PO4 Solid Acid using 1H and 31P Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR)

This study investigated hydrogen-bonded CsH₂PO₄ (CDP) solid acid using ¹H and ³¹P high-resolution nuclear magnetic resonance (NMR). Below the superprotonic phase transition temperature, the temperature dependence of the H NMR spectra was observed with two different hydrogen-bonded networks in the CDP structure. The systematic evolution of the lineshape with temperature dependence indicates hydrogen hopping in the chemical structure of hydrogen-bonded networks in a solid acid lattice. The proton conduction under two types of hydrogen-bonds—interchain and intrachain—in CsH₂PO₄ in the chemical environments of PO₄ tetrahedra is discussed.   

Distinguishing protein aggregates from contaminants in viscous mixtures with holographic video microscopy

We demonstrate how holographic video microscopy detects, counts, and characterizes individual sub-visible protein aggregates ranging in diameter from 0.5 μm – 10 μm in concentrations and viscosities typical of biologic pharmaceutical formulations. Protein aggregates are distinguished from other contaminants that are found in pharmaceutical manufacturing, including silicon oil emulsion droplets, air bubbles, tungsten mental particles and breakdown products of industry standard surfactants. A study was also performed comparing the holographic signatures of protein aggregates and other contaminants in solvents of different viscosities. These results differentiates uniform spherical particles from particles composed of an irregularly shaped aggregation of proteins. Holographic characterization’s unique ability to measure refractive index provides a basis for differentiating protein aggregates from contaminants. For example, silicone oil droplets are clearly distinguished from protein aggregates on the basis of refractive index, even when they have the same size.   

Gripping, Catching, and Conveying with a Soft, Toroidal Hydrostat

This work describes how a toroidal hydrostat can be used to perform three functions found in both living and engineered systems: gripping, catching, and conveying. We first demonstrate a gripping mechanism that uses the inversion characteristic of the toroid to encapsulate and grip onto objects under a uniform hydrostatic pressure. Using this mechanism, we demonstrate gripping forces ranging from 1-80 N, depending (in a predictable way) on the geometry and material properties of the system. We next demonstrate a catching mechanism akin to that of a chameleon’s tongue: the elasticity of the membrane is used to store mechanical energy and drive a rapid acceleration to capture moving objects out of the air. Finally, we show how the toroidal topology can be exploited to construct a soft conveying mechanism that continuously inverts and passes objects through its center—serving a similar function to that of esophageal peristalsis, while eliminating the requirement of a lubricated interface. In general, we show that the use of inflatable topological structures comprising soft polymeric films is a promising approach to designing soft robotic actuators with novel, bio-inspired functionality.   

Comparison of the Helmholtz, Gibbs, and Collective-modes methods to obtain nonaffine elastic constants

We review and compare the Born-Huang and the Lemaitre-Maloney’s theories that lead to analytical expressions for elastic constants, accounting for affine and nonaffine deformations in a lattice. The Born-Huang method is based on Helmholtz energy while the Lemaitre-Maloney’s formalism focus on Gibbs force. Although starting from different perspectives, in the linear elastic limit, and in equilibrium, elastic material constants must be the same in all these methods. This is explicitly verified on examples of linear chains, and numerical simulation of a non-centrosymmetric crystal.   

Hydrodynamics and rheology of a vesicle doublet suspension

The dynamics of an adhesive two-dimensional vesicle doublet under various flow conditions is investigated numerically using a boundary integral method. In a quiescent flow, two nearby vesicles move slowly toward each other under the adhesive potential, pushing out fluid between them to form a vesicle doublet at equilibrium. A lubrication analysis on such draining of a thin film gives the dependencies of draining time on adhesion strength and separation distance, which are in good agreement with numerical results. In a planar extensional flow, a stable vesicle doublet forms only when two vesicles collide head-on. In a microfluid trap where the stagnation of an extensional flow is dynamically placed in the middle of a vesicle doublet through an active control loop, novel dynamics of a vesicle doublet are observed. Numerical simulations show that there exists a critical extensional flow rate above which adhesive interaction is overcome by the diverging stream, thus providing a simple method to measure the adhesion strength between two vesicles. In a planar shear flow, numerical simulations reveal that a vesicle doublet may form provided that the adhesion strength is sufficiently large at a given vesicle reduced area.   

Continuum modeling of colloid-polymer mixtures in microgravity

We present a continuum model of colloid-polymer mixtures in a microgravity environment. Such mixtures are an archetype for phase transition processes, but the variety of observed colloidal structures remain poorly understood. We construct, analyze and numerically simulate a phase-field model for structure evolution in colloid-polymer mixtures. The model consists of the Cahn-Hilliard equation, which describes phase separation processes in multicomponent mixtures, coupled with the Stokes equation for the viscous fluid flow. The results of the model will be compared against experiments performed on the International Space Station, using data available on the NASA Physical Sciences Informatics system.   

Understanding Fundamental Principles of Emergent Collective Behavior from Simple Bristlebots

While it’s known that simple interactions between active constituents reproduces collective phenomena such as flocking and schooling, there is a lack of comprehensive understanding of emergent collective behaviors arising from different kinds of interactions, increasingly relevant in the modern era. Take connected vehicles as an example: increasingly sophisticated sensors and software are being used to make complex decisions, and a basic understanding of how these changes in vehicular interactions in turn affects collective behaviors such as traffic is unknown. We built a model platform for studying various emergent collective behaviors in the real world, using customizable low-cost bristlebots. We find that certain intrinsic behaviors of the bristlebots such as shape and confinement significantly affect collective behaviors. Ultimately, our system provides an accessible, low-cost model to experimentally study and understand fundamental principles of collective behavior, highly relevant in the modern era.   

Energy Stability of Gravitationally Interacting Rods and Dumbbells

We extend classic dynamical results for two or three gravitationally interacting point masses to ideal rods and dumbbells. We derive equilibrium configurations by demanding that the vector of first derivatives of energy at constant angular momentum vanish. We investigate their stability by checking if the spectrum of the Hessian matrix of second derivatives is positive. The additional degrees of freedom allow the objects to store and exchange angular momentum and enable us to elucidate the behavior of non-spherical celestial bodies like asteroids and comet nuclei.   

A Continuous-time, Analog Approach to Boolean Satisfiability Problems

Recently, a continuous-time, deterministic analog solver based on ordinary differential equations was introduced, to solve Boolean satisfiability (SAT) problems, a family of discrete constraint satisfaction problems. As SAT is NP-complete, efficient algorithms would benefit solving a large number of decision type problems, both within industry and the sciences. Here we present a detailed analysis of the performance of this continuous-time solver and several variants of it, implemented on digital machines, on hard random SAT and very hard Ramsey-type coloring problems. We also present a randomization theorem connecting the entropy generation rate of the dynamics with solution time and problem hardness.   

Weighted Network Analysis of Biologically Relevant Chemical Spaces

In cheminformatics, network representations of the space of compounds have been suggested extensively. Among these, the threshold-network consists of nodes representing molecules. In this network representation, two molecules are connected by a link if the Tanimoto coefficient, a similarity measure, between them exceeds a preset threshold. However, the topology of the threshold-network is affected significantly by the preset threshold. In this study, we collected the data of biologically relevant compounds and bioactivities. We defined the weighted network where the weight of each link between the nodes equals the Tanimoto coefficient between the bioactive compounds (nodes) without using the threshold. We investigated the relationship between the strength of the link connection and the bioactivity closeness in the weighted networks. We found that compounds with significantly high or low bioactivity have a stronger connection than those in the overall network.   

Modeling the Response of Rayleigh - Van der Pol Oscillators to Stochastic Excitation Near the Hopf Bifurcation

The Rayleigh-van der Pol equation for nonlinear oscillation has been a useful model for studying critical behavior near a Hopf bifurcation, including effects of external forcing and noise. Moreover, this equation has been found to be a good model of the long-term behavior of an experimental system consisting of the Wien bridge oscillator. We will describe work to study measures of noise variance, phase relations and other features of this combined model and experimental system, extended to coupling of two or more oscillators.   

Granular Crystallization Driven by Dynamic Jamming Front

Mono-dispersed particle systems under moderate vibration or continuous shearing are prone to form crystallized packing structures. Our grain scale simulations also find a collection of initially unjammed mono-dispersed particles would jammed into ordered packing structures by uniaxial compression via a rake. The crystallization degree, indicated by the local order of individual particles and the number of ordered particles, is found to be strongly influenced by the inter-particle friction. For the smooth particles, crystallization ensues the propagating jamming front regardless of the disorder degree of initial particle arrangement. A trivial friction (m = 0.01) suffices to markedly reduce the crystallization degree from above 90% to 68% thanks to the emergence of shear bands. An increased friction (m = 0.3) induces profuse shear bands, contributing to a significant reduction of crystallization degree (below 40%) which is also correlated with the initial disorder degree. As expected, the order-to-disorder transition of jammed structure induced by increasing particle friction dictates the jammed packing fraction and consequently the dynamic of jamming front. We also reveal mechanisms underlying the dynamic crystallization and its degeneration due to the elevated particle friction.   

Self-Entanglement of a Tumbled Circular Chain

The spontaneous knotting of linear chains has been well studied, but little attention has been given to the self-entanglement of chains with more complex topologies. In this work, we perform experiments with granular chains that undergo tumbling motion to investigate the self-entanglement of circular chains, which lack the chain ends essential for forming knots. We study the entanglement probability and types of self-entanglements formed on linear and circular chains, using the well-studied self-entanglements on a linear chain to frame our understanding of self-entanglements on a circular chain. We describe a characterization method that views a self-entangled circular chain as a link of two components and use it to characterize the self-entanglements on circular chains with known topological descriptors from knot theory. By examining the formation pathway of several self-entanglements, we infer a general mechanism for the self-entanglement of circular chains.   

Effect of mechanical coupling on the synchronization regimes of coupled optomechanical systems

The objective of this work is to investigate the effect of mechanical coupling strength on the bistable synchronization regimes of two coupled optomechanical systems. Classically, the system is described by an effective Kuramoto-type model. Quantum mechanically, the system has been defined by a Hamiltonian and can be studied using semi-classical Langevin equation method. Previous research concluded that the quantum fluctuations are responsible for the appearance of the bistable synchronization regime. In this work, it is shown that, at some critical value of mechanical coupling, the synchronization can be bistable even in the absence of quantum fluctuation. Further, the numerical results of systems with detuning and thermal noise are obtained. These results collectively are used to extract a statistical measure that differentiate bistable synchronous states and unsynchronous states.   

Instability power laws and planes from the variational principle

Stable thermodynamic equilibrium of a physical system exists for a finite range of its physical parameters. Beyond this range, the system may transition to a dynamical one or undergo a phase change; determining the stability limit is often challenging for complex and nonlinear systems. We show that, for a broad range of physical systems, the stability limit may be formulated in terms of physical parameters that are independent with respect to an arbitrary variation of the system. Consequently, the stability limit is simply a power law if the thermodynamic potential describing the system comprises only two energy quantities; if comprised of more than two quantities, the stability limit is generally represented as a plane. Our result is shown to be valid for several solid, fluid and gas systems, and is especially useful for determining the stability limit of systems for which no analytical solution exists. As an example of the latter, we experimentally and theoretically investigate the stability limit of a liquid droplet exposed to an electric and gravitational field. The connection with statistical physics is also discussed.   

The conditions leading to dry lumps and dry granular jets when dispersing grains across a liquid-air interface

Avoiding the formation of dry lumps when dispersing powders into water is key for many industrial processes.
This study identifies experimentally the conditions leading to i) the dispersion of individual grains ii) the formation of a dry lump or iii) the formation of a dry granular jet when pouring grains onto a liquid-air interface by varying systematically grain size, density, contact angle and the grain flow rate.
Once a granular island is formed onto an interface, it can grow till reaching a maximum number of grains, above which either individual grains leave the interface or the whole island sinks. These two phenomena are governed by a different scaling law and a critical Bond number can be identified, above which grains disperse. The latter depends on two dimensionless numbers: the relative grain density and on their contact angle. Conversely, the formation of dry granular jets depends on the dimensionless kinetic energy of the grains when impacting the interface and on their contact angle.
A physical interpretation is provided to explain common practical strategies used to promote grain dispersion in industrial processes.
REFERENCES
Ong X. Y., Taylor S. E., Ramaioli M., Langmuir, 2019, 35, 34, 11150-11156   

Processing and Characterization of Ultralow-Binder-Content Particulate Composites

We developed the technique of compaction self-assembly (CSA), through which ultralow-binder-content (UBC) particulate composites were produced, with the binder content (c) in the range from 2 to 8 wt.%. In CSA, a high pressure is applied on premixed binder and granular filler, so that the material is densified and the binder is self-assembled into polymer micro-agglomerations (PMA) at critical load-carrying locations. Such a microstructure minimizes the system redundancy. The flexural strength of the UBC composite is ~25 MPa and the compressive strength is more than 50 MPa. This technology may find important applications for advanced pharmaceutical manufacturing, green cement, energy storage materials, among others.   

Automated Measurement of the Profile of an Avalanching Conical Bead Pile

A conical bead pile subject to slow driving is used as a model system to experimentally investigate critical behavior in a granular system. The pile is composed of roughly 20 000 steel beads, 3 mm in diameter; we drive the pile by adding one bead at a time to the pile apex. We record the changes in pile mass over the course of tens of thousands of bead drops to characterize the distribution of avalanche sizes. To better understand the dynamic effect of individual avalanches, we now capture a profile image of the pile at every bead drop. By automating analysis of the images in Matlab, we can track changes in the overall height of the pile as well as variations in the angle of repose as the pile responds to each bead drop. In our previous work, we have characterized the changes in avalanche size distribution as we tune the cohesion in the system. Specifically, as cohesion is added, the size and number of the largest avalanches in the system increase. Using the new profile measurements, we investigate the effect of cohesion on the overall angle of repose as well as the variation in the angle. We find larger variations in the angle of repose as the cohesion is increased, and we correlate these variations with the mass of individual avalanches.   

Onset of dynamic jamming in granular medium subject to explosion

The dynamic jamming of granular media consisting of glass spheres subjected to explosion is investigated experimentally via a radial hele-shaw cell connected with a vertical shock tube. The evolution of velocity fields as well as the resulting strain rate fields are attained via the imaging technique, namely Particle Image Velocimetry (PIV). As the expanding internal surface accelerates outwards, a highly localized and exponentially decayed flow field is induced and becomes increasingly diffusional with an increased decay length. The end of the acceleration phase of the advancing internal surface corresponds to the transition of exponentially decayed flows to a jammed band with largely constant velocity in the wake of a circular dynamic jamming front across the thickness of which a large velocity gradient is discerned. The onset of a steady dynamic jamming commences from the transition of flow fields. Afterwards, most compression and shear deformations occur throughout the thickness of the jamming front in a highly heterogeneous manner in contrast with the deformation mode in the prior transient period. The statistical analysis of strain fields reveals a characteristic length scale which may well arise from the grain-scale heterogeneity of granular materials.   

Multi-functional Tw-Kagomé lattices: Tuning by pruning mechanical metamaterials

We study phonon and elastic properties of randomly-diluted twisted-Kagome lattices with tunable structures that include auxetics with negative Poisson ratios. Intelligent protocols to tune the Poisson ratio of elastic networks can be used in the design of metamaterials with flexible mechanical properties. Here we introduce two periodic lattice models that access negative values of the Poisson ratio, one of which accesses the full range from -1 to +1. Besides tunable auxetic transitions, these models exhibit rich critical phenomena that include rigidity percolation (where both the bulk B and shear G moduli continuously vanish), jamming (where B undergoes a discontinuous jump and G grows continuously from zero), shear-jamming (where G undergoes a discontinuous jump and B grows continuously from zero), and “double jamming” (where both B and G undergo a discontinuous jump).   

Visualizing the Interfacial Jamming of Nanoparticles on a Liquid Surface

Two-dimensional assemblies of nanoparticles (NPs) are model systems for studying jamming and vitrification, with slowed dynamics, rate dependence, and dynamic heterogeneities. We developed a scanning electron microscopy (SEM) technique to image NP assemblies on a nonvolatile ionic liquid (IL) droplet where areal density, φ, is well-controlled. SEM precisely images the real time location of each NP on the droplet surface, allowing NP dynamics to be determined as a function of φ. The packing structure and interactions between NPs can be rigorously measured as assemblies transition from the liquid state to the glassy or jammed state. Here, monodisperse PEGylated silica NPs and gold NP tracers (100-250 nm) were cast on the surface of 1-ethyl-3-methylimidazolium ethyl sulfate IL and imaged. By measuring the mean square displacement as a function of time, NP diffusion coefficients were found and compared with phenomenological models. Assembled structures were also analyzed at each φ by order parameters <ψ₆> and T*, Voronoi tessellation, and pair correlation functions. The results show that as φ increases, NPs diffuse more slowly and their assembly transitions from a liquid-like structure to a crystalline structure.   

Extreme Energy-Absorbing Metamaterials Based on Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are fascinating materials for energy dissipation, due to their extreme damping behavior emerging from an internal degree of freedom of LC molecules, which is coupled to elastic deformations of polymer network. Here, we report metamaterials composed of LCE beams for extreme energy absorption. We have synthesized LCEs through a two-stage thiol–acrylate reaction to consider the effects of the alignment of LC molecules within LCE beam elements. The energy-absorbing capability of metamaterials consisting of bistable beams with differently arranged LC molecules was characterized at strain rates from 10^-4/s to 10³/s and it followed a power-law relation. We observed that metamaterials based on LCE showed increase of the energy absorption at a higher strain rate. Moreover, the strain-rate dependency could be tuned by LCEs with different degree of alignment. We envision that our study can contribute to harnessing the interplay between snapping-based architectures and enhance dissipation of LCEs to enable the metamaterials with extreme energy-absorbing capabilities.   

Valley Hall In-Plane Edge States as Building Blocks for Elastodynamic Logic Circuits

Although waveguiding is vital in mechanical signal processing and energy harvesting, conventional techniques are prone to significant backscattering from corners or junctions. Inspired by the quantum Valley Hall effect, we create and study topologically protected in-plane waveguides that are immune to backscattering. In addition, the waveguides are exploited to realize multi-interface junctions that act as logic ports with unconventional wave-manipulation capabilities, including asymmetric transport, energy trapping and rerouting, and mechanical signal delaying. These ports can ultimately serve as the building blocks for a practically endless array of elastodynamic circuits.   

Brownian Motions in Two Dimensions under Non-harmonic Potentials: Nonequilibrium Steady-State Dynamics

A Brownian gyrator features a two-dimensional random particle under a harmonic potential, while random thermal kicks are of distinct temperatures along the two axes [Filliger and Reimann, Phys. Rev. Lett. 99, 230602 (2007)]. In our current work, we replace the harmonic potential by other types of potentials, and we investigate the nonequilibrium steady states (NESS) through simulation studies. In contrast to the Brownian-gyrator case, where the average flux circulates along the probability contour, our results for the non-harmonic cases do not follow this signature. Moreover, the violation of this feature exists even near the potential energy minima, where the harmonic approximation work. For the special case with a symmetric quartic potential, we provide a simple argument for this less intuitive behavior.   

Time-reversal-invariant scaling of light propagation in one-dimensional non-Hermitian systems

Light propagation through a normal medium is determined not only by the real part of the refractive index but also by its imaginary part, which represents optical gain and loss. Therefore, two media with different gain and loss landscapes can have very different transmission and reflection spectra, even when their real parts of the refractive index are identical. Here we show that while this observation is true for an arbitrary one-dimensional medium with refractive index n(x) and its time-reversed partner with refractive index n*(x), there exists a universal scaling that gives identical transmittance and reflectance in these corresponding systems. Interestingly, the scaled transmittance and reflectance reduce to their standard, unscaled forms in a time-reversal-invariant system, i.e., one without gain or loss.   

Examining Human Unipedal Quiet Stance: Temporal Correlations in the Jerk Record

We investigated the quality of smoothness during human unipedal quiet stance. Smoothness is quantified by the time rate of change of the accelerations, or jerks, associated with the motion of the foot and can be seen as being indicative of how controlled the balance process is. To become more acquainted with this as a quantity, we wanted to establish whether it can be modeled as a (stationary) stochastic process and, if so, explore its temporal scaling behavior. Specifically, our study focused on the jerk concerning the center-of-pressure (COP) for each foot. Data were collected via a force plate for individuals attempting to maintain upright posture using one leg (with eyes open). Positive tests for stochasticity allowed us to treat the time series as a stochastic process and, given this, we took the jerk to be proportional to the increment of the force realizations. Detrended fluctuation analysis (of various orders) was the primary tool used to explore scaling behavior. Results suggest that both the medial-lateral and anterior-posterior components of the jerk display persistent and antipersistent correlations which can be modeled by fractional Gaussian noise over three different temporal scaling regions.


  

Morphological Properties of Shrinkage-Induced Crack Patterns

We investigate morphological properties of shrinkage-induced crack patterns, which are observed such as on cooling lava and drying soil, through numerical simulations of a phase-field (PF) model. Since our PF model does not require any assumptions related to crack nucleations and numerical lattice configurations, we can investigate the pattern formations that purely depend on material/external parameters. We find that our PF model shows various types of pattern formations depending on a shrinkage rate and material constants, and in particular, the shrinkage rate provides a significantly qualitative difference in the pattern formations. Cellular patterns resulting from sequential fragmentations of straight cracks can be observed when using a slow shrinkage rate, while random network patterns resulting from connections of micro cracks that appear simultaneously can be observed when using a rapid shrinkage rate. We quantify the difference of the pattern formations statistically, and explain the origin of the difference on the basis of a simple continuum theory of a thin layer of viscoelastic material.   

Reaction-diffusion waves interacting with fractals, spirals, and concave & soft obstacles

We simulate the recovery and delay of reaction-diffusion wave fronts colliding with various obstacles in narrow two-dimensional channels by numerically integrating the two-variable Tyson-Fife reduction of the three-variable Oregonator model of the chemical Belousov-Zhabotinsky reaction. We investigate the influence of obstacles on the wave front's shape and its recovery after passing around/through fractals (e.g. Hilbert curve, Peano curve, inverse Sierpinsky carpet), Archimedian spirals, and convex & concave polygons by plotting the wave front's left most point and delay versus time. We find that wave fronts behave the same when propagating through symmetric obstacles (e.g., Hilbert curve and Sierpinski carpet) at specific angles, which is in contrast to non-symmetric obstacles such as the Peano curve. At long times, wave fronts follow the same power-law recovery behavior as previously observed for convex obstacles.
We also construct two types of chemical clocks, using illumination gradients with the light-sensitive reaction term or using space-dependent diffusion constants in the diffusion term.   

Reaction-Diffusion Waves as a Black Hole Event Horizon Analogue

Hydrodynamic systems have been build to model the interaction between electromagnetic waves and black/white hole event horizons using gravity waves in water tanks as they behave analogously to electromagnetic waves traveling through space-time. We present a further scaled-down table-top size experiment using a chemical reaction-diffusion (RD) system, the Belousov-Zhabotinsky (RD) reaction. These RD waves propagate forward without any mass transport. When moving in a channel with a flow-rate gradient it is possible to achieve a situation when the medium's flow rate equals the propagation speed of the BZ wave front, creating the illusion of a stationary wave, the black/white hole horizon.   

Harvesting energy from the thermal motion of gas molecules in gravity

This presentation will describe how to produce energy, e.g. electricity, from the thermal motion of gas molecules under the effect of the gravity. We will describe the motion of the gas molecules in the gravity, and the consequence of such motion. We will shown the conditions for making use of the thermal motion of the gas molecules to produce electricity.
  

Transport and Dynamical Properties of Two-Dimensional One Component Plasma

We present a new theoretical model based on Fick’s law for calculating the diffusion constant for a two-dimensional (2D) one-component plasma in the presence or absence of magnetic field. Both 1/r and ln(r) potentials are employed in conjunction with various values of coupling parameters. Our model predicts diffusion constants in agreement with recent simulation results. We also present detailed comparison of our results with those obtained from other theoretical models. Additionally, we compare our transport coefficients for the 1/r and ln(r) potentials for 2D systems.   

Two-Scale Factor Universality in O2 and H2

Video images are increasingly used in science, supplanting current methods such as light scattering by a statistical evaluation of the images. In this paper, it is shown that light turbidity data, a quantity due to density-induced refractive index fluctuations, can be directly obtained from image analysis. We fitted the turbidity data to a theoretical expression that allowed us to estimate the critical amplitudes of isothermal compressibility and correlation length. Images of an oxygen-filled cell and a hydrogen-filled cell were taken near their respective critical temperatures of 155 and 33 K. Images for Hydrogen were analyzed and values of isothermal compressibility and fluctuation correlation lengths obtained from turbidity fitting were compared against literature values. In order to go very close to the critical point, the fluids are placed under weightlessness using a magnetic levitation device. Our data analysis shows a large sensitivity of the fitting parameters to the refractive index value and to even minute density deviations from criticality.   

A Computational Study of The Villin Headpiece Subdomain HP36: The Effect of Hydration on Side Chain Dynamics in the Hydrophobic Core

Hydrophobic side-chain interactions are considered the major dynamic force in the folding of globular proteins and aggregation of non-globular proteins. Due to its small size, a villin headpiece subdomain HP36 is an ideal system to compute the influence of hydrophobic side-chain interaction on protein stability. Alzheimer’s has been attributed to the interactions between the hydrophobic side chains, such as phenylalanine, to drive beta-amyloid (AB) aggregation. Molecular modeling is used to gain insight into how protein-protein interactions and flexibility of the hydrophobic core are altered with the hydration level of HP36. The calculated free energy profiles will be used to estimate the rate constant for ring flipping and determine the effects of relative solvation on transition frequencies, lifetimes, and populations of the residues. These results can have implications on the hydrophobic residue interactions that drives the aggregation of AB plaques in patients with Alzheimer’s.   

Theoretical modelling of stock price dynamics in stock markets using a phynance approach.

This paper presented time to time price dynamics associated with stock assets within stock markets. Our conjecture was that, stock prices are stochastic and time variant and as such they do attain and posses different values from time to time. We aimed to model the old way phenomenon of stock price dynamics using a distinct model from the physics field. We used the two-forms of Schrodinger equation and identified the one which best describes the dynamic nature of stocks. Our results suggested that, stock price dynamics can well be modelled and presented using the time independent Schrödinger equation (SE) with traceable stock price changes. This supported our conjecture as stock prices are traditionally known to be stochastic in nature and normally they are non-stationary. Additionally, we objectified to suggest another unique way of solving the Schrödinger equation contextualized into our case. We derived the solution using Frobenious method and the method of undetermined factors borrowed from ordinary differential equations. Fortunately, all the chosen methods proved to work well and to provide significant solutions. We therefore concluded that stock prices are non-stationary and recommend the wider use of such physics models.   

Market crashes as second-order phase transitions

We study the behavior of ensemble features of financial markets during crash periods to see if they exhibit the behavior typical for second-order phase transitions. We find that during market crashes the order parameter (defined as the ensemble-average correlation) sharply increases and fluctuations (defined as ensemble volatility) exhibit a spike. In addition, the hysteresis effect is observed for correlations and drawdown (market drop) and a similar effect exists for trading volume and drawdown. These facts point that during crashes the markets not only resemble, but undergo a second-order phase transition. Market phases can be identified on a trading volume vs drawdown diagram as regions with high and low order parameter. While market dynamics has a self-coordinated nature, the two inputs on phase diagram are measurable directly from the markets.   

Policies for allocation of information in task-oriented groups: elitism and egalitarianism outperform welfarism

Communication is probably the most controllable factor that are known to impact on the problem-solving capability of task-forces. In the case connections are costly, it is nedeed a better policy to allocate them to the individuals. We use an agent-based model to study how distinct allocation policies affect the performance of a group of agents whose task is to find the global maxima of NK fitness landscapes. The larger the influence neighborhood of an agent, more information it receives. We find that the elitist policy in which agents with above-average fitness have their influence neighborhoods amplified is optimal for smooth landscapes, provided the group size is not too small. For rugged landscapes, however, the elitist policy can perform very poorly for certain group sizes. In addition, we find that the egalitarian policy, in which the size of the influence neighborhood is the same for all agents, is optimal for both smooth and rugged landscapes in the case of small groups. The welfarist policy, in which the actions of the elitist policy are reversed, is always suboptimal, i.e., depending on the group size it is outperformed by either the elitist or the egalitarian policies.
Reia et al, Eur. Phys. J. B (2019) 92: 205. DOI: https://doi.org/10.1140/epjb/e2019-100345-7.   

Digital sustainability: A high dimensional ML problem

The Brundtland report states sustainability as : “development that meets the needs of the present without compromising the ability of future generations to meet their own needs” (World Commission on Environment and Development 1987, p. 37). However, due to serious asymmetry of knowledge and technology around the world with digital (know how) based economy, a high dimensional definition is needed before we can begin to address this issue. We show that digital sustainability is a sparse high dimensional problem with independent features. We use framework by Friedman, J.H. Data Mining and Knowledge Discovery (1997) 1: 55., to minimize digital sustainability problem and suggest a ML framework.
  

Themodynamical aspects of self-simlar relaxation evolution of dense star clusters

Occurrence of negative heat capacity has been predicted and reported for various systems of particles and objects. Examples are granular gases, plasmas, atomic- and molecular- clusters, self-gravitating systems (black holes, stars, star clusters), ... The Antonov's pioneering work assumed an isothermal sphere (a self-gravitating system described by Maxwellian distribution function of particles at constant temperature) enclosed by an adiabatic wall. The work showed no maximum entropy state can be achieved for the sphere beyond a certain large-density contrast between the center and edge. Without the wall, the system expands and particles flow from the dense core to the ambient. Due to the negative heat capacity and conservation of energy/particle, the core heats up and shrinks while the ambient gets sparse and elongates. Such toy model has helped astrophysicists/astronomers to understand the structures of dense star clusters (e.g. globular cluster). More sophisticated models have predicted the clusters can evolve in a self-similar fashion at the late stage of relaxation evolution. The present work discusses the thermodynamical aspects of the self-similar-evolution model compared to classical models, such as the isothermal sphere, polytropic sphere and King model.   

Steady-state thermodynamics: reservoir independence

For stochastic lattice models in spatially uniform nonequilibrium steady states, a thermodynamic temperature T and chemical potential μ can be defined via coexistence with heat and particle reservoirs. Here we ask whether the values so obtained depend on the nature of the exchange between system and reservoir. For example, a stochastic particle reservoir is usually taken to insert or remove a single particle in each exchange, but one may also consider a reservoir that inserts or removes a pair of particles in each event. In equilibrium, equivalence of pair and single-particle reservoirs is guaranteed by the canonical form of the probability distribution on configuration space. We examine whether this equivalence persists in nonequilibrium steady states.   

Graphlet Degree correlations reveal evidence of partial spatial embedding in complex networks

By virtue of most complex networks being comprised of real world objects or entities that can be assigned a location, the interactions that drive edge formation and network growth are affected by the spatial arrangement of the network's nodes. This means that most networks can be considered "partially embedded" in space, and the degree to which the network is embedded is determined by network growth rules. We utilize recent advances in network alignment techniques based on "Graphlet Degree Similarity" (GDS) to quantify how topological quantities change due to varying levels of spatial embedding in simulated complex networks. We demonstrate that the graphlet degree cross correlation matrix can be used to quantify the level of spatial embedding, and discuss how similar measures can be used to investigate the spatial hierarchy of complex networks.   

Non-equilibrium statistical physics of systems with finite heat baths

In this presentation I analyze the thermodynamics of a system connected to a single, finite heat bath. I concentrate on the case where the work reservoir can only change the Hamiltonian of the system, not the Hamiltonian of the bath, nor the interaction Hamiltonian coupling the system to the bath. In order to apply the tools of conventional stochasteic thermodynamics, I assume that the system is coupled to a second, infinite heat reservoir, in addition to the finite heat bath. I prove that in this scenario the entropy production rate of the joint system-(finite)-bath is lower-bounded by the rate of change of the mutual information between the system and the finite bath. This means that while it is possible to use a semi-static process to simultaneously extract the no-nequilibrium free energy of the system considered by itself, together with the non-equilibrium free energy of the finite bath considered by itself, it is impossible to at the same time extract the free energy stored in the mutual information of those two systems.   

Maximizing free energy gain

Maximizing the amount of free energy that a system extracts from its environment is important for a wide variety of physical, biological and technological processes, from energy harvesting processes such as photosynthesis to energy storage systems such as fuels and batteries. We extend recent results from non-equilibrium thermodynamics to derive closed-form expressions for the maximum amount of free energy that a system can extract from its environment over the course of a fixed process. We also analyze how our bounds on extractable free energy vary with the initial distribution of the states of the system. Simple equations allow us to compare the amount of free energy that can be extracted under the optimal initial distribution with that for a sub-optimal initial distribution. We show that the problem of finding that optimal initial distribution is convex and solvable via gradient descent. We demonstrate our results by analyzing how the amount of extractable free energy varies with the initial distribution of a simple Szilard engine.   

Drop breakup in yield stress fluids

We study the production of droplets inside yield stress fluids (YSFs) using an extrusion-based 3D printer device. Microgel-based YSFs have recently gained traction in 3D printing applications due to their ability to be molded when stresses are applied, followed by immediate solidification to hold shapes when stresses are removed. The capillary breakup of an immiscible Newtonian fluid inside a YSF can be eliminated completely when the yield stress of the YSF is sufficiently high. This contrasts with simple liquids, where the dispersed phase of co-flowing streams always breaks into spherical droplets. There is, however, and intermediate regime where the evolution of capillary breakup and shape relaxation of drops is arrested, resulting in droplets of non-spherical shapes. By carefully tuning the flow and shear parameters, we demonstrate precision control of dispersed drop shapes through simple physical parameters in ‘co-flowing’ streams of a Newtonian fluid and a surrounding YSF.   

Interfacial thermodynamics of spherical liquid nanodroplets: Molecular understanding of surface tension via hydrogen bond network

Surface tension plays a important role in nucleation of atmospheric liquid droplets. Especially, understanding of interfacial thermodynamics of the 1 nm scale nucleus is essential for characterization of nucleation such as the activation energy barrier. Despite theoretical and experimental studies performed for decades, determination of surface tension of the nanodroplet is still a topic of ongoing controversies. Here, we investigate surface tension of spherical nanodroplets by both molecular dynamics and density functional theory, and find that surface tension decreases appreciably below 1 nm diameter, whose analytic expression is derived from the Tolman's equation. In particular, analysis of the free energy of nanodroplets shows that the change of surface tension originates from the configurational energy of molecules, which is evidenced by the disrupted hydrogen bond network of nanodroplet. Our results may further illuminate the molecular mechanisms of the interface-related phenomena associated with molecular fluctuations such as ice nucleation or biomolecule adsorption where the macroscopic thermodynamic quantities are ill-defined.   

Hydrodynamic Analog for Spontanenous Emission Radiation

It has been shown that a drop of fluid can be made to bounce on a vertically oscillating bath of fluid. These droplets, known as “walkers”, couple to the waves they generate and are propelled forward. When a variation of depth in the fluid bath is introduced it creates a difference in potential; droplets crossing the barrier must do so on a transmitted exponentially decaying wave. We have created a system which spontaneously generates walker droplets to simulate particles leaving a potential well. Previous studies of walker droplets have used forcing amplitudes just below the Faraday instability threshold to study the drop’s path about the fluid bath. In this system we use a forcing amplitude well above the threshold in order to generate walker droplets autonomously. The droplets then tunnel across a potential barrier to a damped region where the fluid is below the instability threshold. The formation of these droplets and their resulting kinetic energy is related to the amplitude and frequency of the driving oscillation. We present a statistical study of droplet emmision from a potential well. The system could provide an analog to radioactivity in which particles spontaneously tunnel across a potential barrier, showing promise for future analysis.   

Ejection of individual microparticles from an electrified meniscus

On-demand production of 2D and 3D parts with complex geometries and/or high value material requirements is significant to many industries, and a broad goal of additive manufacturing. State-of-the-art methods for metal, ceramic, and advanced engineering materials typically utilize powder feedstocks, and the dimensional resolution of parts is limited to several particle diameters or larger, which limits precision applications. To address this challenge, we present a method to direct-write individual microparticles onto substrates using an electrohydrodynamic process. A liquid droplet, confined at an orifice and containing microparticles on its surface, is electrified, resulting in the dynamic ejection of individual microparticles from the droplet’s apex. We experimentally detail the electrohydrodynamic process of particle ejection and the parameter regime for which particles print individually. Scaling for ejection speed, liquid entrainment on the particle and satellite droplet formation are also discussed.   

Observing phase separation of temperature-responsive colloids under shear

We use colloidal microgel particles, PNIPAM and PNIPMAM, in a colloid-polymer system to study phase separation. We synthesize particles of various sizes and characterize their temperature-tunable size with differential dynamic microscopy. When these colloidal particles are mixed with a depletant we observe fluid-fluid phase separation as well as gelation. The process of phase separation is observed using a custom-built light-sheet microscope. With a two-color fluorescent labeling scheme we can track single colloidal particles as well as the phases that emerge as an originally-mixed sample phase separates. Furthermore, we observe our samples under shear in both a custom-built cylindrical shear cell with our light-sheet microscope and in a commercial rheometer using brightfield microscopy. Studying the behavior of multiphase systems like our colloid-polymer samples strengthens our understanding of non-equilibrium thermodynamics while providing novel routes for structuring complex fluids.   

Bravais lattice structures settling in viscous media: scaling of terminal velocity with porosity

The settling velocity of porous objects in Stokes flow depends on their geometry. Complex geometries of such objects occur in nature and technology, such as marine snow, particles in pulmonary drug delivery, chemical catalysts and pollen grains. To model the fundamental building blocks of porous geometries, we conduct experiments on sedimentation of various Bravais lattice unit-cell structures at low Reynolds numbers (∼ 10⁻⁴), where each structure consists of equally sized spheres connected by thin rods. For each of these structures, porosity becomes a function of the lattice parameter. We show that the sedimentation behaviour of all Bravais lattices can be collapsed when the settling velocity is scaled using the number of particles in the lattice. A ball-and-stick model using Stokesian hydrodynamic kernel for spheres and slender-body theory for connecting rods, captures the settling behaviour accurately. We perform PIV measurements of flow inside these structures to understand the dependence of permeability on porosity and also the validity of our theoretical model. Our experiments and calculations pave the way for understanding sedimentation of systems closer to reality by introducing disorder in the lattice.   

Wave damping of a sloshing wave by an interacting turbulent vortex flow

We report on the enhancement of the hydrodynamic damping of gravity waves at the surface of a fluid layer as they interact with a turbulent vortex flow in a sloshing experiment. Gravity surface waves
are excited by oscillating horizontally a square container holding our working fluid (water). At the bottom of the container, 4 impellers in a quadrupole configuration generate a vortex array at high Reynolds number, which interact with the wave. We measure the surface fluctuations using different optical non-intrusive methods and the local velocity of the flow. In our experimental range, we show that as we increase the angular velocity of the impellers, the gravity wave amplitude decreases without changing the oscillation frequency nor generating transverse modes. This wave dissipation enhancement is contrasted with the increase of the turbulent velocity fluctuations from PIV measurements. To rationalize the damping enhancement a periodically forced shallow water model including viscous terms is presented, which is used to calculate the sloshing wave resonance curve as a function of the turbulent fluctuations. From these measurements we compute the dependence of the shallow water viscous friction coefficient   

A mathematical model of mitochondrial calcium-phosphate dissolution as a mechanism for persistent post-CSD vasoconstriction

Cortical spreading depolarization (CSD) is the propagation of a relatively slow wave in cortical brain tissue that is linked to a number of pathological conditions such as stroke and migraine. Most of the existing literature investigates the dynamics of short term phenomena such as the depolarization and repolarization of membrane potentials or large ion shifts. Here, we focus on the clinically-relevant hour-long state of neurovascular malfunction in the wake of CSDs. This dysfunctional state involves widespread vasoconstriction and a general disruption of neurovascular coupling. We demonstrate, using a mathematical model, that dissolution of calcium that has aggregated within the mitochondria of vascular smooth muscle cells can drive an hour-long disruption. We determine the rate of calcium clearance as well as the dynamical implications on overall blood flow. Based on reaction stoichiometry, we quantify a possible impact of calcium phosphate dissolution on the maintenance of F0F1-ATP synthase activity.   

Light-Driven Ballistic Nanoparticle Swimmers

Directed high-speed motion of nanoscale objects in fluids can have a wide range of applications. However, directed movement and high speed in the nanoscale are rarely compatible. Light is a convenient source that can drive nano objects to move by applying optical pushing forces due to momentum conservation when photons are scattered off the objects. In theory, optical forces from a planewave can also be “negative” that pull objects against the light propagation direction in a homogeneous medium if some unique optical configurations of the objects can be realized, but this is yet to be demonstrated. Nevertheless, these optical forces are too weak to enable fast-moving swimmers in fluids. Here, we report ballistic plasmonic Au nanoparticle (NP) swimmers with unprecedented speeds (~397,000 mm s^-1) realized by both optical pushing and pulling forces from a single Gaussian laser beam. The Au NP excited by the laser at the surface plasmon resonance (SPR) peak interact with the NP both thermally and optically, leading unique conditions for ballistic movements and “negative” optical forces.   

Enhancing propulsion efficiency of biomimetic elastic propulsor using hybrid actuation

Bio-inspired propulsion of a robotic swimmer can be achieved by periodic actuation of an elastic caudal fin. We use three-dimensional computer simulations to probe how the hydrodynamic performance of a caudal fin can be enhanced by combining two distinct types of fin actuation. The fin is represented by an elastic rectangular plate that is actuated at the base to perform sinusoidal oscillations. The actuation frequency is set to match the fundamental frequency of the plate yielding the maximum bending amplitude. In addition to this plunging actuation, the plate is actuated by a distributed bending moment with the magnitude that periodically changes in time with the same frequency as the fin base. We introduced a phase difference between the base and the internal actuation and investigate the resultant hydrodynamic thrust and efficiency. We compared the hybrid fin actuation to the base actuation and to the internal actuation to identify the parameter space in which the synergetic effects of the two action mechanisms are the most beneficial for swimmer propulsion.   

Scattering of a fast-swimming bacterium off of a surface: Methods

Thiovulum majus is the fastest known bacteria. In its natural habitat, near the penetration depth of oxygen in salt marsh sediment, these bacteria create fluid flows that pull nutrient-rich water through the pore space. As cells navigate the pore space in which they live, they frequently collide with grains of sand. To better understand the ecology of these microbes, we investigate the dynamics of a collision of a T. majus with a solid surface. This presentation focuses on the experimental techniques needed to perform these measurements. These bacteria cannot be grown in pure culture. We first describe how we enrich T. majus from mud, collected from a Massachusetts salt marsh. Next, we analyze the motion of these cells in a microfluidic device. This device confines the motion of cells to two dimensions. We present techniques that allow us to track several thousand collisions between fast swimming cells and the chamber walls. We show that there are two types of collisions. If a cell approaches the wall at a glancing angle, it initially turns to swim parallel to the wall and then returns to the fluid. In the case of head-on collision, the cell becomes hydrodynamically bound to the surface.   

Distinguishing Complex Locomotory Patterns in C. elegans

Caenorhabditis elegans, more commonly known as C. elegans, are transparent nematodes approximately 1 mm long that inhabit soil in temperate environments. C. elegans have 302 neurons that are similar in form and function to that of humans. This similarity and the small number of neurons has spiked the interest of neurological and biological communities. With over 70 phenotypes, it is possible to quantitatively distinguish many C. elegans phenotypes according to different locomotory patterns. Time-Dependent Diffraction by oversampling provides information about the locomotion in the form of a single time-series; nevertheless, visually the different locomotion types are indistinguishable. We investigate the locomotion of two types of C. elegans, Wildtype (N2) and Roller (OH7547), using Recurrence Plots (RP) to analyze the complex dynamics collected. This method of analysis allows us to keep all of the dynamics of motion since every point in the diffraction pattern is a superposition of light rays diffracted from each point on the worm. Through the RPs, we dive deeper into the difference between locomotory patterns to be able to separate these two types of C. elegans.   

Comparison between direct experimental measures of surface excess concentration in surfactant solutions with estimates based on the Gibbs Adsorption Isotherm

Recently, Menger et al [J. Am. Chem. Soc. 131 (30): 10380 (2009)] have suggested that the Gibbs Isotherm Method (GIM) is not the best way to estimate surfactant surface excess concentration from surface-tension measurements at the air-water interface. Important experimental techniques, such as Neutron Reflectivity and the radiotracer method, have arisen in recent years helping to clarify the controversy. This is because they are able to measure the surface excess concentration without the use of the GIM. We review the best available surface-tension data for some ionic and nonionic surfactants, and apply the GIM to them in order to directly compare with Neutron Reflectivity and radiotracer data [Adv. Coll. Int. Sci. 247: 178 (2017)]. As the thermodynamic theory behind the GIM predicts the agreement is good for concentrations smaller than the CMC, both for ionic and nonionic surfactants. In the case of nonionic surfactants, we found also a good agreement for concentrations larger than the CMC.   

Solvent diffusivity and viscosity in graphene oxide membrane for water-ethanol separation

Graphene oxide (GO) membranes were recently suggested for applications in separation of ethanol from water using a vapor permeation method. Understanding microscopic diffusivity of water and ethanol in graphene oxide membranes is important for separations applications. It is also important to study the anisotropy of water and ethanol diffusivity between the direction perpendicular to the plane and in-plane direction. We used quasielastic neutron scattering(QENS) to measure the temperature dependence of the diffusivity of water and ethanol, and its anisotropy by the utilization of Q-dependence of QENS signals obtained from BASIS at Oak Ridge National Lab. We will also discuss how dynamic measurements from QENS can be correlated with atomic force microscopy measurements of the temperature dependence of viscosity in water/ethanol confined in GO.   

Estimating the posture of coiled and overlapping worms using a neural network

The first step in studying postural dynamics is typically animal tracking. A range of deep learning approaches have proven effective for human posture tracking and several groups have adapted these methods to laboratory animals. These methods typically detect key points on the body and they work particularly well for jointed animals where there are clear landmarks. For tracking the nematode worm C. elegans, some challenges remain. We generated a manually annotated data set with tens of thousands of frames including coiled and overlapping worms to train a neural network to find equidistant points along the worm midlines. During training we rely on simple image synthesis augmentations and use priors that incorporate the worm morphology into the loss function. We show that the new algorithm performs well with challenging backgrounds, for coiled worms, and for multiple overlapping worms. We apply the network to mating, a behaviour that necessarily involves overlapping worms.   

Piezoelectric based sensor development to measure sound intensity in aquatic environments.

When aquatic animals are presented with unfamiliar environments they tend to alter their behavior. To gain insight on such behavioral actions and changes there is a need for a submersible sensor designed to measure sound intensity given by the specimen. Our device builds around the property of a piezoelectric element that gives off an electrical signal when undergoing mechanical stress. This small signal is then boosted by means of a pre-amplifier connected to the piezo element within the device’s housing. In experimentation, the device is placed in a position that allows for exposure of the piezo element surface into the aquatic environment. A clear gel containing the animal’s food or prey within is applied on the surface. In an attempt to get food, or attack, the animal will strike, thus apply a force on the piezo. This provides the mechanical stress needed to gain the electrical signal that is recorded into a digital audio workstation (DAW) as audio. DAW is used to analyze the audio in decibels over time. Software is used to process the data as sound intensity produced by the specimen and quantify the striking intensity. Being able to measure these changes in behavioral habits we would be able to study embryonic learning.   

Epigenetic regulation enhances stability of somatic differentiation state

Stem cell treatments, such as those affecting the states of the network of pluripotency genes, are expected to play a central role in future personalized medicine. The genetic regulatory network consisting of the Oct4, Nanog, and TET transcription factors (TF’s) is understood to control cell fate, specifically the transitions between somatic and pluripotent states. In the classical view, the steady states of a dynamical system modeling the genes, mRNAs, and proteins in this network are identified with the above two types of states. In this work, we investigate the role of epigenetic regulation in further stabilizing the two states. Specifically, we present a mathematical model that combines TF’s and DNA methylation, which is one of the most important forms of epigenetic regulation. Our starting point is a mechanistic model formulated in the language of chemical reaction networks that describe promoter binding together with transcription/translation. We conclude that the effective rate of DNA methylation increases stability (relative size of the basin of attraction) of the somatic cell state, while increasing DNA de-methylation rate, which is controlled by Nanog-guided TET protein, increases stability of the pluripotent state.   

Characterizing the spread of the effect of mutations on the protein-protein interaction network

Networks can serve as a powerful tool for dissecting phenomena in complex biological systems. Of particular interest in biology is the effect of mutations on the resulting phenotype. Although numerous prior studies address this for specific cases, a generalized mechanistic theory of the phenotypic effect of mutation-induced perturbations does not exist. Here we identify patterns and principles characterizing the spread of the effect of mutations on the protein-protein interaction (PPI) network. Specifically, we propose network-based laws to identify differentially expressed genes (DEGs) following loss-of-function (LOF) mutation by studying the interplay between the topology of the PPI network and its response following node removal. We utilize cell line gene expression data following LOF mutation to evaluate network measures involving the DEGs and the knocked-out gene (KOG). We find that for 26/53 genes the DEGs form a statistically significant connected subgraph in a PPI network comprised of interactions curated from literature. Furthermore, we find that DEGs are significantly proximal to the KOG in 10/53 cases. This work contributes fundamental systems-level observations which may be exploited for the development of a predictive framework for general use in biological research.   

Human Gene Expression and the Protein-Protein Interaction Network: Identification of Potential Disease Module Association to Differential Gene Expression for Patient-to-Drug Matching

The relationship between human gene expression (GE) and precision medicine applications is central in understanding how patients are affected by and how to better treat a disease. Even though it is of extreme importance, this knowledge is still absent from almost any disease analysis resulting in misdiagnosis and mistreatment based on symptomatic and physical observation criteria devoid of high throughput sequencing technologies. By detecting disease modules in the protein-protein interaction (PPI) network, along with the identification of patient differential GE sets (DGE), we can suggest effective individual treatment options. Here, we collect public RNA sequencing data for a diverse population and arrange unique GE patient profiles. DGEs are computed using machine learning techniques and are identified within the PPI network, where disease modules can be pinpointed. Those modules can be used for disease classification specific to expression levels and cohort phenotype. By elucidating these interactions using network approaches with an individual patient’s features, human diseases can be identified not by peripheral approaches, but by a personal genetic diagnosis; essentially redefining disease diagnostics from a “one size fits all” philosophy, to a “one size fits you” reality.   

Effects of Stochasticity in Biological Oscillators

Many processes need to occur periodically for proper functioning of the cell, despite fluctuations in its environment. A cell must remain robust against irrelevant fluctuations, while simultaneously responding to relevant cues. We study how network topologies optimize the trade-off between maintaining robust oscillations and biosynthetic expenditure.

Many studies concur that motifs with positive and negative feedback are most robust to variations in the deterministic limit [1, 2], and in the stochastic regime [3, 4]. However, there is no mechanistic understanding for predicting the performance of different topologies.

Our findings suggest that deterministic amplitude, along with the distribution of angular velocities along the phase-space orbit of dynamical variables can lead to a mechanistic understanding of this trade-off. Regulation mechanisms and tuning of interaction strengths play a pivotal role in this optimization.

[1] Tsai TY, Choi YS, Ma W, Pomerening JR, Tang C, Ferrell JE Jr, Science, 2008, Jul 4.
[2] Zhengda Li, Shixuan Liu, Qiong Yang, Cell Systems, Vol 5, Issue 1, 26 July 2017.
[3] Woods ML, Leon M, Perez-Carrasco R, Barnes CP, ACS Synth. Biol., 2016, 5(6), pp 459-470
[4] Vilar JMG, Kueh HY, Barkai N, Leibler S, Proc Natl Acad Sci USA. 2002 Apr 30; 99(9):5988-5992
  

Multivalent Binding of Patterned Polymers

Using inspiration from biology, we can leverage multivalent binding to enhance weak, monovalent binding between molecules. Synthetic glycopolymers have been shown to successfully bind to viruses and toxic proteins. This binding indicates that multivalent polymers are a promising tool for inhibiting target attachment to and subsequent infection of cells. Previous work from our group focused on how structural features of uniform binding site polymers create high affinity and specificity to a single target. In contrast, real synthetic polymers might find it desirable to have multiple binding moieties along the chain. Multiple binding site types allow polymers to target multiple species creating broad-spectrum binding or allow for tracking or imaging in addition to targeted binding. Heterogeneity in binding site patterns can also be a byproduct of chemical synthesis methods. We use a reactive-binding, Brownian dynamics simulation and machine learning to examine how patterning of heterogeneous binding sites along a polymer chain controls binding of a multivalent polymer. Our results provide direction for the rational design of multivalent binders.   

Using structural adaptations in plants for desalination

Biologically-inspired abiotic systems are becoming a central pillar in how we respond to critical grand challenges that accompany exponential population growth, uncontrolled climate change and the reality that 96.5% of the water on the planet is saltwater. One fascinating biologic adaptation to saltwater is the growth of mangrove trees in brackish swamps and along the coasts. Through salt exclusion, the mangrove maintains a near freshwater flow from roots to leaves for survival. One abiotic approach to water desalination is capacitive deionization (CDI), which uses electrostatic force to adsorb ions from a feed stream to a pair of charged electrodes. In this work, we use one-step carbonization of mangrove roots with developed aerenchyma tissue to enable highly-permeable, freestanding flow-through (FT) CDI electrodes. We demonstrate the use of the intact carbonized mangrove roots as electrodes in a FT-CDI system. We also show that the structure of carbonized aerenchyma from mangrove roots reduces the resistance to water flow through the electrodes by 65-fold relative to carbonized plant structure lacking this biological adaptation (woody biomass). These findings have implications in a range of fields including desalination, bioinspired materials, and plant-structure functionality.
  

Correlative Atomic Force Microscopy and Photo-Activated Localization Microscopy of Nano-cellulose Fibrils from Tunicate and Poplar

The molecular structure of cellulose in its native nano-fibril form is difficult to characterize and consequently still not well understood. We report simultaneous Atomic Force Microscopy (AFM) and Photo-Activated Localization Microscopy (PALM) of cellulose nano-fibrils (CNFs) from Tunicate and Poplar. PeakForce Quantitative Nanomechanical Mapping (PFQNM) is employed to characterize sample stiffness and modulus mapping. To complement the AFM measurements, the nano-fibrils were exposed to recombinant family 3 Carbohydrate Binding Modules (CBM3) specifically binding to the hydrophobic surface of crystalline CNF. The CBM3 was also tagged with a photoactivatable fluorescent protein PA-mCherry to allow super resolution imaging by PALM. We compare the distribution of CBM binding sites to the nano-fibril topography to draw conclusions about the degree of crystallinity of the elemental CNF and the distribution of amorphous areas within the CNF and at morphological kinks, which show a non-Gaussian distribution related to the crystallographic planes of cellulose.   

The SWCNT-DNA hybrid for imaging and vector delivery applications A. Adesina, S. Pourianejad, T. Ignatova Nanoscience Department, University of North Carolina at Greensboro, Greensboro, NC, USA

Single Wall Carbon Nanotubes (SWCNTs), being quasi-one-dimensional materials, are extensively investigated for intracellular imaging and bio-sensing applications since discovery of the hybridization of ssDNA molecules to SWCNTs.When functionalized, SWCNT can be introduced within the cell and co-localized with the sub-cellular organelles. SWCNTs exhibit resonance Raman scattering, with sharp, high intensity Raman peaks quite different from that of the hybrid or the cell itself.Raman signal of SWCNTs is chirality specific hence it can be used for nanotube identification and to assess their aggregation in cell environment. The long-term intracellular studies of SWCNTs within neural stem cells have been conducted using micro-Raman spectroscopy indicating the viability of SWCNTs as an efficient tracking agent.Additionally, we investigated the affinity of the ssDNA for SWCNT in presence of complementary strand. Here, the results on the equilibrium and dynamic study of DNA displacement from the nanotube surface, temperature dependence, Manning-Oosawa counterion condensation, and pH variation will be presented.   

Investigating Zika Virus RNA Conformational Switching using Molecular Dynamics

Previous attempts at modeling RNA folding for relatively large systems have demonstrated limited sampling of conformation space due to the presence of kinetic traps. In this work, we used a novel method of molecular dynamics simulations, a 2D replica exchange protocol in which RNA replicas cycle through both temperature variations and base-pairing restraint strength using experimentally determined base-pairing patterns. This method refines the energy landscape, biasing the structure to preserve only the most stable conformations at low temperatures.

Using this approach, we designed all-atom models of sections of the 3’ untranslated region of Zika Virus RNA in order to predict pseudoknot formation between the 3’ dumbbell region and other nearby structures. The dumbbell is theorized to be a driving mechanism that allows switching between the translation and replication phases of the RNA lifecycle. From our simulations, we were able to infer the kinetic and structural favorability of proposed pseudoknots, and investigate potential intermediaries required to trigger switching.   

Restraint-Biased 2D Molecular Dynamics Simulations

Although recent experimental and computational advances have strived to understand RNA structure-function relationships, the roles of many RNAs are dependent upon a complex network of motifs, including hairpins, internal loops, pseudoknots and long-range contacts, that are not easily captured by these methods. Ab-initio modeling using all atom molecular dynamics simulations is limited by the available energy functions and the required sampling of an extensive conformational space. We have developed a hybrid replica exchange method that incorporates secondary structure information from experiments in the form of piecewise linear-harmonic distance restraints (bias forces) to efficiently fold RNA in 3D detail. This sharpens the free energy landscape by reducing the number of conformations that satisfy an optimum 3D fold. Replica exchange conducted in both temperature and position space, where replicas with variable bias forces and temperatures run in parallel and periodically swap, also allows molecules to overcome kinetic barriers surrounding local minima. A range of possible structures are uncovered from higher temperatures, but only the most stable conformations are retained at lower temperatures.   

Flow Analysis and UniverScope: complementary techniques to monitor spheroid growth

The Bioimaging & Optofluidics laboratory has developed an encapsulation technique that allows to produce multicellular spheroids as model tissue at a very high throughput and controlled manner.¹ To monitor the growth of spheroids, we use two complementary techniques. The first one relies on an image-free analysis of the light absorbed by the spheroids; the second one consists of building a microscope that allows parallelized imaging of spheroids in physiological conditions. These two methods are aimed at measuring the spheroid radius as a function of time and at gaining insight into the fate of cells within the spheroids.


^{1. Kevin Alessandri,, Laetitia Andrique, Maxime Feyeux, Andreas Bikfalvi, Pierre Nassoy & Gaëlle Recher. “All-in-one 3D printed microscopy chamber for multidimensional imaging, the UniverSlide”. Scientific Reports. Volume Article number: 42378 (2017).}   

Expanding Without Exploding: Measuring and Modeling the Biomechanics of Pollen Hydration

To carry male gametes to female plants, pollen grains must undergo desiccation. They subsequently rehydrate before extending a tube to fertilize the egg cell. Hydration involves dramatic changes in both the turgor pressure and cell wall properties of the pollen. Little is known about the mechanics of hydration, especially the mechanisms that allow the pollen grain to tolerate the large forces that occur. Here we combine mathematical modeling with experimental data to clarify the role of a membrane-bound tension-sensitive ion channel (MSL8) during hydration. This channel is known to be essential for pollen survival during hydration, but its exact role is not known. New data presented here reveal that MSL8 contributes to rapid stabilization of the pollen grain volume during hydration. In the absence of MSL8, the pollen grain continues to expand indefinitely, often leading to overexpansion and bursting. We develop a mathematical model of rehydration dynamics embodying the interplay between turgor pressure, cell wall mechanics, membrane mechanics, and tension-sensitive channels. This model finds, consistent with the data, that the absence of tension-sensitive channels causes continued expansion of the cell.   

Interaction of Electromagnetic Radiation with DNA

Cells are known to be affected significantly due to constant exposure to electromagnetic radiation. DNA can be damaged with electromagnetic waves in a variety of ways. Cellular multiplication plays an important role in the survival of life. This multiplication involves a chain of DNA replication going through several steps including auto-repair mechanisms to protect DNA from damages. These repair mechanisms range from specific "locate and repair" systems to "damage tolerance" repairs. With many damage events per cell per day the errors that do make it through the repair process are seen as mutations to the genetic code and may be expressed as changes in morphology. Applying an appropriately modified Jaynes-Cummings model with an extra potential we re-investigate the DNA replication considering the entangled states of the nucleotide. Interaction of DNA with radiation is studied as an interaction of matter with electromagnetic waves. This approach is used to measure the probability of changes in morphology due to the interaction of DNA with radiation.   

Polymerization Characteristics of Different Microtubules’ Interaction with Tau Protein and Taxol

We previously reported that the interaction of one of the microtubule-associated protein (MAP), tau protein, is significantly different on two different types of microtubules; brain microtubules versus human breast cancer microtubules in vitro. These two types of microtubules are distinct from one another due to structural differences that exist in beta-tubulin isotype distributions. In addition, microtubules are known as one of the best targets for anti-cancer drugs such as Taxol. It is known that the direct interaction of Taxol with tubulin or microtubules changes the polymerization specifications of them. Due to the existence of Tau protein in brain cells and some breast cancer cells, it is important to study how the presence of both Tau protein and Taxol may affect the polymerization specifications of these two structurally different microtubules. In this study, we explain the outcomes of our experiments in identifying the distinct polymerization properties of Tau protein and Taxol interacting with different beta-tubulin isotypes in these two types of microtubules.   

A Nano-Technology Approach to Screen the Charge of Molecular Motors

A better understanding of the nature of the electrostatic interactions between molecular motors and microtubules requires a thorough examination of, not only the charge of microtubules, but molecular motors as well. Several reported studies have been conducted to screen and evaluate the charge of microtubules in vitro. However, our knowledge about the electric charge of molecular motors is still limited to the evaluation of the association rate they express in conjunction with microtubules.
With the goal of screening and evaluating the charge that molecular motors carry, we have designed a nanotechnology-based method. In this approach, the charge of motors can be screened in vitro by monitoring their behavior inside a uniform electric field. This novel in vitro approach minimizes the cellular factors that can potentially interfere with evaluating the charge of motors, leading to a better capability of evaluating the charge of these bio-motors in a more exclusive framework.   

Optical Imaging of Magnetic Particle Rotation in High Viscosity Fluids

The purpose of this experiment is to study the oscillation and rotation of nanoparticles in fluids of different viscosities. The investigations have practical applications to the medical field, specifically drug delivery through high viscosity fluids like mucus. Magnetic barium hexaferrite (BaFe12O19) and iron oxide (Fe3O4) particles were suspended in distilled water or various glycerol concentrations. The mixtures had a concentration of 2.50mg/ml for the BaFe12O19 and 1.00mg/ml for Fe3O4. Magnetic particles were exposed to oscillating or rotating magnetic fields and imaged with an optical microscope. Time-varying magnetic fields ranging from 10Hz to 180Hz are created by pairs of home-made wire coils that insert into the microscope. Magnetic field amplitudes can be varied from 0-12 mT. The resulting measured frequency of the particle oscillation or rotation equaled the drive frequency when the drive frequency was less than half the frame rate. For high viscosity fluids, higher magnetic field strength was necessary for particle motion. Further investigation will need to be done to determine how the viscosity, particle size, and drive frequency impact the movement of the particles, going from oscillating at the driving frequency to no particle motion.   

The Dynamics of Human Society Evolution

Human society is an open system that evolves by coupling with various known and unknown (energy) fluxes. How do these dynamics precisely unfold? Energetics may provide further insights. We expand on Navier-Stokes’ approach to study non-equilibrium dynamics in a field that evolves with time. Based on the ‘social field theory’, an induction of the classical field theories, we define social force, social energy and Hamiltonian of an individual in a society. The equations for the evolution of an individual and society are sketched based on the time-dependent Hamiltonian that includes power dynamics. In this paper, we will demonstrate that Lotka-Volterra type equations can be derived from the Hamiltonian equation in the social field.
Keywords: evolution; energetics; social field theory; capital; capabilities.   

Ecological mechanisms of direct and indirect bacteriotherapies in generalized Lotka-Volterra systems

Over the last two decades, an association between microbiome composition and some human diseases has been unambiguously established. The correlation between gut microbe composition and these diseases has prompted medical interest into bacteriotherapies, which seek to modify the gut microbiome composition in the hopes of treating the correlated disease. In this work we use generalized Lotka-Volterra (gLV) models to probe the ecological mechanisms through which these bacteriotherapies function. We first describe direct bacteriothapies, which drive a microbiome to a target state via an instantaneous influx of foreign microbes (e.g. probiotics or fecal microbiota transplantation). Then, we present a novel control framework for indirect bacteriotherapies, which drive a microbiome to a target state by deliberately modifying its environment (e.g. diet, acidity, or nutrients). These dual control methods for gLV systems, interpreted as bacteriotherapies, could eventually inform personalized medicine for the microbiome.   

Competing for Resources: on the Emergence of Property Rights

A game theory approach to the evolution of animal conflicts has shown that choosing an initial asymmetric feature, such as first come first served, to settle a contest is evolutionarily stable, as it avoids the costs of animal fights. In this context, we investigate the optimal strategies of a population of non-aggressive agents competing for multiple resources. Resources provide different payoffs and can only be exploited by one agent at a time. If an agent tries a resource that turns out to be occupied, it then looks for another resource, which has a cost. We show theoretically and numerically that this system admits two types of Nash equilibria. In an under-crowded system, resources are equally shared between agents; our reinforcement learning simulations find multiple optimal solutions, where agents each exploit different sets of resources but all earn the same average payoff. The average strategy of these agents matches with the theoretical mean-field solution. Over-crowded systems are instead conducive to the emergence of inequality; some agents can earn more than the others by establishing themselves as property owner of a medium-payoff resource. In the reinforcement learning simulations, such lucky agents emerge naturally from their random learning experience.   

A model to explain the propagation of small dysfunctional mitochondrial DNAs in budding yeast

Mitochondrial DNA (mtDNA) in budding yeast is unstable, resulting in a high rate of spontaneous respiratory-deficient mutants [1]. These respiratory-deficient mutants (called petites) can, however, grow on fermentable media. A petite cell often contains small, incomplete mtDNA fragments, that replicate and pass on from generation to generation with no known function in the cell. These selfish mtDNAs often do better than wild type mtDNA and outcompete (suppress) them if they are present together in a cell [2]. The features that allow these selfish mtDNAs to outcompete wild type mtDNA, as well as the potential for mechanisms that select against selfish mtDNAs remains to be fully characterized. To address these questions we have characterized a set of spontaneous petite mitochondrial genomes, and developed a model to explain what features of their structure inform their suppressivity when mixed with wild type mtDNA.

[1] Dimitrov, et al. (2009). Polymorphisms in multiple genes contribute to the spontaneous mitochondrial genome instability of Saccharomyces cerevisiae S288C strains. Genetics, 183(1), 365–383.
[2] Zamaroczy, et al. (1981). The origins of replication of the yeast mitochondrial genome and the phenomenon of suppressivity. Nature, 292(July), 0–3.   

Information costs in the control of protein synthesis

Efficient protein synthesis depends on the availability of charged tRNA molecules. With 61 different codons, shifting the balance among the tRNA abundances can lead to large changes in the protein synthesis rate. Previous theoretical work has asked about the optimization of these abundances, and there is some evidence that regulatory mechanisms bring cells close to this optimum, on average. We formulate the tradeoff between the precision of control and the efficiency of synthesis, asking for the maximum entropy distribution of tRNA abundances that is consistent with a desired mean rate of protein synthesis. Our analysis, using data from E. coli, indicates that reasonable synthesis rates are consistent only with rather low entropies, so that the cell’s regulatory mechanisms must encode a large amount of information about the “correct” tRNA abundances.
  

Contours information and the perception of various visual illusions

The simplicity principle states that the human visual system prefers the simplest interpretation. However, the conventional coding models could not resolve a contradiction between the global minimum principle and the local minimum principle. By quantitatively evaluating information content along the contours under our framework, it is shown that the most plausible interpretation of a stimulus (the one correctly predicted by local minimum principle) possesses the lowest information globally, hence resolves the contradiction. This model could also explain the perception of various visual illusions including Kanizsa illusions, Ehrenstein illusions and Rubin’s vase via estimating the lower bound of the spread parameter of the von Mises distribution governing human visual expectation. This provides new insight into the celebrated simplicity principle and could serve as a fundamental explanation of the perception of illusory boundaries and the bi-stability of perceptual grouping.   

The robust bioinformatic analysis of the protein sequences with phase behavior

Liquid-liquid phase separation (LLPS) of many proteins is critical in the biological function of membrane-less organelles. Reversible nature of biomolecular PS in cells suggests that phases of proteins and nucleic acids are like to be tightly regulated. Post-translational modifications and single-residue mutations have been shown to lead to the dissolution of biomolecular droplets or phase transition into aggregated forms. Based on the wealth of experimental data, it reasonable to expect that the PS of proteins is highly sequence-specific.
To reveal common motifs of protein sequences that promote PS, we have carried out large-scale statistical analysis of phase separating protein deposited in the LLPSDB database. For each motif, the most significant interactions have been identified (e.g. charge, hydropathy, polarity, pi-stacking). The bioinformatic analysis provides us with crucial knowledge about sequence conservation, secondary structure preferences, aggregation tendency, post-translational modification prediction, and binding affinity.
The long-term goal of our research is the creation of a publically accessible database that would be used by both experimentalists for designing controlled mutations and for computationally-oriented scientists for developing new modeling tools.   

Reentrant Liquid Condensation of Ribonucleoprotein–RNA Complexes

Intracellular Ribonucleoprotein (RNP) granules are membrane-less liquid condensates that dynamically form, dissolve, and mature into a gel-like state in response to a changing cellular environment. RNP condensation is largely governed by attractive inter-chain interactions mediated by low-complexity domains. Using an archetypal disordered RNP, fused in sarcoma (FUS), here we employ atomistic simulations to study how RNA, a primary component of RNP granules, can modulate the phase behavior of RNPs by controlling both droplet assembly and dissolution. Electrostatic interactions are found to be the primary driving force behind condensate formation. Monotonically increasing RNA concentration initially leads to droplet assembly via complex coacervation and subsequently triggers an RNP charge inversion, which promotes disassembly. We construct phase diagrams based on Droplet density and Shannon entropy calculations, wherein three distinct regimes can be identified based on RNA and peptide concentrations. Increasing salt concentration is found to inhibit the formation of liquid condensates and narrow the coexistence region. The internal organization and dynamics of the condensates are investigated as a function of RNA/peptide concentrations, RNA chain length and salt concentration.
  

Liquid-liquid phase separation driven compartmentalization of reactive nucleoplasm

Organization and regulation of intracellular biochemistry are achieved by compartmentalizing biomolecules through the liquid-liquid phase separation process (LLPS) and its associated chemical reactions into membrane-bound and membrane-less organelles. The LLPS of proteins/RNA within the nucleus plays a crucial role in the gene regulation of eukaryotic cells. Here, we present a reaction-diffusion model of transcription which connects the phase separation of proteins-RNA mixture inside nucleoplasm with transcriptional and catalytic reactions. Our model shows the existence of a variety of complex protein-RNA patterns that arise from the competition between different mechanisms including LLPS, biomolecular interactions, and biochemical reactions. The protein-RNA pattern formation is strongly dependent on the interplay of time-scales for diffusion, transcription, and translation. Under appropriate kinetic regimes, we can dramatically accelerate or slow down the phase-separation process, thereby leading to arrested phases.   

Liquid-liquid phase separation driven compartmentalization of reactive nucleoplasm

Organization and regulation of intracellular biochemistry are achieved by compartmentalizing biomolecules through the liquid-liquid phase separation process (LLPS) and its associated chemical reactions into membrane-bound and membrane-less organelles. The LLPS of proteins/RNA within the nucleus plays a crucial role in the gene regulation of eukaryotic cells. Here, we present a reaction-diffusion model of transcription which connects the phase separation of proteins-RNA mixture inside nucleoplasm with transcriptional and catalytic reactions. Our model shows the existence of a variety of complex protein-RNA patterns that arise from the competition between different mechanisms including LLPS, biomolecular interactions, and biochemical reactions. The protein-RNA pattern formation is strongly dependent on the interplay of time-scales for diffusion, transcription, and translation. Under appropriate kinetic regimes, we can dramatically accelerate or slow down the phase-separation process, thereby leading to arrested phases.
  

Complex Coacervate Core Micelles for Protein Delivery

Proteins are an increasingly important class of therapeutics. There are, however, several challenges in the development and production of protein based therapeutics. These include relative temperature sensitivity and propensity to aggregate at therapeutically relevant concentrations, both of which contribute to the their relatively short shelf-life and short circulation times. Previous work has developed protein delivery platforms that increase circulation time while maintaining protein activity, but suffer several shortcomings such as low-levels of protein loading and controlled release. To address these limitations, we have investigated the use of block-polyelectrolytes to form complex coacervate core micelles (C3Ms) to be used as a protein delivery vehicle. While C3Ms allow for controlled protein loading and release, they also have the potential to improve protein stability at therapeutically relevant concentrations and increased temperatures. In this work, we identified protein and polymer design parameters that govern complex coacervation of model proteins to form C3Ms that remain phase separated under physiological conditions and study how incorporation of proteins in C3Ms influences protein stability and delivery to cells.   

The role of epigenetics and microenvironment in breast cancer evolution

Cancer is a disease driven by aberrant signaling pathways that control and maintain the malignant phenotype. Among the different aspects that drive these aberrant behavior are epigenetic changes that favor the uncontrolled growth and longer survival of malignant cells. These molecular changes affect microenvironment and cellular function subjecting cells to stress due to the lack of sufficient nutrients (glucose) and oxygen. On the other hand, estrogens have been found to play a major role in promoting the proliferation of both normal and neoplastic cells and they are believed to stimulate the occurrence of neoplastic mutations. Here, we incorporate these biological processes into a quantitative model to understand how microenvironmental conditions affect genetic dynamics and phenotypic diversity. Our results suggest that the evolution of malignancy and heterogeneity in a tumor arise and can be controlled by local epigenetic changes. This yields support to the believe that controlling local microenvironmental aspects might prevent the further development of a tumor.   

Geometry and mechanics of a model epithelium with irregular cells and a clonal inclusion

An important role in the modeling of epithelial tissue mechanics has been played by vertex models, with cells idealized as polygons, and tricellular junctions as vertices joined by straight interfaces. Numerical simulations of these models in the presence of cell divisions display geometrically irregular cells, similar to those of epithelia, even when cell properties are homogeneous. Yet existing theoretical analyses are mostly confined to the mechanics of regular hexagonal lattices. We develop an analytical description of geometrically disordered vertex models. We first quantify, in numerical simulations, geometrical properties such as the distributions of cell areas and perimeters, and mechanical properties such as the tissue bulk and shear moduli, with different sources of disorder, e.g different division rules or simply relaxation in the presence of noise. We then develop a simple mean-field description that accounts for these properties. Finally, we apply our analytical description to a simple case of interest in different biological contexts: a clonal group of cells with material properties that differ from the surrounding tissue and that may also grow at a different rate. We compare our results with data obtained on epidermis differentiation in flies.   

Chemotaxis of Tumor Spheroids in 3D Extracellular Matrices

Cytokine-mediated tumor migration (chemotaxis) plays a crucial role in cancer metastasis. Previous studies have explored the chemotactic behavior of single cells embedded within 3D extracellular matrices (ECM). However, due to the vastly different nature of single cells and tumors, a single cell 3D model cannot recapitulate the in vivo tumor invasion behavior. In this talk, we will present a 3D tumor spheroid model for tumor chemotaxis studies. Breast tumor spheroids using MDA-MB-231 cells were engineered and embedded within a collagen matrix. Tumor spheroid dynamics were imaged and analyzed when subjected to well-defined cytokine (Epidermal growth factor, hEGF) gradients provided by a microfluidic platform. Differential behavior of single tumor cell and spheroid chemotaxis will be discussed.   

On the mechanical principles of biofilm formation

Bacteria, once thought to be solitary and asocial, can grow in large, dynamic, multi-cellular communities known as biofilms, which consist of bacteria embedded in an extracellular matrix. However, the manner in which these three-dimensional, surface-adhered communities are built in space and time still remains to be shown. Using the human pathogen Vibrio Cholerae as model biofilm former, we examine the growth of biofilms from single founder cells to mature three-dimensional colonies. We find that biofilms grow from disordered, two-dimensional layers into three-dimensional structures with long-range order consisting of vertically aligned cores and horizontally aligned periphery. Using agent-based modelling, we find that verticalization occurs due to mechanical instabilities at the cell-scale and we quantitatively test these predictions by varying the cell lengths, surface roughness and osmotic conditions.   

Probing tissue mechanical properties in Drosophila embryos using magnetic tweezers

The amnioserosa is an single-cell-deep epithelial cell sheet that contracts during dorsal closure in Drosophila melanogaster embryos. Displacement can be easily observed by microscopy and strains can be inferred from UV laser ablation experiments. The material properties of the amnioserosa and surrounding cell layers, however, remain elusive, which challenges a complete understanding of the driving forces in developing tissues. We have constructed an experiment to quantify mechanical properties of the amnioserosa cells during dorsal closure using magnetic tweezers. Complex shear moduli of the tissues are directly measured by exerting calibrated magnetic forces and observing displacements of bound magnetic beads.   

Interpretation of Phase Boundary Fluctuation Spectra in Biological Membranes with Nanoscale Organization

In this work, we use a computationally less expensive and more automated Support Support Vector Machine algorithm to detect simple and complex interfaces in atomistic and coarse-grained molecular simulation trajectories of phase separating lipid bilayer systems. We find that the power spectral density of the interfacial height fluctuations and in turn line tension of the systems depends on the order parameter used to identify the intrinsic interface. To highlight the effect of length scale used to identify the interface on the fluctuation spectra, we perform a convolution of the boundaries identified at molecular resolution with a 2D Gaussian function of variance equal to the experimental resolution limit. The region of fluctuation spectra that scales according to capillary wave theory formalism relies on the complexity of the interfacial geometry, which may not always be detected at experimental resolutions. We propose that the different q-regimes in the fluctuation spectra can be used to characterize mode dependent interfacial tensions to understand the interfaces beyond the linear line tension calculations.   

External Forces generated by the Attachment between embryonic Tissue and Egg Shell affect Gastrulation in Insects

Gastrulation is a critical step during the development of multicellular organisms in which a single-layered tissue converts into the multi-layered germband. This shape change is characterized by tissue folding and large-scale tissue flow. The myosin-dependent forces that underlie this process have been increasingly investigated; however, thus far, a possible interaction between the moving tissue and the rigid shell surrounding the embryo has been neglected. Here, we present our quantitative findings on the physical mechanisms governing gastrulation in the red flour beetle, Tribolium castaneum. We investigated the forces expected within the tissue given the myosin distribution observed by multi-view light-sheet microscopy and discovered that an additional external force must be counteracting this tissue-intrinsic contractility. We then identified that a specific part of the tissue tightly adheres to the outer rigid shell. This attachment is mediated by a specific integrin whose knock-down leads to a complete loss of the counter-force. Moreover, in the fruit fly Drosophila melanogaster, knock-down of another integrin leads to a severe twist of the germband, suggesting that the integrin-mediated interaction between tissue and vitelline envelope may be conserved in insects.   

Size and shape control dynamics in an early diverging animal

Animals have traditionally been characterized as clonal cell populations with complex tissue architectures which have specific developmental programs that guide the morphogenesis of organs and in turn whole animals. On the other hand, very early diverging metazoans display simple tissue architecture with a lesser division of labour that allows high plasticity and scalability. High plasticity in such tissues allows dynamic shape changes at time scales of ~five minutes and a scalable tissue architecture allows size variation of over two orders of magnitude. For such tissues, size and shape are deeply interdependent due to physical constraints. Using high throughput long term scanning microscopy, we collect datasets on shape and size variation in lab culture conditions and quantify their temporal trajectories. Using perturbations to these 'natural' trajectories, we decouple size and shape parameters to decipher the principles that the system uses to exercise control over growth. Utilizing large datasets and growth-based models, our study provides abstract principles that would have governed size and shape control in very primitive `animal-like' multicellular clusters.   

Single-Chain Variable Fragment Bearing Silica Nanoparticle Evaluation in HER2+ Over Expressing Tumor Tissues Using Optical Super-Resolution Microscopy

Active targeting of therapeutics or imaging probes to cancer cells is the primary pharmacological strategy in cancer therapy and diagnostics. Recent years have witnessed a boom in nanotechnologies, such as antibody-functionalized nanoparticles, in cancer theranostics which provide unique advantages over standard treatments. However, nanomedicine has is currently suffering from a lack of characterization and quantitative methods that provide understanding of the complex interactions in tumor tissues. The challenge in making quantitative methods for large samples usually lies in cost and availability of equipment. However, optical microscopy is a well-established and relatively low-cost alternative which is equipped, due to various super-resolution techniques, with the capability to study objects smaller than 100 nm. Here we demonstrate the use of coordinate base colocalization and nearest neighbor distance analysis with optical super-resolution microscopy to determine the quality of nanomedicine targeting using HER2 overexpressing tumor tissues.   

Exploring the Dynamics of Biological Macromolecules at Angstrom Scale

This research project plans to study Guanylate Kinase (GK) using nano-rheological techniques. These biological macromolecules will be immersed in solutions with different solvents in order to observe how the conformational dynamics of the molecules change.
An oscillatory force will be applied by attaching enzymes to a gold-coated surface and to gold nanoparticles. An additional gold-coated surface will be placed on top of the setup in order to create a parallel plate capacitor configuration. An oscillatory voltage will then be applied across the capacitor to drive the gold nanoparticles and exert a force on the enzymes. The setup will detect the ensemble averaged movement of the enzymes at the Angstrom scale in order to measure the conformational dynamics.
The specific goals of this research project include the following: creating a one-of-a-kind experimental setup which will be useful for a plethora of other experiments in the future, deforming enzymes and recording their levels of deformation, changing what enzymes and solvents are used, and comparing data from the experiments. The results of the enzymes’ viscoelastic behavior which will be useful for further research in the fields of Biophysics, Biology, and Chemistry.   

Effect of Carbon Nanotubes on The Propagation of Neural Signals

Studies have shown that carbon nanotubes scaffolds promote the growth of neurons along the nanotube axis and also enhance neural signaling. In addition carbon nanotubes have been successfully incorporated into cells as transmembrane channels and as bridges connecting with the intracellular and intercellular microtubule scaffolds which function as cellular transportation highways. Based on laboratory experiments in the literature, it is clear that carbon nanotubes will play an ever increasing role in neurological research acting to enhance properties or to serve in a sensing capacity.
To investigate the effects of nanotubes on neuron properties, we report finite element based simulations of voltage pulse propagation along neural membranes in the presence of incorporated carbon nanotubes. Results will be used to help determine the origin of neural enhancement in neuron-nanotube complexes and to look for mechanisms by which a nanotube proximal to a neuron may act as a sensor allowing imaging of neural signals.   

Dynamical Model of Cytokines in Rheumatoid Arthritis in the Presence of Additive Noise

A dynamical model from Baker et. al* is used to explore the possibility of predicting onset of Rheumatoid Arthritis. The model is based on nonlinear interaction of pro- inflammatory and anti-inflammatory cytokines. In the model, a parameter measures the effect of the pro-inflammatory cytokine concentration on further growth of pro- inflammatory cytokines. As this parameter increases, a healthy state jumps to a disease state through a fold bifurcation. By adding noise to the model, we observe precursor flickering and large-scale fluctuations that might appear clinically as early clues to incipient onset.
*M. Baker, S. Denman-Johnson, and M. R. Owen, “Mathematical modelling of cytokine-mediated inflammation in rheumatoid arthritis” Mathematical Medicine and Biology (2013) 30, 311–337)
  

The Quantum Theory of Entanglement and Alzheimer's

The Quantum mechanics is inscrutable. Nevertheless, it baselines other inscrutable realities. Science is about exploration of Nature by hook or by crook. The system of a centipede used as an example in [1], is analogous to us all, we cannot deny to exist at the tips of our 20 nails. The hands of Nature connecting non local realities may be sub-quantum, and therefore be beyond our manipulative skills. The argument of Dr. Yanhua Shih in [1] is coauthored with Dr. Yoon-Ho Kim. Their findings agreed with the key feature of Popper’s neglected predictions, revealing a concern about the non-local instantaneous interferences. The multiple existences of the particles, we say, reflect such multiple interferences, addressing dark matter in [2] consistent with Feynman's 1957 conference presentation stated in [1]. [1] Goradia SG (2019) The Quantum Theory of Entanglement and Alzheimer's. J Alzheimers Neurodegener Dis 5: 023. [2] Goradia SG (2015) Dark Matter from Our Probabilistic Gravity. J of Physical Science and Application 5: 373-376.   

A synthetic transcriptional counter

Elementary sequential logic circuits have been proposed in synthetic biology. The long-term goal is to obtain more complex circuits, capable of finite-automaton computation, in analogy to central processing units in digital computers. We design here a single bit counter, i.e. a parity checker, that can be implemented with current technology by a genetic circuit. When excited by a sequence of N sufficiently separated pulses of an external input (which might be a chemical inducer or a physical signal such as light at a specific frequency), the network will produce an output to indicate whether the number N of pulses seen so far is even or odd. Our circuit is the key component of a distributed-computation m-bit counter, in which a count modulo 2^m of the observed number N of pulses is stored and displayed. The m-bit counter can be in principle assembled from a set of m single-bit counters, together with additional gates that implement "carry" operations. In this design, communication among cells could be biologically implemented by diffusing chemicals (for example, quorum-sensing molecules).   

Dose-rate effects in the radiation induced mutation of Drosophila

We proposed a mathematical model to study the biological effects of radiation which we call WAM model. (1) An important feature of the model is that it can describe the dose-rate dependence of the effects of radiation. We succeeded in reproducing the existing data of dose/dose-rate dependence of mutation frequencies of 5 species including animals and plants. (2) We hope to develop the model to desceibe the whole process from mutation to cancer generation.
In 1926, Herman Muller conducted the famous expetiments of the radiation-induced mutation in Drosphila, which shows a linear increase with the the total dose. Later, Purdom and McSheehy irradiated Drosophila with much lower dose rate to find out if there is a dose-rate dependence in the mutation frequency.(3) Here, we reexamine their date with WAM model and found that the data is reproduced well with our model. We will discuss the possible dose-rate dependence of the radiation-induced mutation frequency in Drosophila.

References
(1) Y. Manabe, T. Wada, et al., J. Phys. Soc. Jpn. 84 (2015) 044002.
(2) T.Wada, et al., J. Nucl. Sci. Tech. 53 (2016) 1824-1830.
(3) C.E.Purdom and T.W.McSheehy, Int. J. Rad. Biol. 7 (1963) 265-275.   

Understanding Base Pairing Interactions in Aqueous Environment

Base pairing plays a pivotal role in DNA functions. But while the complementarity between Watson-Crick matched bases is conventionally believed to arise from the different number of hydrogen bonds in G|C pairs versus A|T, the energetics of these hydrogen bonds are heavily renormalized by the aqueous solvent. We computationally extracted the solvent components of the free energy, entropy and enthalpy for canonical and some wobble and stacked base pairs. For all of them, the solvent's contribution to the base pairing free energy appears to be exclusively destabilizing. While the direct hydrogen bonding interactions in the G|C pair is much stronger than A|T, the thermodynamic resistance produced by the solvent also pushes back more forcefully against G|C pair formation compared to A|T, generating a difference in thermodynamic stability of only ~1 kcal/mol between them. We have profiled the density of water molecules in the solvent adjacent to the bases and observed a decrease in the solvent's entropy as a result of a "freezing" transition where waters are recruited into the gap between the bases to compensate for the unsatisfied hydrogen bonds between them.   

Stochastic Lattice Model of the RAD51 Nucleoprotein Filament Formation on Single-Stranded DNA

Homologous recombination (HR) is one of the most enigmatic processes in DNA metabolism and is a fundamental driver of evolution. Its central step involves the search for homology between two DNA molecules and the subsequent exchange of the DNA strands.
We use a Monte Carlo model to study the statistics and dynamics of the recombinase filament nucleation, extension, and disassembly.
We use our developed computational toolbox for analyzing single-molecule data to build a predictive model to test different binding hypotheses   

In silico studies of “DNA Flossing” through a Double-Nanopore system

We carry out in silico study of “DNA Flossing” (X. Liu et al., bioRxiv: http:/dx.doi.org/10.1101/77817) in a model Double Nanopore Device, where the DNA conformations oscillates back and forth from one nanocavity to the other while translocating through a double nanopore when the linear segment between the nanopores gets scanned. We simulate the conditions for efficient multiple scans as well as the reduction of the variance in measurement of genomic length from these scans. In particular, we vary the dimensions of the cavities, the distance between the nanopores, and nature of the feedback forces to optimize the measurement of genomic lengths through flossing mechanism.   

Loss of CTCF loops on human interphase chromosome organization

Chromatin looping, mediated by CTCFs, is a widespread principle in chromatin folding. Despite many studies that eluciate the importance of the insulator protein CTCF and cohesin complex in regulating genome folding, how chromatin loops determine the overall genome organization is unclear. We created a Chromosome Copolymer Model (CCM) to investigate the impact of CTCF loop loss on human interphase chromatin organization. We show that chromatin folding associated with loop domains is extended with a decrease in the number of CTCF loops. Surprisingly, the degree of epigenetic compartmentalization increases. In contrast, local topological associating domain (TAD) is only affected by removal of adjacent CTCF loop. Furthermore, deletion of some CTCF loops causes abnormal contacts between certain insulated loci with adjacent TADs merging, while others do not, indicating that CTCF loop is required to form a subset of TADs. The TADs whose formations are mainly driven by underlying epigenetic states have minimal dependence on CTCF loops. Moreover, our results employing complete CTCF depletion are in good agreement with experiments, supporting that cohesin is necessary for the formation of CTCF loops.   

Role of Liquid-Liquid Phase Separation in Organization and Regulation of Chromatin

Genomic DNA is densely compacted in the nucleus of eukaryotic cells into chromatin. However, mechanisms that regulate the assembly and compaction of the genome remain unclear. Here we employ atomistic simulations to study how histone tail-driven interactions drive liquid-liquid phase separation (LLPS) of archetypal DNA-histone mixtures, resulting in formation of mesoscale condensates. The role of histones H1, H2a, H2b, H3 and H4 are individually explored under a range of physiological salt concentrations. Histone H1 is found to play a critical role in chromatin organization by promoting LLPS, denser compaction and slower dynamics within the condensates. Electrostatic interactions are found to be the primary driving force behind condensate formation and consequently histone acetylation is found to inhibit LLPS. We construct phase diagrams based on Droplet density and Shannon entropy calculations, wherein distinct regimes can be identified based on histone concentrations. The spatial organization and dynamics of the droplets are investigated as a function of histone concentrations, DNA chain length and salt concentration.   

Nuclear chromatin patterns: modeling dynamics of intra-chromatin interactions and its impact on structure organization

The description of chromatin organization and its dynamics, at a large scale, are functionally important factors in the genome regulation function. Growing evidence suggests that chromatin within the nucleus has a liquid-like behavior mediated by phase separation into micro-droplets with distinct transcriptional states. The formation and spatial arrangement of chromatin droplets within the nucleus depending on their transcriptional states either active (euchromatin) or inactive (constitutive and facultative heterochromatin) genes are important features of the nuclear architecture. Understating mechanisms that control the dynamics and spatiotemporal regulation of droplets formation is a possible way to elucidate the relationship between nuclear architecture and gene regulation. Here, we introduce a mesoscale liquid model of nucleus (MELON) that incorporates dynamic of interactions between A-B-C chromatin compartments of the nucleus, as well as the affinity between constitutive heterochromatin and Lamina at the nuclear envelope and nucleus deformation. Using MELON framework, we show that phase separation together with surface tension effects and nuclear shape deformation is sufficient for recapitulating large-scale morphology and dynamics of chromatin along the life cycle of cells.
  

United we stand, divided we fall: Response to antibiotics in a bacterial swarm

Bacterial swarms consist of millions of bacterial cells collectively colonizing a nutrient rich agar plate. Pseudomonas aeruginosa is a bacterial species in which swarming is modulated by the interplay of surfactant driven fluid flow and chemotactic motility of individual cells, leading to the formation of a sparse branched pattern over the agar plate. The branches of the swarm do not intersect due to repulsive forces from the surfactant gradients. However, when an antibiotic drop is added near one of the growing branches, its growth gets arrested and the neighbouring branches intersect to join this branch. This is quite non-intuitive as the bacterial cells move towards the branch exposed to antibiotic as opposed to escaping the area to protect themselves. Moreover, this behaviour requires a complex signalling mechanism as the bacterial cells cannot directly sense the antibiotic as they lack the receptors specific to the antibiotic (gentamicin). We present experimental data and a mathematical model to unravel the mechanism behind this behaviour. Our findings reveal that stress on the bacterial community drives them into collaborating with each other to demonstrate collective resistance and avoidance of antibiotic toxicity.   

Delay embedding of low-dimensional attractors of local field potentials from optogenetic data

We investigated the effect of acute cocaine injection in conjunction with dopamine (DA) receptor antagonists on the medial prefrontal cortex (mPFC). In his study we used D1-like receptors antagonist SCH 23390 or D2-like receptors antagonists sulpiride. The goal of the study was to determine the changes in the gamma oscillations and their relationship to short term neuroadaptation that may mediate addiction. In this study we used 17 mice and recorded optogenetically evoked local field potentials (LFPs) in vivo. The optical stimulus was a brief 10 ms laser light pulse. Data preprocessing involved computing a Euclidian distance-based dendrogram to separate the 100 trials for each animal in disjoint clusters. The classifier is not perfect as it gives a minimum 20% overlap between the control data and DA receptor antagonists trials. We used the delay-embedding method to reconstruct neural activity attractor. The lag time was determined both using the correlation time and the mutual information methods.   

System identification in the brain: inferring ARMA dynamics from sensory data

Sensory information reaches the brain as a stream with non-trivial correlations across time. In a generative model, these correlations can be seen as the result of a dynamical system acting on a white noise source signal. Learning the parameters describing this system enables a variety of applications, from detecting changes in the input dynamics to inferring dynamical rules in the environment.

Here we present biologically plausible neural networks for performing system identification from time series data. The starting point is the mutual information between the past and the future, which in the case of one-dimensional Gaussian signals is equal to a kind of cepstral norm. By searching for the autoregressive moving-average (ARMA) filter that minimizes this mutual information, we develop algorithms that learn an inverse model to the dynamical system generating the data. Employing update rules based on Givens rotations we ensure that our algorithms work online, an essential ingredient to maintain biological plausibility. We also look for implementations that rely on local learning rules, such that synaptic updates only require information that is available to them from pre- and post- synaptic activity.   

Dynamics of multi-domain bio-macromolecules by neutron scattering

Conformational ensembles of synthetic polymers and intrinsic disorder in proteins are both aspects of the varying degree of order and disorder that are crucial for the properties of macromolecules. Neutron scattering techniques, in particular small angle scattering and neutron spin echo, have an important contributions to understand conformation and dynamics of macromolecules with regards to polymer physics. The possibility for altering and defining accessible conformational spaces through localized or intermediate and long-range interactions of segments along a polypeptide chain are limited by chain stiffness and local hydrodynamic friction. Our research aims for a deeper understanding of this challenging topic. We will present the dynamics of several bimolecular species based on results of neutron scattering and the comparison with secondary and segmental relaxations, Rouse and reptation dynamics in polymers, with emphasis on the significant difference between dynamics of random coil of synthetic polymers and the dynamics of globular proteins.   

Decoupling Effect between Dynamics of Proteins and their Hydration Water at Elevated Temperature

Water, as a life solvent, is primary to the energy transfer, biomacromolecules assembly, biological reaction and so on. Particularly, the dynamical transition at ~200 K in proteins, which hydration water is essential for, leads the state of proteins from rigid, nonfunctional state to flexible, functional state. So far, a large number of studies have suggested that the dynamical transition at ~200 K is coupled to the activation of translational motion of the hydration water on the protein’s surface, i.e., the dynamics of proteins and that of their hydration water are coupled. However,by performing elastic neutron scattering experiments of atomic dynamics of proteins with different secondary structures under various instrumental resolutions, we surprisingly find that the dynamics of proteins and that of their hydration water molecules show a resolution-independent and a resolution-dependent behavior at elevated temperature, respectively.These results suggest thatthere actually exists the decoupling effect between dynamics of proteins and that of their hydration water at elevated temperature, and gives a new comprehension in protein dynamical transition.   

A new heteropolymer theory to describe conformation of Intrinsically Disordered Proteins

Intrinsically Disordered Proteins (IDPs) perform many vital functions and yet have no native structure. This is in contrary to the commonly held notion that protein sequence determines the unique folded structure that in turn dictates the function of the protein. IDPs exhibit an ensemble of conformation, instead of a unique folded structure, making them amenable to tools of polymer statistical physics. Presently, homopolymer theories are commonly used in order to describe the conformation of IDPs. However, IDPs consist of amino acids of different chemical properties, including positive and negative charges. We present a novel heteropolymer theory to describe the sequence specific conformational ensemble of IDPs. Our theory provides rich ensemble average distance maps that are more informative about conformation than coarse-grain measures like radius of gyration or scaling exponents. We will showcase application of this novel analytical theory – benchmarked against all-atom simulations – to multiple naturally occurring IDPs. We highlight how biology can achieve variability with the ability to induce drastic conformational changes by making concise changes in the sequence – termed as hot spots -- such as post-translational modifications (PTMs), often used as a regulator.   

Into the Vast Unknown: Structure-Function Relationships in Uncharacterized Bacteriophage Proteins

With an estimated global population of 10³¹ particles, bacteriophages (viruses that infect bacterial hosts) are Earth’s most abundant biological entities. Billions of years of evolution have granted phages unparalleled genetic diversity and given rise to a vast collection of genes. We have sequenced and analyzed the genomes of over 3,000 phages and sorted their hundreds of thousands of encoded proteins into phamilies. The functions of over 70% of these protein phamilies are unknown, with a complete dearth of functionally informative sequence-based homology for guidance. We hypothesize that these hypothetical phage proteins represent an unexplored reservoir of genetic, functional, and possibly structural novelty. To address this, we have developed a research pipeline to structurally and functionally characterize subsets of phage-encoded proteins. We have successfully advanced a number of proteins through various stages of this pipeline, which combines bioinformatics, molecular biology, biochemistry and biophysical approaches. This presentation will address our progress on elucidating structure-function relationships in these bacteriophage proteins.   

The Impact of Deep Eutectic Solvent on the Structural Stability of Myoglobin

The stability of Myoglobin (Mb) in hydrated Deep Eutectic Solvent (DES) is investigated as a function of temperature and DES concentration. Determining the effect of DES on protein structure can lead to understanding the properties of DES, and allow their use to expand to cost-effective, and simple synthesis applications. Mb in 2 DESs: 35% wt glyceline-water and 5% wt reline-water and the resulting structure were compared to Mb in water. Small Angle X-ray Scattering results show that Mb’s tertiary structure remains relatively unchanged in both DESs until the temperature reaches 80^°C (for glyceline) and 70^oC (for reline) at which globular structure of Mb starts to gradually unfold. However, in water, Mb’s tertiary structure unfolds slowly and linearly with temperature until reaching a major transition at 80^°C. In addition, circular dichroism spectroscopy measurements show that Mb loses substantially more secondary structures with increased temperature in DES mixtures than in water solvent, yet DES allows better recovery of Mb secondary structure.   

Adaptive Sampling of Markov-state models with MD Simulations to assess the rates of biologically relevant processes

It is a challenge for classical molecular dynamics to directly simulate the time and length scales of biologically relevant processes. Incomplete sampling makes the calculation of macroscopic observables such as rates for major state transitions difficult to estimate correctly. A number of techniques bias the energy landscape to accelerate processes that are difficult to reach with direct simulation. However the altered landscape introduces artifacts that obscure rate estimates. Here we show how adaptive-sampling workflows that iteratively redirect swarms of trajectories can enhance the sampling of the slowest kinetic processes. Markov-state models are then adaptively refined with continual updates from these parallel swarms resulting in unbiased, well-sampled estimates of kinetic rates. We demonstrate how this method can be applied to provide long-timescale kinetics for atomistic simulations of protein-protein interactions.   

Data–Driven Modeling of Discrete Synchronization in Firefly Swarms

Fireflies are a landmark example of synchronization in nature. During mating season, males of synchronous firefly species flash in unison within swarms containing tens of thousands of individuals. This magnificent display inspired numerous mathematical models that aim to explain how global synchronous patterns emerge from local interaction rules. However, experimental data to validate these models has been rarely reported. To address this gap, we obtained quantitative measurements of the spatiotemporal flashing pattern of synchronous firefly swarms. Using the North American synchronous firefly species Photinus carolinus as a model system, we recorded the flashing patterns of the fireflies in their natural habitat as well as within light controlled environments, and for the first time, we were able to perform a 3D reconstruction of the synchronous flashing positions of thousands of individuals. This talk will focus on analysis of the spatiotemporal patterns observed in our experiments and connecting it to mathematical models that account for the species specific discontinuous flash pattern, short range spatial correlations, and spatial mixing due to movement of individuals within the swarm.   

A Quest for a Science of Social Dynamics

Humans are social beings. Social interactions inform the state of an individual in society. We proposed a new social field theory in order to define the state in terms of Hamiltonian of an individual. In a multi-dimensional social field, social interactions are conceived in terms of the energetic exchanges of various fluxes including information. Such exchanges govern the change of states of an individual. The transport equations of change for an individual and ensemble of individuals include source terms and power dynamics. For an ensemble of individuals, the equation takes a form of an implicit Fokker-Planck equation. A target of this paper is to formulate the “equations of motion for social systems” Wolfgang Weidlich has long sought for. Lotka-Volterra equations can be derived from the proposed equations of motion for the social field.   

Physics of the Brain. Treatment of Neurological Diseases via the Brain Waves, Using the Multi-photon Pulsed-operated Fiber Lasers in the Ultraviolet Range of Frequencies.

The novel study of the brain waves (BW)[1] in connection to neurological diseases is proposed. It is based on the pulsed-operated (amplitude modulation) multi-photon (frequency modulation) fiber-laser interaction with the brain neuro-topion (the neurological disease area).[2] The repetition frequency, Ω, (5-100 pulses per second) matches the frequency of a particular brain wave, Ω_BW. The tunable fiber laser frequencies, Δω (multi photon operation), are in the ultraviolet frequency range, thus enabling monitoring of the electrical charge dynamics in the neuro-topion of a particular disease within the 10s of milliseconds.


[1] Tae Kim et. al. Cortically projecting basal forebrain parvalbumin neurons regulate cortical gamma band oscillations, Proceedings of the National Academy of Sciences, vol. 112 no. 11, 3535–3540, (2015).

[2]V.Stefan, B.I.Cohen, C.Joshi, Science, 243, 4890, (1989);Stefan et al.,Bull. APS 32, No.9,1713, (1987);Stefan,APS-March-2015,#P1.00099; Stefan,APS-March-2016, #M1.00310; APS-March-2017,# M1.00291; V. Alexander Stefan, Neurophysics, Stem Cell Physics, and Genomic Physics: Beat-Wave-Driven-Free Electron Laser Beam Interactions with the Living Matter, (S-U-Press, La Jolla, CA, 2012).

  

Dynamics of timescales on complex lattices

A popular idea in biology is that the intrinsic timescale of an individual “unit” plays a crucial role in the information processed by the system as whole. For example, it has been proposed that the intrinsic timescales of single neurons in different brain areas are related to functional differences between these areas. However, disentangling between intrinsic and collective timescales remains a highly nontrivial task, and could benefit by drawing intuition from simple physical toy models. To this end, we consider the prototypical model of collective temporal behavior: kinetic Ising models, where identical units are connected with a given topology, and neighboring units stochastically interact with one another. We analyze how the behavior of such models is altered when considering many aspects relevant to their computational implementation, namely, finite temporal resolution, topological connectivity, and finite system size. Not coincidentally, these considerations have biological analogues. For example, the clock speed of processor is functionally similar to an “inverse refractory period” of a neuron. While locality of interactions can be exploited for parallel simulation of physical systems, the diversity of topologies in biological systems is key to their expressive power.
  

Development of control in brain networks over temporal and spatial scales using graph models

Regions of the human brain vary in their capacity to control whole brain activity, in large part due to their location in the underlying structural network of interconnections crisscrossing the cortex. Recent work suggests that this capacity for control differs across spatial and temporal scales of the brain’s dynamics and can be formally probed using the Laplacian eigenspectrum of the brain’s structural network. Yet how such spatiotemporal control might differ from one human to another, potentially supporting and explaining differences in cognitive function, remains unclear. Here, we address this question by measuring several summary statistics of spatiotemporal control from human brain network architecture, as reflected in diffusion tensor imaging data acquired from 882 youth between the ages of 8 years and 22 years. We found that distinct features of network topology are correlated with a region’s capacity to enact distinct control strategies, and we investigate these relationships as a function of discrete timescales, from markedly slow modes of dynamics to relatively swift modes of dynamics. Our results provide insight into how local variation in connectivity gives rise to distinct processes of global control as a function of timescales over modes of activity.   

Kinesin Model for Brownian Dynamics Simulations of Stepping Efficiency

The motor protein kinesin plays an integral role in cell function, transporting, for example, cargo from the center to the periphery of a cell. Kinesin molecules have been shown experimentally to walk along microtubules in a hand-over-hand stepping motion, carrying their cargo eight nanometers per step. However, details of the stepping process are still under investigation. Kinesins are difficult to study with atomistic simulations due to the size of the proteins and the long time-scales involved. In this work we develop a 3D model of kinesin stepping on a rigid microtubule substrate that can be simulated efficiently with Brownian dynamics simulations. The interactions governing the motor protein conformations and the interactions between kinesin sites and the microtubule sites are designed to reproduce important aspects of the biological system. We perform simulations spanning many kinesin steps to investigate the stepping efficiency of the motor protein for different neck linkers. We find that neck linkers close to the wild-type length yield a stepping efficiency of about 90%, in agreement with experimental data. In addition, we find that increasing the neck-linker length leads to a decrease in efficiency, as has also been observed in experiments.   

Correlating synaptic vesicle motion to cytoskeletal structure of the presynapse and axon.

Two neurons communicate via release of neurotransmitter from a presynapse of one neuron to a post-synapse of another. This neurotransmitter is carried from the inside of the presynapse to the membrane of the presynapse using a presynaptic vesicle. The transport mechanics of presynaptic vesicles thus influences presynaptic communication. Presynaptic vesicles undergo many types of motion, but it is still unclear how their motion corresponds to the overall structure of the synapse. High resolution fluorescence microscopy has recently been shown to be an effective tool to understand vesicle motion. We take advantage of recent three-dimensional microscopy experiments and develop a three-dimensional computational analysis method of single vesicle motion in live cells. We use the computational method to classify types of motion, then distinguish different types of motion relative to the known structure of the synapse observed. We show that this approach can distinguish types of vesicles motion relative to the synapse. This method also allows us to take a closer look at the vesicles when the motion is directed.   

A micromechanical model for interactions of curvature sensing septin filaments with membrane

Cells reconfigure their shape in micron-scale in response to internal and external forces. Septins are filament-forming GTP-binding proteins that can sense curvature in micron-scale by preferentially accumulating in membrane regions with high curvature. Yet, the underlying mechanisms by which a nanoscopic septin hetero-octamer senses curvature in micron-scale remains unclear. We combine biophysical modeling and experiments and propose a micromechanical model for interactions of septin filaments with membranes. We propose that the rate of attachment of septins filaments to membranes is controlled by a combination of membrane’s thermal and stored elastic energy, while the rate of detachment is controlled by the bending energy of the curved membrane-bound septin filaments. Moreover, our model predicts a qualitative change in septins curvature sensing as they assemble into micron-scale filaments in larger concentrations. We verify these predictions experimentally, by measuring the attachment/detachment kinetics under different bulk concentrations of septins on beads of different radii, covered with lipid bilayer.   

One-dimensional contact process with both temporal and spatial disorder

We investigate the non-equilibrium phase transition in the contact process under the influence of both temporal and spatial disorder using large-scale Monte-Carlo simulations. Spatial disorder alone is known to lead to an exotic infinite-randomness critical point [1]. More recently, it was shown that temporal disorders also produces unconventional critical behavior dubbed infinite-noise critical behavior [2]. We perform a series of simulations adding weak temporal disorder to a spatially disordered space and vice versa. We find that the critical behavior remains unchanged by these perturbation, in agreement with the generalized Harris criteria [3]. We then focus on the case where both temporal and spatial disorder are strong.

[1] J. Hooyberghs et. al, Phys. Rev. Lett. 90, 100601 (2003). T. Vojta, M. Dickison, Phys. Rev. E 72, 036126 (2005).
[2] T. Vojta, J. Hoyos, Europhys. Lett. 112, 30002 (2015).
[3] T. Vojta, R. Dickman, Phys. Rev. E 93, 032143 (2016).   

Modeling Swimming Microorganisms using Macroscopic Experiments

The swimming of microorganisms is typically analyzed using biological experiments or numerical simulations because the Reynolds number is much less than one. Our research group uses model macroscopic experiments with typical length scales of ~ 10 cm, but match the low Reynolds number of microoganisms by using a highly viscous silicone oil. The fluid has a viscosity that is 10⁵ larger than water but with approximately the same density. We can therefore build laboratory scale robotic swimmers and model microorganisms that are typically ~10 μm. We can also explore fundamental theories such as our project to build a laboratory scale three-link swimmer to test theoretical predictions (Purcell 1977, Hatton et al. 2013).   

Single molecule force spectroscopy to determine specific receptor densities on live cancer cells under varying conditions

We have used atomic force microscopy to study the single-molecule interaction between the ligand and receptors in live cancer cells under different conditions. We particularly focused on discoidin domain receptors (DDR), an important player in cancer metastasis. DDRs are receptors that dimerize in response to collagen and initiate a signaling chain within the cell. To mimic the cancer environment, we constructed gel substrates of varying stiffness and collagen concentration. The goal of the research is to determine receptor levels, dynamics and interactions as a function of cell type and environment. Using an improved AFM analysis method, we were able to determine not only binding probability and kinetic parameters, but also ligand densities. We will present preliminary results from this research.
  

Photoprotective properties of a eumelanin building block: Ultrafast excited state relaxation dynamics in 5,6-dihydroxyindole

Skin plays the significant role in protection of body from ultraviolet radiation. The protection is primarily done by a type of pigment called eumelanin. Eumelanin chromophores absorb the energy of the radiation, and through internal conversion, dissipate them quickly in the form of heat. The process takes place in femtosecond time scales. A bottom-up approach is used to study the photophysics of eumelanin which focuses on individual building blocks to gather the overall information of the complex system. Time-Resolved Photoelectron Spectroscopy (TRPES) is implemented to understand the underlying excited state relaxation mechanism in 5,6-dihydroxyindole . This method allows to see the associated dynamics interms of photoelectron kinetic energy.   

Fabrication of CFO@C Core/Shell Nanoparticles by Pulsed Laser Ablation

A considerable research work is going on worldwide among the therapeutic scientific communities to use magnetic nanoparticles (NPs) for drug delivery system. Challenges to use those NPs for such in vivo applications successfully, are- biocompatibility and functionality of those NPs and control over the complex drug release system. In this project, we synthesized carbon coated cobalt ferrite (CFO) core shell nanoparticles (CSNPS) by ablating a solution of toluene and cobalt ferrite NPs with varying number of pulsed laser shots. The structure of the CSNPs was further investigated through X-ray diffraction, UV-VIS, Raman spectra and also with the high-resolution transmission electron microscopy (TEM). The XRD analysis shows that the inner structure of CFO is intact after the laser shots. Raman spectra confirms that with increasing laser shots the G band peak for Carbon becomes more prominent which infers to the crystallinity of carbon. Preliminary TEM line scan data concludes that laser ablation technique is quite efficient to manufacture carbon coated magnetic metal oxide CSNPs.
  

Kinetic compensation effect due to the variation in the concentration of an additive

As part of a systematic study on the kinetic compensation effect, we use kinetic Monte Carlo simulations to study the effects of the change in the concentration of a chemical species in the Arrhenius parameters - effective activation energy E_a, and preexponential factor ν - during the thermal desorption of a binary mixture of interacting and non interacting adsorbates from two dimensional ordered and disordered surfaces. A chemical additive, such as a catalyst, can have an effect in the overall rate at which a process occurs. In this study we quantify the transient variations in the Arrhenius parameters when the concentration of one of the chemical species in the mixture, which is treated as an additive, is varied. The purpose is to observe if a compensation effect and/or isokinetic relation occur when this 'experimental parameter' is altered. We expect our results to help advance the understanding of the microscopic origins of compensation effects in our system of study but also in other fields where these effects have been reported.   

Probing structure-property relationships of BODIPY dimers on the efficiency for symmetry breaking charge transfer

Symmetry breaking charge transfer (SBCT) is the process in which a pair of identical chromophores are coupled such that post excitation an exciton (electron-hole pair) can dissociate between the two chromophores. The product is an uncoupled, spatially separated electron and hole pair, reducing the probability of recombination. The resulting uncoupled charges from SBCT are beneficial for photoelectrochemical and photovoltaic applications. BODIPY dimers have been shown to undergo SBCT in femto-to-pico-second timescales¹. The dipyrrinato dimer analogues have also been studied and shown to undergo SBCT where they are ligated via the nitrogen termini through a zinc (II) center². Herein the BODIPY and dipyrrinato dimers are analyzed to understand the effect of different bridging methods and increased substituents on the efficiency of SBCT. Efficiency in the context of this analysis is the rate of formation and longevity of the SBCT state. We find that direct linkage where torsion is not sterically hindered results in the fastest SBCT formation and that orthogonally fixed geometries result in the slowest decay times of the SBCT state.

1. Whited M. T. et al. Chem. Commun., 2012, 48, 284 – 2846.
2. Trinh C. et. al. J. Phys. Chem. C. 2014, 118, 21834 – 21845.   

Stimulated Raman spectroscopy for characterizing two-dimensional crystals

Two-dimensional dielectric materials such as hexagonal BN and mica give very weak signals in conventional spontaneous Raman scattering using visible excitation lasers. In this work, we report a micro-spectroscopy setup for stimulated Raman scattering (SRS), which gives a substantial increase in overall sensitivity. Pump and Stokes beams with temporal width of a few ps were prepared by spectrally slicing 200 fs pulses from an optical parametric oscillator. The pulse slicer acting as a narrow band-pass filter consisted of a grating (1200 grooves/mm), a cylindrical lens, a mirror, and a mechanical slit that were arranged in the 4f-configuration. Whereas the Stokes pulses were fixed at 1040 nm with a bandwidth of 0.7 nm, the pump pulses were varied from 734 nm to 998 nm for the Raman shift in the range of 400 ~ 4000 cm^-1. The pump beam intensity-modulated by interaction with the Stokes beam in thin hexagonal BN crystals was detected with a photodiode connected to a lock-in amplifier. The sensitivity of the SRS setup and its spatial resolution in raster-scan imaging will be presented.   

Visualization of orbital free models of Kinetic Energy density in solids

The metaGGA functional for describing the exchange-correlation (XC) energy in density functional theory (DFT) is conventionally constructed as a functional dependent on the density, density gradient, and kinetic energy density (KED). The addition of the KED makes metaGGA’s a more accurate functional than ones that use the density and its gradient alone but also more computationally expensive for some applications such as ab initio molecular dynamics simulations. The calculation of the XC energy in meta-GGAs can be made less expensive by replacing the explicit orbital dependence in the KED with expressions involving only the particle density and its gradient and Laplacian. We test the validity of recent deorbitalization strategies in the literature by visualizing their predictions for the KED and related quantities, and comparing these to exact calculations. For an effective test, we perform these calculations on ionic, semiconductor, simple metal, and transition metal solids to see how well the KED for these different binding topologies can be represented by a single metaGGA model in terms of the scaled gradient and Laplacian of the density. The calculations of exact KED and electron density are done with the ABINIT DFT plane-wave pseudopotential code.   

Bayesian Calibration assisted by Markov Chain Monte Carlo in DFT+U for Iron Compunds

Density Functional Theory (DFT) revolutionized condensed matter physics by mapping the electron problem solved by the Schrodinger’s Equation to a series of mean field electron independent equations. In DFT, the most crucial approximation is from the exchange correlation functional. Although many have attempted the development of accurate functionals, they all fail to reproduce the behavior of strongly correlated materials when kinetic energy is as large as the Coulomb interaction. To solve this problem, local exchange correlation functionals are corrected by a Hubbard term. This is DFT+U which depends on two extra parameters, U (on-site coulomb interaction) and J (on-site exchange interaction). These parameters are usually fitted from experimental values or obtained by using density functional perturbation theory. Either way the quality of the parameter values can only be assessed by comparing to experimental data. In this work, we use uncertainty quantification methods to study the dependence of different experimental observables where we evaluate the relevance of errors by utilizing Bayesian Calibration facilitated by Markov Chain Monte Carlo simulation with a number of strongly correlated materials.   

Accurate effective potential for density amplitude and the corresponding Kohn-Sham exchange-correlation potential calculated from approximate wavefunctions

It is well known that direct inversion of an accurate but approximate density leads to KS exchange-correlation potential which has spuriously large deviations from the exact one. On the other hand, a different approach which employs wavefunction directly to obtain KS exchange-correlation potential is found to lead to potentials close to the exact one. This approach utilises the Levy-Perdew-Sahni expression for the effective potential used in the equation for the square root of the density.
We present why the Kohn-Sham exchange-correlation potential obtained by using an approximate wavefunction in the Levy-Perdew-Sahni expression for effective potential is accurate while the direct inversion of density associated with the same wavefunction may lead to
pathological features in it. By analysing the Levy-Perdew-Sahni equation, we show that quantities like effective and exchange-correlation potentials, calculated from approximate wavefunctions are free from spurious features and close to the corresponding exact ones. The
study also suggests possibility of a new approach for the calculation of accurate densities from approximate wavefunctions. These densities are closer to the exact ones than those obtained from the wavefunction directly.   

Mechanical and electronic properties of mechanically-bent monolayer transition metal dichalcogenides (MX2) in the ground state using SCAN density functional

As an alternative to graphene, transition metal dichalcogenides (TMD) have gained a lot of interest as promising candidates for future flexible nano-electronics due to the mechanical and electronic properties related to their high flexibility [1]. Though the TMD thin layers have a promising future, traditional methods of tuning the band gap such as doping with impurities or contact engineering suffer strong Fermi-level pinning or even damage the materials. However, due to the high bending and in-plane stiffness, thin monolayer MoS₂ can be bend mechanically to tune the bandgap as well as reducing Fermi level pinning to some extent [2]. In this work [3], we extend the study to exploring the other TMD monolayers corresponding to transition metals from groups IV to X in the periodic table in the ground state, using the recently developed meta-GGA SCAN.

[1] D. Akinwande, et al., Extreme Mechanics Letters 13, 42 (2017).

[2] L. Yu, A. Ruzsinszky, and J. P. Perdew, Nano Lett. 16, 2444 (2016).

[3] N. K. Nepal, et. al., Phys. Rev. Material 3 (7), 073601 (2019).   

A simple self-Interaction correction to the RPA+ correlation energy

The exchange energy of the Random Phase Approximation is exact for a many-body ground state, but the correlation energy is often overestimated. The error comes from the poor description of the short-range correlation. The RPA+ approximation ^[1] largely reduces this error for atoms, for the jellium surface and for the uniform electron gas by adding a local or semi-local correction. While the RPA+ is accurate for the total energies of atoms, it fails for single-electron systems like stretched H²⁺, and systems where spin-polarization plays a significant role such as ionization energies, electron affinities, and atomization energies. In this work, we have introduced a simple correction to the RPA+ correlation energy to make the new gRPA+ ^[2] approximation exact for single-electron systems. We are assessing this computationally feasible approximation beyond RPA for various molecular test sets and we analyze the impact of self-interaction correction.

[1] S. Kurth and J. P. Perdew, Phys. Rev. B 59, 10461 (1999).
[2] T. Gould, A. Ruzsinszky, J.P. Perdew, Phys. Rev. A 100, 022515 (2019)   

Quasiparticle energies of the uniform electron gas

The uniform electron gas (UEG) model is the simplest model of a metal, a large volume of electrons subject to a uniform background of positive charge. Existing parametrizations of the UEG correlation energy allow us to perform calculations in Kohn-Sham density functional theory (KS-DFT). While the Kohn-Sham and generalized Kohn-Sham orbital energy eigenvalues are not single-particle excitation energies [1,2], how well do they predict the occupied bandwidth of the uniform electron gas or of alkali metals like Na? In this work, we aim to answer this question using the local spin density approximation (LSDA), the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE GGA), and the PBE0 hybrid functional (with 25% of Hartree-Fock exchange). We benchmark our DFT values against the results of higher-level theories and experiment.




[1] E. Jensen and E.W. Plummer, Phys. Rev. Lett. 55, 1918 (1985).
[2] J. E . Northrup, M. S. Hybertsen, and S. G. Louie, Phys. Rev. Lett. 59, 819 (1987).   

Pressure-induced New Oxidation States

The formalism of the oxidation state of atoms in compounds is a key concept in chemistry. Finding novel compounds containing elements with unusually high oxidation states allows a deeper understanding of chemical behavior of elements. On the other hand, high oxidation state compounds usually bring new types of bonds with interesting physical and chemical properties. Thus, the preparation of compounds with unusual oxidation states becomes an attractive topic in chemistry and condensed-matter physics. Gold (Au) is a well-known fascinating element, but still hides interesting surprises to be discovered, especially for oxidation state. Here, we propose that high pressure becomes a controllable method for preparing high negative oxidation state of Au through its reaction with lithium. Au acts as a 6p-element in dense lithium aurides. Moreover, we identify two hitherto unknown stoichiometric compounds, AuF4 and AuF6, exhibiting typical molecular crystal character, in which Au demonstrates +4 and +6 oxidation states. On other hand, we also achieve several compounds with unusual stoichiometry such as IrF₈, IF₈, and BaF₅.   

Charge Transport Properties of Biomolecules

Double-stranded DNA (dsDNA) and dsRNA hold great promises in molecular electronics. We characterize the charge transport properties of dsRNA for different sequences and compare them with similar sequences of dsDNA in two extreme charge transport regimes – incoherent charge hopping regime and coherent electron transport regime. We find that the relative conductance of A-form dsRNA and B-form dsDNA depends on the mechanism of charge transport. This is attributed to various structural differences in dsDNA and dsRNA. We also study the effect of stretching and propose a method to detect conformational changes using electrical measurements. Despite the twist-stretch coupling of dsRNA and dsDNA being different under external force, dsRNA shows similar structural polymorphism to dsDNA under different pulling protocols. Our atomistic MD simulations show that overstretching dsRNA along the 3’ ends (OS3) leads to the emergence of S-RNA whereas overstretching along the 5’ ends (OS5) leads to melting of dsRNA. Using the dsRNA morphology from pulling MD simulations, we use a multiscale method involving ab initio DFT calculations and Kinetic Monte Carlo (KMC) simulations to estimate the conductance of dsRNA and find that the conformational changes drastically affect its conductance.   

Tuning the adsorption properties of transition metal dichalcogenides (TMD) monolayer by bending

Due to their exceptional catalytic performance, there has been a growing interest in understanding the adsorption properties of transition metal dichalcogenide (TMD) monolayers. Previous studies focused on tuning the adsorption energies and distances in these 2D materials using mechanical strain as well as doping with impurities. Besides that, 2D materials are highly flexible. Moreover, some recent works [1,2] have shown the importance of mechanical bending for tuning various mechanical and electronic properties of TMD monolayers. Using first-principles calculations, we report the effect of mechanical bending on the adsorption properties of the monolayers using a single hydrogen atom as the adsorbate.


[1] L. Yu, A. Ruzsinszky, and J. P. Perdew, Nano Lett. 16, 2444 (2016).
[2] N. K. Nepal, et. al., Phys. Rev. Materials 3 (7), 073601 (2019).   

Modelling the electronic kinetic energy density and Pauli potential by orbital free density functional theory.

In Kohn Sham density functional theory, the kinetic energy (KE) functional is described by fictitious Kohn-Sham (KS) orbitals. This causes a computational bottleneck for large systems that require many KS orbitals. Much recent research is going into Orbital-Free Density Functional Theory (OFDFT), which models the Kinetic Energy as a functional of density and other ingredients that are derived from density directly, avoiding the need for orbitals. There are reasonable OFDFT models for Kinetic Energy at the meta-GGA level, such the Perdew-Constantin model [1], that properly treat the nonnegativity constraint for the Pauli contribution to the KED, which describes the correction to the von-Weizsäcker KED, which describes the KE of a single electron pair. However, an issue arises of Pauli potentials that are not physically reasonable and difficult to find convergent solutions for. Our goal is to construct meta-GGA level models with potentials which vary smoothly. We test them against calculations of the exact Kohn-Sham KE density and potential for atoms, with atomic densities constructed from the fhi98PP code. As another design goal we will try to incorporate the calculation of linear response.
[1] J. P. Perdew and L. A. Constantin, Phys. Rev. B 75, 155109 (2007).   

Stable Structures and NMR Analysis of (Na)n with pore size in hard carbon by DFT calculations

The lithium ion batteries (LIB) which are a typical secondary batteries, is widely used as an energy storage system by having high voltage, good charge and discharge cycles. However, due to problems with the resources and costs of lithium in LIB, there is a growing interest in resource-rich sodium-ion batteries (NIB) with equivalent electrode potential. In recent years, a NIB using a Na metal oxide as the positive electrode and hard carbon (HC) as the negative electrode has attracted attention in particular. To develop a NIB with high capacity, high efficiency, long life, and acceptable safety, it is essential to elucidate the state of the sodium ion and the mechanism of charge and discharge on the electrode. The states of sodium electrochemically inserted in HC samples have been experimentally reported using solid 23Na NMR. In this study, in order to study the correlation of NMR shift of Na clusters (n = 1-8) with pore size in HC and structure, DFT calculations are performed at B3LYP/6-31G(d) level using the Gaussian 16. From NMR analysis it can be seen that the value of chemical shielding shifts significantly depending on the strength of bonding between Na atoms and the size of clusters.   

Investigation the nature of CO2 binding to borophene χ3

Global warming following the release of greenhouse gases, especially CO2, is one of the important areas of research to adsorption of CO2 to remove it from the environment. But the CO2 binding to the surfaces is weak, and scientists want to correct this defect.
Here we used borophene χ3 as a novel 2D material with the hole density (η=1/4) and investigated the nature of CO2binding to its surface. DFT calculations and plane-wave basis set with the ultrasoft pseudopotentials were used in the quantum espresso. The GGA approximation with PBE functional was applied for the electron exchange-correlation energy. The VDW interactions were taken into account using the DFT-D2 method. The binding site for CO2 was determined and the adsorption energy and electronic properties of the χ3-CO2were analyzed.
The calculated binding energy (~0.2eV) and distance of CO2 to the surface (~3.5Å) indicated weak adsorption. The charge density differences calculation shows the charge depletion between the boron sheet and CO2. The Bader charges analysis shows that the CO2 tends to capture electrons from boron atoms. during the adsorption. Therefore, because of the electron deficiency of the surface, the CO2 physisorbed. Therefore, to achieve stronger adsorption, the surface modifications were needed.   

Theoretical study of charge and discharge process and NMR in Na ion battery anode material Sn

Sn can be used as an anode material for secondary battery Na ion batteries because it can work with Na with a composition ratio of up to Na₁₅Sn₄ by alloying with Na. For practical application, it was reported from both experiments and calculations that there were multiple plateaus in the charging curve of Na-Sn system based on the structural change in the sodiation/desodiation process of Na-Sn system. In this study, we investigated the detailed mechanism of charge and discharge processes in Sn anodes by theoretically clarifying the structural changes, charge and discharge curves, and ²³Na NMR chemical shift properties of the anode material Sn in Na ion batteries.
We used VASP-5.3.2 to optimize the structure of Na-Sn systems with different composition ratios during the charge/discharge process, and calculated their formation energy. The boundary composition ratio was clarified from the charging curve obtained by estimating the change in potential with respect to the Na/Sn composition ratio.
We calculated chemical shift tensors for Na-Sn system, Na and NaCl by GIPAW method. As a result of theoretical calculation of the 23Na chemical shift, it was possible to obtain a number of shielding effect values corresponding to the number of independent Na sites in each Na-Sn composition.   

Alternative diffusion Monte Carlo methods for fermions

Current continuum diffusion Monte Carlo methods for fermions are limited by the sign-problem, or, fixed-node approximation, computationally inefficient in cases that require far more than single determinant to reliably describe leading correlations. We devise and critically examine some alternative approaches that enable maintaining Fermi statistics within continuum DMC simulations.   

Self-Interaction correction using Fermi-Lowdin orbitals: Methodology and Parallelization

Density functional theory is a popular electronic structure method that can handle comparatively much larger systems than quantum chemical methods. However, density functional approximations suffer from self-interaction errors which limits its reliability for certain properties. The Perdew-Zunger (PZ) self-interaction correction (SIC) method removes self-interaction on an orbital by orbital basis and is computationally expensive. Recent implementation of Fermi-Lowdin orbital based SIC (FLOSIC) is a promising method that removes many computational complexities of PZ-SIC. We present some details of the FLOSIC methodology and present parallelization strategies using MPI+MPI and recent shared memory features of MPI-3 to make efficient use of large supercomputers. Using this implementation, we achieved parallel efficiency above 80% for 3360 processors. Applications of the FLOSIC method using the FLOSIC code on water clusters and carbon fullerenes will be presented.   

Synthesis of carbon nanodots from caramelized glucose solutions

Carbon nanodots are a class of zero-dimensional carbon-based nanoparticles that have been used increasingly in the fields of bioimaging, drug delivery, and optronics. Despite their popularity, carbon nanodots remain difficult to isolate when synthesized by a bottom-up approach, especially through the thermal treatment of carbohydrate solutions due to the presence of many molecular byproducts which result from the Maillard reaction [1].
We investigated multiple possible methods of purification of glucose solutions [2]. Glucose solutions were heated at 120 °C for a period of 48 hours to produce carbon nanodots. We found that dialysis combined with solid phase extraction preserved the photoluminescent properties of dissolved carbon nanodots, while eliminating measurable traces of other chemical byproducts, as measured by Fourier-Transform IR spectroscopy, Carbon-NMR, and Hydrogen-NMR.

1. Essner, J. B., Kist, J. A., Polo-Parada, L., & Baker, G. A. (2018). Artifacts and Errors Associated with the Ubiquitous Presence of Fluorescent Impurities in Carbon Nanodots. Chemistry of Materials, 30(6), 1878–1887.
2. Obadiya, T., Uppala, H., & Sidebottom, D. (2019). Fluorescent Carbon Particles formed from Concentrated Glucose Solutions. In MRS Advances (Vol. 4, pp. 67–72).   

Synthesis and Characterization of Barium Titanate and Carbon-based core-shell nanoparticles

Core-shell nanoparticles (CSNP) have diverse applications such as biology and electronic field of studies. Among so many oxide nanoparticles, Barium Titanate nanoparticles are of current interest due to their good electronic and ferroelectric properties. There are some studies based on the synthesis and characterization of oxide-oxide and oxide-sulfide based core-shell nanoparticles. Here we are trying to synthesize and characterize oxide-carbide based core-shell nanoparticles, where the core is made of Barium Titanate and the shell is made of Carbon. There are various synthesis approaches like hydrothermal synthesis, sol-gel method, emulsion polymerization etc to produce core-shell nanoparticles, here we have synthesized these core-shell nanoparticles with a unique method. The shell is made of Carbon which is a biocompatible element and it can be used as a connector between organic and inorganic materials. Shell made of carbon shows interesting optical properties and they can be useful for biosensing and optoelectronic applications. We used different methods like XRD, RAMAN, SEM, and TEM to characterize and analyze these CSNP. Some of the findings will be discussed here.
  

A Gd@C82-based single electron transistor device with ferroelectric-like switching behavior

Ferroelectricy arises from permanent dipole and has led to technology innovation in memory devices. However, maintaining ferroelectricity at nanoscale is challenging, greatly limits its application in nanoscale devices. Here we report the gate-controlled switching between two sets of single-electron characteristic stability diagrams in the electrical transport of a Gd@C₈₂-based single molecular device. It is operated in a hysteresis-like loop with a coercive gate field of up to 0.5 V/nm. The theoretical calculations attribute the two diagrams to the energy levels of two trapping states of the Gd atom in the C₈₂ cage, which possess two different permanent electrical dipole configurations. The switching thus originates from the electrical field driven dipole flipping and demonstrate the ferroelectricity at the single molecule level.   

Evolution of plasmonic response of individual size-selected Au clusters from nanoscale to atomic-scale

We have studied the evolution of plasmonic response of individual size-selected Au clusters by scanning transmission electron microscopy-electron energy loss spectrum (STEM-EELS) from nanoscale to atomic scale. The clusters are prepared and selected by megnetron sputtering time-of-flight cluster source to get an atomic number precision. To distinguish surface and bulk plasmon mode, we put the electron beam at the edge or center of clusters, where we can get a pure surface mode or a mixure of surface and bulk mode. Both surface and bulk mode exist when the gold clusters are large enough. However when the atom numbers decrease to 887, the bulk mode disappears abruptly while the surface mode still appears. The surface mode excitation probability of a single incident electron shows a square relationship versus atom numbers from 70000 to 300, but deviates from it when the atom number reaches 175. What’s more, a new peak higher than surface mode appears when the gold clusters are smaller than Au₅₀₀. After a slight redshift from Au₇₀₀₀₀ to Au₈₈₇, an obvious blue-shift happens in the range Au₈₈₇ to Au₃₀₀, showing a combined influence between quantum size effect and classical size effect of electron mean free path.   

Infrared Spectroscopy Investigation of Ammonium Sulfate at Low Temperatures

We present a comprehensive study of ammonium sulfate between 30 and 300K us-
ing Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry
(DSC) and neutron powder diffraction (NPD) to investigate the paraelectric to ferro-
electric phase transition. Heat capacity measurements taken with DSC show a clear
anomaly at the phase transition (223 K). This phase transition in ammonium sulfate
is associated with temperature dependent hydrogen bonding and presents an interest-
ing and challenging research area.¹ We characterize the hydrogen bonds at different-
temperatures and compare our NPD at room temperature with previously reported
results². We then analyze FTIR spectral features of the sample and demonstrate their
temperature dependent displacement.

[1] Malec, L. M., Gryl, M., & Stadnicka, K. M., Inorg. Chemistry, 57, 4340–4351
(2018).
[2] E. O. Schlemper and W. C. Hamilton, J. Chem. Phys. 44, 4498 (1966)   

Activation of CO2 at chromia-nanocluster-modified rutile and anatase TiO2

Converting CO₂ to fuels is required to enable the production of sustainable fuels and to contribute to alleviating CO₂ emissions. In considering the conversion of CO₂, the initial step of adsorption and activation by the catalyst is crucial. In addressing this difficult problem, we have examined how nanoclusters of reducible metal oxides supported on TiO₂ can promote CO₂ activation. In this paper we present density functional theory (DFT) simulations of CO₂ activation on heterostructures composed of clean or hydroxylated extended rutile and anatase TiO₂ surfaces modified with chromia nanoclusters. Our results highlight that a metal oxide support modified with reducible metal oxide nanoclusters can activate CO₂, thus helping to overcome difficulties associated with the difficult first step in CO₂ conversion.   

Logarithmic Expansion of Swellable Organosilica Material

Swellable organically modified silica (SOMS) is a matrix of crosslinked polysilsesquioxanes that undergoes rapid reversible swelling upon contact with organic solvents. This material has applications in environmental remediation, catalysis, and personal care products. We demonstrate that SOMS exhibits a logarithmic stress-strain relationship when it swells. Using a two-piston force sensor, we measured the swelling force exerted by various samples of SOMS when treated with acetone. As the mass of SOMS inside the force-measurement apparatus increases, the force generated by the swelling also increases, up to a maximum value of approximately 180 N at 650 mg of SOMS. However, the rate-of-change of the force generated by SOMS decreases at larger masses, implying that SOMS is dynamic rather than static. A data collapse of two different SOMS samples (Osorb® and Cyclasorb® ) shows that the underlying behavior of SOMS is uniform across sample types.
  

Carrier Injection Mechanisms in Diketopyrrolopyrrole Organic Semiconductors

Organic semiconductors based on diketopyrrolopyrrole (DPP) are a new class of small molecule semiconductors which are air-stable, possess relatively high mobilities, and have been recently shown to undergo excitonic singlet fission. Realizing high quality (Ohmic) contacts to DPP is essential for the fabrication of high quality devices. In this work we use a combination of scanning Kelvin probe microscopy and electrical transport measurements to spatially map the work function in several DPP-based semiconductors and study the carrier injection mechanism in thermally evaporated DPP films.
  

Novel phase transitions as bridges for broken ergodicity in confined colloidal prisms

We used Monte Carlo simulations to study the assembly behavior of square and hexagonal prisms under quasi-2D confinement separations, within a hard-wall slit. Our results for hexagonal prisms revealed two types of first order phase transitions at increasing concentration: 1) solid-solid transition (6-fold→4-fold symmetry solid) occurring through lattice symmetry breaking, and 2) solid to dense-liquid (disorder) to solid. The predicted dense-liquid has a density intermediate to those of the two solid phases and high translational/orientational mobility. We showed that similar phase behavior can be expected for other n-gonal prisms (n > 6). For square prisms, we observed a solid-solid phase transition where a square lattice spacing rearrangement gives rise to a polycrystalline phase with multiple locally ordered domains. These unusual phase transitions are attributed to the broken ergodicity associated with a dynamically disconnected rotational phase space accessible to the particles. As an experimentally viable strategy to dynamically bridge those rotational states observed for hard-slit phase behavior, we also investigated and validated a soft-repulsive wall model.   

Non-Adiabatic Molecular Dynamics of Molecules in the Presence of Strong Light-Matter Interactions

The mixing between the light and matter characters modifies the photophysical and photochemical properties. In this work, a theoretical model and an efficient numerical method for studying the dynamics of molecules strongly interacting with quantum light are developed based on non-adiabatic excited-state molecular dynamics. The methodology was employed to study the cis-trans photoisomerization of a realistic molecule in a cavity. Numerical simulations demonstrate that the photochemical reactions can be controlled by tuning the properties of the cavity. In the calculated example, the isomerization is suppressed when polaritonic states develop a local minimum on the lower polaritonic state. Moreover, the observed reduction of isomerization is tunable via the photon energy and light-molecule coupling strength. But the fluctuation in transition dipole screens the effect of light-matter, which makes it harder to tune the photochemical properties via the coupling strength.   

Strong plasmon-molecule coupling at the nanoscale revealed by first-principles modeling

Strong light-matter interactions in both the single-emitter and collective strong coupling regimes attract significant interest due to emerging applications in quantum optics as well as opportunities for modifying material-related properties. Exploration of these phenomena is theoretically challenging, as polaritons exist at the intersection between quantum optics, solid state physics, and quantum chemistry. In this presentation, we shed light on nanoscale polaritons in small strongly-coupled plasmon-molecule systems by using time-dependent density-functional theory (TDDFT) [1]. By analyzing the electron-hole transitions involved in the excitation process, we dissect the symmetric and antisymmetric polaritonic modes caused by strong coupling between plasmon and molecular excitation, resulting in Rabi oscillations in time domain. Our results indicate that cavity quantum electrodynamics description holds down to resonators of a few cubic nanometers in size. In a broader perspective, first-principles methods enable parameter-free in-depth studies of polaritonic systems for emerging applications.

[1] T. P. Rossi, T. Shegai, P. Erhart, and T. J. Antosiewicz, Nat. Commun. 10, 3336 (2019).   

High-order harmonics generated from a quasi-one-dimensional hexagonal solid

High harmonic generation has attracted enormous attentions.
Here we report a completely new kind of harmonics
in a quasi-one-dimensional and hexagonal barium titanium sulfide:
Under circularly polarized laser excitation, harmonics are
generated only at first, fifth, seventh and eleventh orders. These
magic harmonics appear only with circularly polarized light, not
with linearly polarized light. Neither cubic nor tetragonal cells
have magic harmonics even with circularly polarized light.
Through a careful group-theory analysis, we find that two
subgroups of symmetry operations unique to the hexagonal symmetry
cancel out third and ninth harmonics. This feature presents a
rare opportunity to develop HHG into a crystal-structure
characterization tool for phase transitions between hexagonal and
nonhexagonal structures.

Refs.
[1] G. P. Zhang and Y. H. Bai, Phys. Rev. B {\bf 99}, 094313
(2019).
[2] Lei Jia, Zhiya Zhang, D. Z. Yang, M. S. Si, G. P. Zhang,
and Y. S. Liu, Phys. Rev. B {\bf 100}, 125144 (2019).

[3] G. P. Zhang, M. S. Si, Mitsuko Murakami, Y. H. Bai and
T. F. George, {\it Generating high-order optical and spin harmonics from
ferromagnetic monolayers}, Nat. Comm. {\bf 9}, 3031 (2018).   

Deciphering photoacidity by following electronic charge distribution changes along the photoacid Förster cycle with picosecond nitrogen K-edge x-ray absorption spectroscopy

Photoacids are molecular systems that show a strong increase in acidity in the first electronic excited state. The underlying mechanisms for photoacidity and photobasicity have until now remained unsolved. We use picosecond N K-edge x-ray absorption spectroscopy to determine how the transient electronic-structure changes and hydrogen-bond dynamics determine the acidity of a prototypical photoacid, 8-aminopyrene-1,3,6-trisulfonate (APTS), in aqueous solution. We follow in time the characteristic spectroscopic signatures of N-H σ* and N-lone pair interactions of the proton donating functional amine group as well as aromatic pyrene π* anti-bonding orbitals of APTS along the different stages in the Förster photocycle. With our flatjet system for x-ray absorption spectroscopy in transmission and with the picosecond x-ray pulses at BESSY II (in multibunch mode), we elucidate how UV excitation converts the photoacid into the conjugate photobase form on a time scale of 150 ps, followed by electronic excited state fluorescence decay on nanosecond time scales. With these results we demonstrate that a systematic electronic-structural approach to the ultrafast dynamics of photoacids in aqueous solution can be established.   

Microscopic study of proton kinetic energy anomaly for confined water

Several anomalies, related to structural and dynamical transition, have been reported for water at different thermodynamic conditions and environments. Of particular interest, the reported anomalies of the proton mean kinetic energy, Ke(H), in nanoconfined water, as measured by deep inelastic neutron scattering (DINS), are a longstanding problem related to proton dynamics in hydrogen-bonded systems. We used classical MD method to deduce Ke(H) by calculating the proton vibrational density of states, H-VDOS, for the case of water inside single wall carbon nanotubes (SWCNT) of varying diameters. The mean vibrational density of states (VDOS) of protons in water nanoconfined inside single wall carbon nanotubes (SWCNTs) is calculated as a function of temperature and SWCNT diameter, D_CNT. The calculated VDOS are utilized for deducing the mean kinetic energy of the water protons, Ke(H), by treating each phonon state as a quantum harmonic oscillator. The calculation depicts a strong confinement effect as reflected in the drop of the value of Ke(H) at 5K for D_CNT < ~12Å, while absent for larger diameters. The results also reveal a very significant blue and red shifts of the stretching and bending modes respectively compared to those in bulk ice, in agreement with experiment.   

Evidence of topologically non-trivial phase trough unusual Shubnikov–de Haas oscillations

Topological features in bulk related phenomena are still an uncharted territory as opposed to edge and surface effects, making Shubnikov-de Haas (SdH) oscillations an interesting phenomenon to explore in topological systems. Here we investigate the bulk magneto-oscillations in connection with topological bands of 2D systems. We describe our systems by the BHZ model [1] and use a convenient trace formula [2] to describe the density of states of broadened Landau levels within a phenomenological Drude-like transport description. Our preliminary results show unusual beatings in the SdH oscillations for the topological regime, which are absent in the trivial regime. These can be traced back to the coexistence of both electron- and hole-like carriers, present in the “Mexican hat” band structures. This suggests SdH oscillations as a possible bulk probe for the non-trivial band topology of 2D systems.

[1] B. Andrei Bernevig, Taylor L. Hughes, Shou-Cheng Zhang, Science 314, 1757 (2006).
[2] S. I. Erlingsson, J. V. I. Costa, D. R. Cândido, and J. C. Egues, Analytical results for the SdH oscillations in Rashba-Dresselhaus 2DEGs, (Unpublished)   

Stacking-type dependence of topological phase transitions in a superlattice of topological and normal insulators

We consider a superlattice of topological and normal insulating layers. It is known that topological phase transitions occurs with the variation of the interlayer hopping when two-dimensional quantum spin Hall insulator is hybridized with the normal insulators in the AA-type stacking. We examine how the nature of topological phase transition changes depending on the stacking type. The resulting phase diagram exhibits rich phases such as semimetal and topological ones. The properties of the edge states are also investigated in the sample of finite width. Finally we discuss the topological features of alternately stacked layers of topological and normal insulators.   

The physical basis of using MRI to detect cancer

Nowadays, the most effective way of detecting cancer in a patient is using MRI (Magnetic Resonance Imaging). This technique is based on the measurement of hydrogen nuclear spin signal of water molecules inside the body. During the development of the MRI technique, it needed to overcome two major difficulties: (1) How to produce a contrast between water molecules inside the cell and the extra-cellular water? (2) How to differentiate the water signals contributed by cancer cells from those contributed by normal cells? These difficulties were resolved mainly through the discovery that the relaxation times of water protons inside the cells are very different from those of the extra-cellular water. Furthermore, the relaxation times of water molecules in normal cells are found to be significantly different from those in the cancerous cells. In this presentation, I will give a concise review of the evidence for these discoveries. Finally, I will discuss the possible physical basis that may account for the relaxation time difference between different cells.   

Influence of self-heating on noise of SiGe heterojunction bipolar transistors at cryogenic temperatures

Cryogenic low noise transistor amplifiers (LNA) at microwave frequencies have long been of interest for radio astronomy and more recently for quantum computers. While high electron mobility transistors have been the primary device of choice for these applications, SiGe HBTs are increasingly competitive due to increases in cutoff frequency and their relatively high yield. However, an understanding of the influence of self-heating on noise at cryogenic temperatures is lacking. In this work, we report noise measurements on SiGe HBT LNAs from 4 – 50 K. The influence of self-heating on the temperature of electrons at the base-emitter junction is evaluated by comparing measurements in vacuum with those in liquid Helium baths. Our work helps to identify a path for SiGe HBTs to achieve noise figures competitive with those of HEMTs.   

Transfer of Linear and Angular Momentum from Evanescent Fields of an Optical Fiber to Isotropic and Anisotropic Dipolar Spheres.

We calculate the force and torque on a transparent isotropic dipolar sphere due to the higher-order evanescent field modes propagating outside a few-mode optical fiber immersed in water. After applying these results to find the orbital trajectory and spin dynamics of an isotropic sphere, we then use perturbation theory to calculate the orbital trajectory of a weakly anisotropic sphere.   

Capacitive Compactness and Surface Charge Density Variation in Pluronic (F-127) Micelles using Small-Angle X-ray Scattering

The spatial extent of a charged colloid or an electrode is usually characterized by its Debye length which is independent of important physical features including colloidal charge and electrostatic ion correlations. To describe the colloidal stability of electrified nanoparticles more accurately we need tools to study the structure of the electrical double layer. Colloidal particles can be used in diverse technological applications involving colloidal stability of charged solutions, or storage capacity of electrical energy in batteries or supercapacitors. Capacitive compactness is a novel description of the diffuse electrical double layer extension in terms of an effective capacitor and has ability to consider important physical characteristics of the solute. We have chosen Pluronic F-127 in this work because it is a temperature responsive triblock copolymer with well-known adsorption and colloidal properties. In this work, we studied the surface charge density of Pluronic F-127 micelles at different concentration and pH to understand how interparticle interactions relate to capacitive compactness. Small-angle x-ray scattering results were used to analyze the micelle-micelle structure factor and relate it to pH dependent changes in the capacitive compactness.   

Nano Spray-Dried Block Copolymer Nanoparticles and Their Transformation into Hybrid and Inorganic Nanoparticles

Block copolymers (BCPs) self-assemble into highly ordered structures with periodicities of 5-50 nm. When BCP self-assembly is confined in macro and nano spheres various morphologies can be achieved by tailoring BCP chemical composition, solvent, and process parameters, making them attractive materials for applications like drug delivery and catalysis. So far, BCP NPs were mainly fabricated through emulsion-based methods. However, there is need to develop novel, potentially scalable, BCP NP fabrication methods which will enable new NP morphologies.
We present a new route for BCP NP fabrication through BCP confinement in sprayed nano-droplets using Nano Spray-Drying (NSD). PS-b-PMMA and PS-b-PVP were NSD from toluene and chloroform. SEM and TEM were used to study the effects of BCP chemistry, solvent, and NSD process parameters on BCP NP morphology. Following NSD, BCP NPs were suspended in non-solvents and solvent annealed with chloroform to tune internal NP morphology. Finally, hybrid organic-inorganic particles were created through Sequential Infiltration Synthesis (SIS), a method that enables growth of metal-oxides within polymers. Using SIS, alumina and zinc-oxide were selectively grown in BCP polar domains, transforming the BCP NP into hybrid or BCP-templated inorganic NPs.   

Fabrication of low dimensional oxide thin films using cuprates as sacrificial layers

The characteristics and functionalities of complex oxide caught a lot of attention recently. For oxide perovskite thin film, an available modulation is to form heterostructure. Applying different perovskite allows strain manipulating and other effects that would change the characteristic which form a unique system. Unfortunately, not all kinds of complex oxide are acceptable to heterostructure. To give a wider range of combination for oxides, freestanding membrane was developed. La_0.7Sr_0.3MnO₃ and Sr₃Al₂O₆ are the two commonly used buffer layer. By etching the buffer layer, the thin film that deposit after the buffer layer ca n be separated from the substrate. In this work, we used YBa₂Cu₃O_7-x as a buffer layer, this method takes less time etching but maintain a fine freestanding membrane for later procedure. Furthermore, the process allows us to separate materials like La_0.7Sr_0.3MnO₃ with acid etchant because of the time etching YBa₂Cu₃O_7-x takes a few minutes while etching La_0.7Sr_0.3MnO₃ takes a few hours. Our goal is to develop a universal freestanding method that could applied to all kind of materials which could give access to more combination of complex oxide thin films.