Session M71: Poster Session II (11:15am - 1:15pm)

High-Throughput Prediction of Stress-Strain Curve of Thermoplastic Elastomer Model Block Copolymers with Various Chain Structure by Combining Hierarchical Simulation and Deep Learning

Thermoplastic elastomer (TPE) is a typical industrial product, in which the microphase separation of block copolymer is utilized, and the stress-strain (S-S) curve is a key issue to design such products. The polymer chain and phase separated structure affect the S-S curve. However, it is not simple to find the relation between the structures and S-S curve. To tackle the problem, we applied hierarchical simulation and deep learning technique. We studied various type of AB type block copolymers, where A blocks and B blocks form glassy and rubbery domain respectively. S-S curves of wide variety of volume fractions and structures were investigated by hierarchical simulation of consistent field theory and coarse-grained molecular dynamics (CGMD). Furthermore, we applied 3D-convolutional neural network (3D-CNN) to make regression between the structures and S-S curves obtained by CGMD simulation. The predicted S-S curves of untrained structures using trained 3D-CNN showed good agreement with simulation, and high-throughput prediction could be realized comparing to the computationally intensive simulation.   

Viscoelastic Properties of Iron Oxide–Poly(ethylene oxide) Nanocomposites via Nanoindentation Experiments

Current research aims to characterize the viscoelastic properties of iron oxide (Fe₃O₄) nanoparticle (NP) infused poly(ethylene oxide) (PEO) nanocomposites. Iron oxide NPs with varying surface chemistries (hydroxyl, amine, and polyethylene glycol diphosphate surface groups) were used to prepare nanocomposites with varying concentrations of iron oxide less than 1% by weight. Nanoindentation experiments were carried out on hot pressed cylindrical samples using a Hysitron TI950 Triboindenter with a Berkovich tip. Samples were subjected to stress relaxation experiments where loading rates were varied while hold times and unloading rates were held constant. Loading curves were fit to identify the influence of loading rate on loading response as a function of NP surface chemistry and NP concentration.   

Probing architecture-dependent polymer deformation in extreme shear flows

It is increasingly recognized that molecular architecture plays a significant role in controlling rheology and mechanical stability in applications involving polymers extremely high shear rates. However, in situ measurements of polymer deformation in such high-rate flows have been extremely limited, preventing a broader understanding of the high-rate behavior of architecturally-complex polymers. We report new devices and methods that enable scattering and rheology measurements in situ at shear rates exceeding 10⁶ s^-1. We have employed these methods to understand how polymer architecture influences key rheological properties at extreme shear rates. Specifically, measurements on a well-controlled series of linear, branched and star architectures reveal the primary mechanisms by which architecture controls polymer deformation and scission. Most importantly, we find that star polymers exhibit significantly greater mechanical stability at extreme shear rates relative to linear counterparts, owing primarily to reduced deformations encountered as a result of the faster relaxation dynamics of polymer arms. Overall, our results highlight the importance of characterizing molecular structure in situ to aid the rational design of polymer architectures for high-shear applications.   

Rheology of Complex Fluids at High Shear Rates using Scattering and Microfluidic Rheometry

Structure-property relationships of fluids may be governed by self-association behavior influenced by flow. We developed microfluidic methods to minimize fluid volumes, to test high shear rates (up to 1 000 000/s), and to obtain simultaneous measurements of viscosity and structure. For fd-virus solutions and wormlike-surfactant solutions we measure alignment, form and structure factors, and we interpret these results in light of rod-like scattering models, in an effort to understand length distributions of the surfactant solutions. A high degree of alignment is observed over a wide range of shear rate; at intermediate rates, wormlike solutions exhibit semidilute behavior and at higher rates dilute behavior is observed, and we relate this change to differences in structure. For protein solutions, we measure aggregation of protein particles during flow past surfaces of controlled surface chemistry, to which protein adsorbs.   

Thermal Reversible Gelation of TEMPO-Oxidized Cellulose Nanofibers Dispersed in Propylene Glycols

TEMPO-oxidized cellulose nanofibers (CNF) have attracted a great deal of research interests recently, due to their environmentally friendly nature, excellent mechanical properties and abundant surface functional groups for direct applications or modifications. They also show good dispersibility in water due to these hydrophilic functional groups, and strong electrostatic repulsion between the fibers. Our lab has successfully dispersed CNF in ethylene glycol (EG) and propylene glycol (PG), having different hydrophobicity. The CNF/EG suspension was found to behave similar to the aqueous CNF suspension, while CNF/PG suspension exhibited unusual gelation behavior beyond overlap conc., as revealed by the long-time oscillation monitoring. This thermal-irreversible gelation process can be greatly accelerated at high temperature, and the formed gel can be reversed back to viscous liquid upon severe stirring. We argue that the PG is a poor solvent for CNF compared to EG and water, and thus with relatively higher hydrophobicity. The increasing temperature appears to increase the hydrophobicity of PG, leading to more hydrophobic aggregation between CNF and thus stronger gelation tendency.   

Ultra-low interfacial tension 3D printing of high definition silicone structures

Jammed particulate systems can be used as support materials in 3D printing applications. Their capacity to yield and re-solidify in response to the application and removal of stress allows printing nozzles to be translated through them and printed inks to be deposited within them. These properties of jammed materials have been leveraged to 3D print soft structures having complex geometries. However, the interfacial tension between the jammed system and printed ink can drive the printed structures to break-up, limiting the minimum feature size that can be printed. To prevent these interfacial instabilities and stably 3D print structures with fine features, we have investigated pairs of inks and jammed support materials having ultra-low interfacial tension. In this presentation, we will describe our research on 3D printing inks and jammed support materials made from chemical variants of polydimethylsiloxane. Preliminary results on the phase behavior of these materials, the rheology of the relevant jammed support materials, and the printed structures will be discussed.   

The Importance of Non-Universalities in Entangled Polymer Melts During the Startup of Steady Shear Flow

The importance of non-universality in inception of shear flow at large strain rates is examined using the discrete slip-link model (DSM). An expression for the Rouse relaxation time, τ_R, as a function of entanglement activity and number of Kuhn steps is found from a master curve of strain maxima. Unlike tube models τ_R is predicted by the DSM instead of added as an adjustable parameter. We find that DSM predicts only a weak dependence of τ_R on chemistry. DSM predictions are then compared to a large set of experimental data for melts and solutions, where agreement is found over three decades of Rouse-Weissenberg number, Wi_R. In contradiction with conclusions based on tube models, we find no significant non-universality in the scaling of either strain maxima or stress overshoot. Finally, our results show that the overshoot stress and strain scale as Wi_R^0.33, while the undershoot and steady-state stresses scale as Wi_R^0.1.   

Inhomogeneous Elastic Yielding after Planar Extension under Confinement

Entangled melts can be demonstrated to terminate homogenous uniaxial extension before reaching the steady flow state, due to localized yielding. Additionally, such melts break up after stepwise extension in absence of any ongoing extension. This is coined elastic yielding by us since its first report. To further explore the nature of the elastic yielding, we conduct lubricated squeezing to achieve planar extension with confinement. It is shown¹ that elastic yielding still occurs in an inhomogeneous manner, “breaking up” without breakup. This result adds to the current understanding presented in Chapter 12 of the book “Nonlinear Polymer Rheology”.²

1. Yuan, R.; Wang, S.-Q. Inhomogeneous chain relaxation of entangled polymer melts from stepwise planar extension in absence of free surface. J. Rheol. 2020, 64, (5), 1251-1262 DOI 10.1122/8.0000093.
2. Wang, S.-Q., Nonlinear Polymer Rheology: Macroscopic phenomenology and Molecular foundation. Wiley: Hoboken, NJ, 2018.   

Deformation Model of Chains and Networks with Extendable Bonds

The nonlinear stress-strain behavior of polymer networks, manifested in the monotonically increasing instantaneous modulus, is a product of the nonlinear deformation of individual network strands. This nonlinear network response to external deformations is described in the framework of a network model relating macroscopic stress-strain response to force-elongation behavior of individual network strands with finite bending rigidity and extendable bonds. The developed approach correlates network shear modulus, bond deformation modulus and extensibility ratio with the strands’ Kuhn length, bond elastic constant, and their dimensions in undeformed and fully extended states. The model’s accuracy is tested in coarse-grained molecular dynamics simulations of chain and diamond network deformations covering both linear and nonlinear deformation regimes. The model is used to describe different deformation regimes of DNA molecules and biological networks of fibrin, collagen, and neurofilaments and to obtain elastic constants characterizing these systems’ mechanical responses.   

Deformation of Hybrid Networks

Hybrid networks - networks consisting of different types of strands which could differ by their degree of polymerization (DP), chemical structure, and rigidity (Kuhn length). Examples of hybrid networks include biological networks and gels made by crosslinking biopolymers with various binding protein and networks made by crosslinking graft polymers through their side chains. Here we report on a theoretical model and coarse-grained molecular dynamics simulations of hybrid networks made of two types of strands. The model is built on the phantom network model of nonlinear springs which properties are derived from the force-deformation response of individual chains and are characterized by the Kuhn length, DP, and bond deformation potentials. The developed approach self-consistently accounts for entropic elasticity, bond deformation, and continuous redistribution of stress between different network strands as they undergo nonlinear deformations. The model predictions are tested by molecular dynamics simulations of hybrid network deformations. In particular, simulations confirm a breakdown of the simple mixture rule and the “weakest link” concept in the nonlinear deformation regime.   

Updating classical swelling theory with loops: experiments and real elastic swelling theory

Understanding the effect of loop defects on polymer network properties is an important step to improving the accuracy of predictions. Equilibrium swelling ratios were measured for a set of gels with known loop fractions, extending prior work on loop counting with network disassembly spectroscopy. As expected, loopier gels demonstrated a higher degree of swelling due to a reduction of elastically effective strands. Building off of the Real Elastic Network Theory (RENT) developed for predicting linear viscoelasticity as a function of loops, the Real Elastic Swelling Theory (REST) incorporates RENT as the elasticity model in the classical Flory-Rehner swelling equation. To avoid error in estimation of the Flory-Huggins chi parameter, a non-dimensional master equation was derived, demonstrating a collapse across all good solvents. For gels with loop densities low enough to accurately assume loops were not correlated, REST was more accurate than models that do not account for loops. Despite the increased accuracy in REST, persistent deviations suggest the need for further improvements in elasticity models, especially in the high defect regime.   

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

The di-epoxy diglycidyl ether of bisphenol A (DGEBA) is reacted with the curative diethanolamine (DEA, HN(CH₂CH₂OH)₂). 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 (or gelation) 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. This results from the complex series of reactions associated with the zwitterion driven addition reaction characteristic of many epoxy/tertiary amine systems. Here we measure the gel point across a wide range of temperature. For the extended tests necessary at low temperatures, a simple bubble viscometer is used, while traditional rheology is employed for high temperature tests.   

Role of Chain Walking and Hopping on Anomalous Self-Diffusion in Linear Associative Polymers

Associative polymers have been shown to exhibit apparent superdiffusion on mesoscopic length scales, attributed to transitions between diffusive modes such as chain walking, hopping, and cluster motion. Here, the effect of sticker density on walking and hopping dynamics in linear associative polymers is probed via Brownian dynamics simulations and forced Rayleigh scattering (FRS) across a wide range of length and time scales. Unexpectedly, the FRS results show superdiffusive scaling for all sticker densities probed, suggesting an ability to hop even for chains with up to 15 stickers. Simulations reveal that hopping can occur in chains with high sticker density due to their enhanced propensity to form loops, which partially counteracts the effect of the additional stickers and increases the likelihood for complete dissociation from the network. The simulations also show that, surprisingly, superdiffusive scaling can arise from walking alone, which is attributed to an increase in strand fluctuations upon unbinding of a sticker compared to shorter timescales when caging restricts motion. Scaling arguments are developed to predict the characteristic walking and hopping diffusivities, finding good agreement with simulation.   

Probing the distribution of localization lengths in amorphous solids via wavelength-dependent elasticity

The amorphous solid state exhibited, for example, by randomly crosslinked macromolecular systems has two distinguishing features: (i) it arises via a continuous transition controlled by the crosslink density; and (ii) as a result of the intrinsic randomness, the state is described by a distribution of single-particle localization lengths. Owing to the continuity of the transition, in its vicinity the localization-length distribution is concentrated predominantly at intermediate lengthscales, i.e., lengthscales larger than microscopic but not truly macroscopic. We report on the development of an elasticity theory for the amorphous solid state that is valid not only in the limit of long wavelength strains but also for strains at wavelengths corresponding to intermediate lengthscales. The corresponding wavelength-dependent shear modulus is sensitive to the distribution of localization lengths, diminishing monotonically with decreasing lengthscale -- a physical reflection of the idea that elasticity at a given legnthscale is primarily supported by particles localized on that or shorter lengthscales. The dependence of the shear modulus on wavelength therefore provides an experimental pathway to probing the distribution of localization lengths.   

Hydrolysis-induced large swelling of polyacrylamide hydrogels

Polyacrylamide hydrogels are widely used and studied. The feature of hydrolysis in polyacrylamide gels is often ignored. After neutral polyacrylamide gels are hydrolyzed under alkaline conditions, they become polyelectrolyte gels and exhibit large swelling capability. In this work, we establish a non-equilibrium thermodynamic theory to describe large deformation of polyacrylamide gels in the context of hydrolysis. In particular, our theory accounts for the auto-retardation of the reaction. A thermodynamic consistent reaction kinetics is proposed. The theoretical model is validated by comparing with experiments. Our work might be important for the utilization of polyacrylamide gels.   

A Study of Mechanical Behavior of Ethylene Vinyl Alcohol (EVOH) in Contrast to Poly(Lactic acid) (PLA) and Polypropylene (PP)

Semicrystalline polymers are usually brittle below their T_g, with PLA and PP being well-known examples. Based on our recent understanding of ductility in glassy polymers¹, we have recently demonstrated how to acquire a semi-crystalline PLA that is super tough, optically clear and rigid at 120 ^oC, well above its T_g.² The interplay between crystallization and vitrification is further explored based on the example of EVOH that appears to contrast sharply with PLA in all aspects, with an emphasis on the effect of pre-deformation.

1. Wang, S. Q.; Cheng, S.; Lin, P.; Li, X. A phenomenological molecular model for yielding and brittle-ductile transition of polymer glasses. J Chem Phys 2014, 141 (9), 094905 DOI: 10.1063/1.4893765.
2. Razavi, M.; Wang, S.-Q. Why Is Crystalline Poly(lactic acid) Brittle at Room Temperature? Macromolecules 2019, 52 (14), 5429-5441 DOI: 10.1021/acs.macromol.9b00595.   

Crystallization Induced Stress Decay in Crosslinked Shape-memory Networks

Polymer crystallization within crosslinked elastomeric networks can reduce mechanical tension, forming the basis for semicrystalline shape-memory polymers. However crystallization is path-dependent and is sensitive to both mechanical strain and thermal undercooling. By utilizing in situ wide-angle x-ray scattering, we investigated the coupling between isothermal crystallization and stress decay under fixed strains ranging from 25 to 100%. Our results showed that significant stress reduction can occur at only a few percent crystallinity (<0.05). Depending on the annealing process, the dependency of stress decay could either match or surpass the dependency predicted by Flory in crosslinked natural rubber. This study contributes to understanding the interrelationship between crystallization, stress reduction, and material stiffening to support further development of semicrystalline shape-memory networks.   

Comparison of Stress- and Strain-Induced Polymer Crystallization

Polymer processing is featured with stress-induced crystallization with its microscopic mechanism in competition with stress relaxation. We have set up a platform of molecular simulations on strain-induced polymer crystallization, and studied bulk homopolymers, random copolymers, polymer solutions and blends [1-5]. We further developed a polymer stress relaxation model to gain insights into difference between uniaxial stress- and strain-induced crystallization of homopolymers at various stretching rates and temperatures. We investigated crystallization kinetics and the morphological features with low, medium, and high stretching temperatures. The results show that the stress relaxation of the local chain segment and the overall orientation of the sample has obvious influence on the crystallization behavior at low temperature. The differences between stress- and strain-induced crystallization will facilitate our better understanding of the stress relaxation in polymer processing at the molecular level. The financial support of National Natural Science Foundation of China (Grant No. 21734005) is appreciated!   

Threshold Tie Molecule Concentration for the Brittle-Ductile Transition in Linear Polyethylene

Linear polyethylene (PE) can be either brittle or ductile, depending on molecular weight and crystallization history. The brittle-ductile transition (BDT) occurs when the polymer is able to form sufficient tie molecules (TM), polymer chains that connect different crystallites across the amorphous layer. The Huang-Brown model predicts the concentration of TM by calculating the fraction (P) of polymer chains whose end-to-end distance in the melt is equal to or greater than a critical distance in the semicrystalline solid, defined by the sum of two crystal and one amorphous layer thicknesses. The threshold TM concentration needed for the BDT, P_BDT, is experimentally evaluated by observing an abrupt change in the breaking strain of narrowly-distributed (D < 1.2) PEs with varying molecular weights and their bimodal blends, synthesized by ring-opening metathesis polymerization followed by catalytic hydrogenation. TM concentrations are further varied by either quenching (Q) or slow-cooling (SC) the PE from the melt. The results show that the P_BDT of SC-PE is lower than that of Q-PE, and a further comparison with literature data for hydrogenated polybutadiene suggests that increased crystallinity lowers the P_BDT of PE, favoring ductility.   

Microrheology of topologically-active DNA solutions

DNA naturally exists in three distinct topologies - supercoiled, relaxed circular, and linear - with enzymes that convert one topology to another. Changes to DNA topology can give rise to changes in the viscoelastic properties of dense DNA solutions. Thus, topological conversion of DNA offers a way to design active biopolymer solutions that can alter their rheological properties in programmable ways. Here, we create highly entangled solutions of DNA molecules that actively undergo enzyme-driven conversion from supercoiled to linear topology over the course of several hours. We perform time-resolved particle-tracking microrheology during the conversion to examine how changes in topology affect the viscoelastic properties of DNA solutions in real-time. We show that these ‘topologically active' biopolymer solutions exhibit time-dependent non-equilibrium dynamics and viscoelasticity that can be tuned by the concentration of the constituents. For example, solutions undergo viscous thickening as molecules convert from supercoiled to linear topology. In future work, we will examine different DNA lengths, concentrations, topologies, and enzymatic activities to create a class of programmable topologically active materials.   

Characterization of molecular mobility in covalent adaptable networks

Covalent adaptable networks (CANs) are crosslinked polymer networks that contain molecular moieties capable of covalent bond rearrangement upon exposure to a specific stimulus. This molecular restructuring imparts greater rheological control over traditional thermosets, allowing the creation in recent years of materials with superior stress relaxation, fatigue resistance, shape memory, and actuation properties. While molecular scale mobility is a key characteristic of CANs, very little work exists explicitly examining the phenomenon of transport through these networks. In this work, diffusion of small molecules and macromolecules in a dynamic network is characterized. Fluorescence recovery after photo-bleaching (FRAP) is used to evaluate differences in diffusivities of fluorescent molecules within a nucleophile-catalyzed thiol-thioester exchange CAN compared to a traditional thermoset of similar composition and crosslinking density. The effects of fluorophore size compared to network mesh size (crosslinking density) are evaluated for both the CAN and traditional thermoset. This work provides insights into the molecular underpinnings of CANs’ rheological behavior, with applications in membranes, drug delivery, and separations.   

The dynamics of probe particles inside a polymer network

The diffusion of probe particles inside a polymer gel-like medium is a subject of great interest. Examples include protein diffusing through mucus membrane, nuclear pore complex or small molecules diffusing in polymer films and many more. We use coarse-grained molecular dynamics simulations to investigate the dynamics of the probe inside a polymer network made on a lattice representing a ordered polymer gel. Our study shows that the dynamics of the probe particle inside the polymer gel become increasingly subdiffusive with increasing probe size, network rigidity, and the probe-polymer affinity. Additionally, velocity autocorrelation functions have negative dips due to the viscoelastic nature of the gel. The van-Hove correlation function shows fat non-Gaussian tail for the sticky bigger probe particle as it locally stretches the network but the fat tail vanishes on increasing the rigidity of the network. Our simulation results are in qualitative agreement with a series of experiments on probe diffusion in polymer gel.   

Soft adhesive latches to control the kinematic response of a recoiling band

Mantis shrimp, salamanders, and chameleons are just a few of nature’s overachievers relying on latch-mediated spring-actuation (LaMSA) to produce ultra-fast, powerful movements. Though latches are fundamental to LaMSA, the study of latch control is currently in its infancy and warrants further understanding to engineer LaMSA systems. We examine the impact of materials properties on the performance of contact adhesive latches. We conduct high-speed adhesion tests measuring the force and contact area as a probe-tipped polyurethane band is stretched to make contact with the adhesive latch. Altering the band strain input allows us to control the deformation mode observed during high-speed imaging of interface separation. Energy not dissipated by the adhesive latch is converted into kinetic energy of the recoiling band. By tuning the adhesive properties of the latch, we can control the maximum velocity (23 ms^-1) and acceleration (6.2 x 10³ ms^-2) of the recoiling band. The insights gained from this work will provide a better understanding of how LaMSA systems rely on latches to store and mediate energy to achieve small, power-amplified movements.   

Tracer Diffusion of Polystyrene in Poly(styrene-ran-styrenesulfonate)

In this study, we investigate the tracer diffusion of deuterated polystyrene (dPS) in random copolymers of poly(styrene-ran-styrenesulfonate) (PS-SS_x) by forward recoil spectrometry (FRES). The sulfonation level of the random copolymer PS-SS_x ranges from 0.2 to 1.0 mol% sulfonation. At x < 0.7 mol%, dPS is fully miscible with PS-SS_x and the tracer diffusion is characterized by standard Fickian diffusion. Between 0.7 mol% ≤ x ≤ 1.0 mol%, dPS is partially miscible with PS-SS_x, and the tracer diffusion profile is composed of a surface peak and a Fickian diffusion tail. Interestingly, as the sulfonation level increases from 0 to 1.0 mol%, the tracer diffusion coefficient decreases by a factor of three. The decrease in diffusion coefficient with increasing sulfonation level is attributed to an increase of monomeric friction coefficients. This is further supported by rheological experiments, which reflect a slowing down of chain dynamics with increasing degree of sulfonation.   

Characterization of Janus Micromotors Using Dynamic Light Scattering

Recent advances in understanding the active motion of Janus micromotors has established their use as autonomous devices for applications spanning from drug delivery to environmental remediation. While their design is relatively simple, their non-equilibrium enhanced motion is incredibly complex and difficult to characterize. Traditional characterization methods of active motion in colloidal systems rely on optical microscopy (OM) which is cumbersome and requires manual analysis to identify anomalies that could be mistakenly classified as autonomous motion. To address these issues, a high-throughput dynamics characterization method utilizing dynamic light scattering (DLS) has been developed. The established method of data analysis decouples the rotational and translational diffusion of these micromotors and their influence on active motion. This allows for changes in diffusive properties to be characterized as a function of fuel concentration, solution aging time, and particle concentration. Utilizing DLS for the characterization of active motion will enable mechanistic studies of the non-equilibrium dynamics of Janus micromotors.   

Selective Lithium Ion Transport in Mixed Electron- and Ion- Conducting Radical Polymer-Based Blends

Interest in polymers capable of mixed electron and ion conduction has increased in recent years due to their use in a variety of organic electronic devices. Radical polymers have received less attention than conjugated polymers in this field despite their promise as solid-state charge conductors. To address this, we developed a radical polymer-based system by blending a radical polymer, poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PTEO) with poly[poly(ethylene oxide) methyl ether methacrylate)] (PPEGMA) and lithium hexafluorophosphate. Consistent with previous reports, PPEGMA had a room temperature ionic conductivity of 10^-4 S cm^-1; one of the highest polymer-based lithium-ion conduction values reported. Additionally, PTEO had an ionic conductivity of 10^-6 S cm^-1. A blend of the two polymers at equal weight ratios had a room temperature ionic conductivity similar to PPEGMA and electronic conductivity similar to PTEO. This was due to the clear microscale phase separation between the two polymers, which yielded pathways of distinct ion and charge conduction (i.e., through PPEGMA and PTEO domains respectively) with Li⁺ mostly incorporating in the PPEGMA domains. This work elucidates the impact phase separation has on mixed conduction in radical polymer-based blends.   

Crosslinked PEDOT Thin Film Electrode for Microbial Bioelectrochemical Systems

Microbial bioelectronics combines microbes and electronic devices for diverse applications including energy harvesting, chemical production, and health monitoring. The biotic-abiotic interface in bioelectronics can connect the biology and electronic worlds and facilitate cell-material communication. Materials with nice conductivity, mechanical property, and biocompatibility at the interface such as poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) have been employed in bioelectronics. Recent studies electropolymerized PEDOT:PSS to encapsulate microbes, but the microbe-material interaction and biofilm formation were not fully investigated. Here, we used a photocrosslinking method to coat PEDOT thin film on indium tin oxide (ITO) electrode for proliferation and immobilization of exoelectrogenic microbe, Shewanella oneidensis. The electrode was fabricated by crosslinking PEDOT:PSS with 2-hydroxyethyl acrylate under long-wavelength UV light. Microbe viability and attachment were excellent on the crosslinked thin film electrode, and the film greatly improved current density comparing to plain ITO electrode. This work studies the microbe immobilization and biofilm formation on a crosslinked PEDOT thin film electrode and can be applied to future bioelectronics design.   

Verdazyl-based Radical Polymers and their Performance in Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) have been implemented in a wide array of sensing and biomedical applications. These devices typically employ conducting polymers based on a doped conjugated polymer design motif. Our work employs non-conjugated radical polymers in these devices due to their ambient stability, scalable syntheses, tunable electronic properties, and optical transparency relative to conjugated macromolecules. Here, we describe the design, synthesis, optoelectronic, and electrochemical properties of a next-generation radical polymer. Specifically, we have evaluated the performance of polyoxo-3-(2-mercapto ethyl)-1,4-dihydro-1,2,4,5-tetrazin-6-one (PVEO), which is an open-shell macromolecule whose backbone is based upon an ethylene oxide repeat unit structure. This macromolecular backbone motif imparts a low glass transition temperature to the radical polymer, which we have previously established is critical for high-performance electronic properties to be had. Spin characterization of PVEO revealed a Lorentzian-shaped curve, indicating a high degree of radical-radical coupling. This further implies that the presence of a high density of redox-active sites would potentially provide sufficient conductivity required for the operation of the OECTs.   

Charge and Ion Transport in Radical Polymer-based Organic Electrochemical Transistors

Radical polymers are composed of a non-conjugated macromolecular backbone with open-shell sites that are present on the side chains of each repeat unit. Previously, we have demonstrated that this macromolecular design motif alters the route by which charge is transported in solid-state electronic devices when compared with the more common conjugated polymers utilized in organic electronics. Here, we report on the application of a specific high-performance radical polymer, poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PTEO), as the active layer component in an electrolyte-gated organic electrochemical transistor (OECT) device structure. Specifically, we establish the differences in charge and ion transport (i.e., mixed conduction) in these inherently glassy radical polymer systems relative to many oft-used OECT semicrystalline macromolecular materials. Moreover, we highlight the unique synergies that exist between mixed conduction that exist in non-conjugated radical polymer systems. Finally, we manipulate the degree of crosslinking within the radical polymer thin films after casting of the film (i.e., through the use of photoinitiated crosslinking) in order to demonstrate a mechanically-stable, solvent-robust OECT that achieves high device performance as well.   

Evaluation of Ion Penetration Dynamics in Conjugated Polymers Using Bilayer OECTs

Recently, conjugated polymer based organic mixed semiconductors have attracted increase interests due to their simultaneous ionic-electronic transport properties, enabling unique operations in electrochemical devices. Accurate determination and control of ion penetration is vital to design and screen promising mixed semiconductors. Here, we demonstrate a simple method to electrically evaluate ion penetration dynamics in various conjugated polymers using a bilayer organic electrochemical transistor (OECT) structure, where the top poly (3, 4-propylenedioxythiophene) (PProDOT) and buried PEDOT:PSS layers serve as the ionic buffer and electronic transport layer, respectively. The specific separation of these two conducting pathways allows precise measurements of ion penetration timescales. Upon increasing the hydrophilicity of the top buffer layer, significant accelerate of response timescales was observed for high hydrate anions (Cl^-), which only changed slightly for low hydrate alternatives (TFSI^-). These results are consistent with the calculated diffusion constants using cyclic voltammetry. Our findings offer a simple and direct method to evaluate ion penetration dynamics in a wide range of polymers and can also serve as a general tool for re-establishing OECT operating models.   

Developing a Flexible Electrochemical Sensor for Trace Detection of Energetic Materials

Manually inspecting packages which may contain explosives presents inherent hazards from unplanned detonation. We are developing a sensor for remote explosives detection using a 3,4-propylenedioxythiophene (ProDOT)-based copolymer channel organic electrochemical transistor (OECT). Specifically, we describe the OECT behavior using a range of electrolyte ionic strengths especially at highly dilute (< 1 mM) ionic strengths. The ProDOT channel provides an example of accumulation mode sensing, benchmarked against standard poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) based OECT sensors. Additionally, a functionalized polyethylene-glycol dimethacrylate molecularly imprinted polymer (MIP) barrier is placed atop the channel to serve as a selective layer. Minimizing false positives, the additional layer produces a species selective barrier; this directly impacts OECT dynamics allowing us to precisely isolate the polymer morphology in species transport. This sensor will describe OECT versatility in both depletion and accumulation mode operation, while holistically characterizing electrolyte concentration effects on transport roles in sensor functionality.   

Comparison of doping mechanism and efficacy between polymers doped by novel Lewis acid dopant BCF and F4-TCNQ

Doping of polymers has been a longstanding area of research in an effort to enhance their electrical properties, yet typically used π-electron acceptors such as F4-TCNQ are limited by solubility and insufficiently deep electron affinity. Alternative dopants based on Lewis acids have attracted attention recently due to promising results on a range of polymers, but the doping mechanism is not yet fully understood. We have studied the Lewis acid dopant BCF with three polymers, using a range of optical techniques and comparing with F4-TCNQ doped samples. Absorption spectroscopy has revealed polaron formation as well as confirming a protonation-based doping mechanism. FTIR spectroscopy has shown to be particularly useful in such protonation-based doping by revealing evidence of the specific protonation site and its effect on the polymer. Finally, pump-probe measurements help to uncover whether different doping mechanisms affect ultrafast charge dynamics and polaron excitation in doped polymers. By pumping and probing the polaron directly, we are able to monitor the excitation and evolution of the polaron within 50 ps of excitation for both dopant types. These results help to further the understanding of a novel type of dopant and test their suitability for use in organic electronics.   

Structure-thermal/mechanical property relationship of semiconducting polymer thin films

The past decades have witnessed surging developments of semiconducting polymer thin films (<100 nm) in the flexible/wearable electronics field. Despite much efforts in enhancing their electronic performances, the fundamental understanding of their thermal and mechanical behaviors has been limited. This is mainly because of their heterogeneous rigid backbone/soft side-chain structure and a lack of characterization techniques. Here, we deconvoluted the backbone and side-chain effect on thin-film thermal and mechanical properties through thin-film dynamic mechanical analysis (DMA) and film-on-water (FOW) tensile tests. A model diketopyrrolopyrrole (DPP)-type polymer is selected to show the effect of chemical moiety choice on thin-film glass transition temperature (Tg) and modulus. It is noticed that the inclusion of thiophene groups in the polymer backbone increases the backbone Tg, while a longer side-chain length can sufficiently depress the backbone dynamics. A Tg predictive model is developed to capture such an effect and provide a design strategy for the engineering of new semiconducting polymers.   

Conjugated Radical Polymers for Organic Electrochemical Transistors

OECTs hold great potential in myriad applications, including neuromorphic devices, circuit elements, and bioelectronic sensors because OECTs are capable of transducing and amplifying ionic fluxes into the electronic signals with a high signal-to-noise ratio. The most oft-used electronically-active polymer implemented in OECTs is the commercially-available poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS); however, modifying the structure and electrochemical properties of PEDOT is difficult. Herein, we develop 3,4-propylenedioxythiophene (ProDOT)-based polymers which have much more readily tunable properties and chemical versatility relative to the EDOT-based polymers. In addition, we incorporate the nitroxide radical group (TEMPO) to the poly(3,4-propylenedioxythiophene) conjugated backbone to improve the electron and ion transport. The spectroelectrochemical analysis shows that as increasing positive potentials is applied, a polaron band appears at around 900 nm followed by the appearance of a bipolaron band as the potential increases to about 1.2 V. This work establishes the OECT performance of the P(ProDOT-TEMPO) polymer and determines the mixed ion-electron transfer mechanism for the conjugated radical polymer.   

Molecular switches for organic electronics: A computational approach to oligothiophene-azobenzene hybrids

Molecular switches based on azobenzene (azo) are defined as light-responsive molecules which can change between two configurational states under light stimuli. Responsive azo monolayers can be used to modulate the work function, i.e. they tune the properties of the interfaces – at the electrodes or nanoparticles. In this work, we investigate what happens to the structures, optical properties, and the charge hopping within azo-bithiophene (azo-BT) hybrid monolayers depending on the light stimulus using various computational approaches. Two types of hybrids that differ in the order of azo and BT counting from the anchor group are modelled: azo-BT and BT-azo. One of them (BT-azo) has been studied experimentally by Karpe et al. [1], whose data are used for the model validation. We describe trans- and cis-isomers for each hybrid and conclude that the isomers of BT-azo are more stable. The optical spectra for trans-isomers are similar, whereas the cis-states are characterized by different positions and intensities of the absorption bands. The charge hopping is evaluated for the coupled dimers within Markus-Hush theory of electron transfer.
[1] Karpe S. et al. Chem. Commun. 46 (2010) 3657   

Influence of injection layer on transient electroluminescence lifetime in OLED during switch-off

We present a study of the transient electroluminescence (TREL) of a polyfluorenes-blend based light emitting diodes during the turn-off cycle. We observed a bi-exponential decay of the transient electroluminescence, a fast decay with a lifetime of tens of nano-seconds, followed by a slow decay with a lifetime of hundreds of nano-seconds. We attribute the slow decay to the transit time via drift of the mobile charges between the space charge regions during discharge. We tested this hypothesis using various methods that affect the electric field in the bulk region. In particular, we modified the injection efficiency by varying the thickness of a LiF electron injection layer. The variation of the injection layer thickness influences the width of the space-charge region and hence influences the division of the voltage drop between the space charge regions and the bulk. In all cases, the electric field inversely correlated with the measured decay time, supporting our assumptionThese observations are supported by simulation based on the drift-diffusion model.   

Fabrication of flexible inkjet-printed copper electrowetting valve for capillary-driven microfluidic devices

Research in microfluidic valves has led to advances in the control of liquids in capillary-driven microfluidic devices. Herein, we fabricated low-voltage-activated microfluidic valve on a flexible substrate based on the principle of electrowetting on dielectric. Using inkjet printing technique, copper-based electrodes were deposited on poly(ethylene terephthalate) substrate, followed by coating a thin layer of poly(perfluorooctyl methacrylate) (pPFOM) on top of electrodes via initiated chemical vapor deposition (i-CVD). pPFOM coating thicknesses were well controlled from 20 nm to 200 nm. A droplet was placed on top of the electrode and a voltage was applied in between of the droplet and the electrode to polarize the pPFOM thin film, thus cause a contact angle decrease of the droplet. The results show that the contact angle decreasing process has a faster response under thinner pPFOM coating thickness and higher applied voltage. Finally, the valve can be actuated under 2V, which is among the best results in the literature. Furthermore, a microfluidic device with the designed pattern is demonstrated for potential bioelectronic applications.   

Rylene-imide Incorporated Nonfullerene Electron Acceptors with Enhanced Photovoltaic Performance

In organic solar cells, rylene diimides and rylene imides, exemplified by perylene diimide and naphthalene diimide, are common polymeric and molecular electron-deficient building blocks known for their high electron affinity, high electron mobility, enhanced electrochemical redox stability, and tunable optoelectronic properties. Despite their ubiquity, photovoltaic performance of rylene-imide nonfullerene acceptors (NFAs) has lagged behind fused-ring electron acceptors (FREAs). Recently, we synthesized a new series of FREAs and non-FREAs featuring naphthalene-imide end-capping units in donor-acceptor architectures. These NFAs, bis(naphthalene-imide)arylenelidenes (BNIAs), were found to exhibit enhanced electrochemical redox stability, high carrier mobilities and high photovoltaic performance (>10%) when blended with electron donor polymer PBDB-T. The blends were found to have similar electron mobilities between FREAs and non-FREAs, suggesting the enhanced photovoltaic performance of the FREAs were not due to electron mobilities, but differences in blend morphology and photophysics. These results demonstrate the potentials for rylene imide electron acceptors as a design strategy towards more efficient and electrochemically robust materials for organic solar cells.   

Effects of Selenophene Substitution in NDI-based n-Type Semiconducting Polymers for All-Polymer Solar Cells

We have investigated how substitution of selenophene for thiophene in the alternating naphthalene diimide (NDI)-thiophene copolymer (PNDIT-hd) resulting in NDI-selenophene copolymer (PNDIS-hd) influences the individual polymer properties and the performance of their binary blends with donor polymer PBDB-T in all-polymer solar cells (all-PSCs). The two polymers have identical electronic structures and similar bulk electron mobility; however, PNDIS-hd has a narrower optical bandgap of 1.70eV than that of PNDIT-hd (1.77eV). PBDB-T:PNDIS-hd all-PSCs were found to combine a high power conversion efficiency (PCE) of 8.4% with high external quantum efficiency (EQE=86%) and a high fill factor of 0.71, which are significantly enhanced compared to the corresponding PBDB-T:PNDIT-hd devices with 6.7%PCE and 73%EQE. The improved photovoltaic properties of the selenophene-containing acceptor copolymer are due to enhanced light harvesting, favorable molecular packing in blends and reduced charge recombination losses. These findings demonstrate that selenophene substitution for thiophene in donor-acceptor copolymers is an effective strategy that enhances the intrinsic polymer properties as well as the performance of their blend solar cells.   

Functionally Graded Semiconducting Polymer Thin Films as Organic Thermoelectrics

Molecularly doped semiconducting polymers have demonstrated great potential in organic thermoelectrics (TEs) for thermal energy management. Along with the efforts to increase material’s figure of merit, the use of functionally graded materials (FGMs) where TE properties are spatially controlled opens pathway to further improve TE device performance. However, experimentally fabricating FGMs has been a challenging task. In our study, we utilize the facile processability to modulate electronic properties through molecular doping of conjugated polymers to fabricate and characterize thin film of organic FGMs with gradient in dopant composition. Our work focuses on understanding the structural and TE properties (Seebeck coefficient and electronic conductivity) across the FG polymer films. Using 1D thermoelectric coupling model, we also predict the thermoelectric cooling performances based on our experimental transport properties. Cooling temperature, ΔT, and coefficient of performance are calculated through linear constitutive relations coupled with conservation of charge and energy. The results demonstrate that ΔT of graded samples are significantly improved compared to that of uniform profile. This study provides guidelines to further development on more complex FGMs.   

Ion Dynamics and Conductivity of Polymerized Ionic Liquids in the Confining Geometry: Investigating the Roles of Molecular Structure and Counterion

In the current research, thin film of polymerized ionic liquid with different molecular structures prepared by solvent casting on a silicon wafer substrate. Broadband dielectric spectroscopy (BDS), vibrational spectroscopic methods (FTIR, Raman Spectroscopy), and X-ray scattering were employed to explore the effect of molecular structure on the charge transport and dynamics of the polymerized ionic liquids (PILs) in the one-dimensional confinement in comparison with the bulk. The effect of interfacial interactions, film thickness, and confinement on the ion was determined. The conductivity of the confined samples governed by molecular structure, interfacial interaction, and film thickness. This study provides new insight into understanding the ion dynamics of the PolyILs in confining geometries.   

Electrical conductivities of ternary polyelectrolyte complexes based on PEDOT and sulfonate polymers with various processing treatments

A polyelectrolyte complex of poly(3,4-ethylenedioxythiophene) (PEDOT) and polystyrene sulfonate (PSS) has shown an electrical conductivity of >10³ S/cm through post-deposition processing and visible transparency of >80%. The composite, PEDOT:PSS in short, is therefore widely used in various industries. In this work, we synthesized ternary polymer systems using PEDOT, PSS, and various third components, including poly(vinyl sulfonate) (PVS), by in-situ polymerization of monomers in presence of the sulfonate polymers. We then studied the effects of compositions and processing parameters on the electrical properties of the ternary composites. We confirmed that the conductivity of as-synthesized composite films reached ~20 S/cm when PSS:PVS=1:1, which was three times higher than that of the as-prepared binary PEDOT:PSS (~7 S/cm). We also applied various processing methods on the ternary polymer systems by varying the dipping solvents and studied the UV-Vis-NIR absorption characteristics and the electrical conductivities. Different degrees of property changes were observed depending on the processing conditions.   

A non-conjugated polymer with flexible backbones bearing spin-delocalized pendant radical groups

Radical polymer thin films have shown high electrical conductivity values when locally ordered domains form after thermal annealing. However, most efforts in the radical polymer studies have focused on polymers bearing spin-localized nitroxide radical groups, and macromolecules bearing spin-delocalized radical groups have received little attention. To open new insights into the solid-state charge transport mechanism in radical polymer thin films, a polysiloxane-based polymer bearing galvinoxyl radical groups has been synthesized. Density functional theory (DFT) calculations predicted that the spin delocalization behavior of the galvinoxyl group would result in a higher charge transfer rate compared with nitroxide radical systems. It is determined that the flexible backbone endowed the polymer with a glass transition temperature around 0 °C, and this feature allowed the molecules to pack into ordered structures after thermal annealing. Furthermore, the conductivity of this radical polymer was quantified after being cast into a thin film. Thus, these studies provide a strategy to direct molecular packing and facilitate charge transport in radical polymers with delocalized open-shell sites, which can aid in deciphering the charge transport mechanism in radical polymer thin films.   

Mechanisms of ion transport in lithium salt-doped polymeric ionic liquid electrolytes

Recent experimental results have demonstrated that polymeric ionic liquids (polyILs) doped with Li salts exhibit enhanced ionic mobilities and lithium ion transference numbers with increasing salt concentrations. In this study, we used atomistic molecular dynamics simulations on a model system of lithium salt doped 1-butyl-3-methyl-imidazolium bistriflimide ionic liquids and poly(1-butyl-3-methyl-imidazolium bistriflimide) electrolytes to identify the molecular mechanisms underlying such findings. Our results mirror qualitatively the experimental results on the influence of salt doping on the ion mobilities. Further, a surprisingly stronger dependence (coupling) between the lithium ion mobilities and polymer segmental dynamics is observed relative to the coupling between the anion diffusivities and polymer dynamics. We present results for ion coordination and hopping characteristics to rationalize such behaviors and identify the mechanistic origins of the properties of this emerging class of polymer electrolytes.   

Tuning Ionic Conductivity and Mechanical Properties of Network SPEs

Solid polymer electrolytes (SPEs) are regarded as a promising solution to next-generation lithium metal batteries. However, the trade-off between ionic conductivity and mechanical properties remains a challenging obstacle. In this poster, we display how molecular engineering polymer networks can tune such properties for LMB applications. We designed, synthesized and characterized different types of PEO-based networks including star-type single networks, comb-type single networks and interpenetrating networks (IPNs) with poly(carbonate) or poly(ionic liquid) chemistry. Our results showed that the network mesh size plays a significant role in tuning ion diffusion as well as mechanical properties. Salt concentration and polymer backbone chemistry were also established to have an impact on ionic diffusion. Compared with poly(ether) systems, the incorporation of poly(carbonate) can help liberate cations and thus improve the cation transport efficiency. Additionally, the introduction of free mobile polymers or mono-functionalized dangling chains can facilitate ionic conduction. Moreover, the comb-type network structure has enhanced mechanical properties compared to the star-type network.   

Charge Density and Hydrophobicity-Dominated Regimes in the Phase Behavior of Complex Coacervates

We investigate the phase behavior of coacervates formed from a library of 54 acrylamide-based copolymers with charge densities ranging from 40-100% and nonionic aliphatic sidechains ranging from zero to twelve carbons in length. The phase behavior of the coacervates is characterized by optical turbidity, revealing three distinct regimes of complex coacervate behavior. For polymers with hydrophilic sidechains, the salt resistance of the polymers decreases with decreasing charge density, while for polymers with the longest hydrophobic sidechains, the salt resistance increases with increasing hydrophobic content. For polymers with intermediate length hydrophobic sidechains, however, there is a critical charge density at which the phase behavior switches from being dominated by the charge density to being dominated by the hydrophobic interactions. Comparison with the solubilities of the component polyelectrolytes suggests that the phase behavior of these materials cannot be understood simply in terms of changes in the polymer-water interaction parameter, motivating an urgent need for models of coacervate phase behavior that accurately reflect the molecular-scale interactions at play.

Funding: American Chemical Society Petroleum Research Fund (58034-DNI7)   

Effects of lithium chloride on the solid state properties of polyzwitterions*

Polyzwitterions (PZIs) have ideal properties such as ionic conduction, thermal stability, and mechanical stability, making them prime candidates to be used as solid polymer electrolytes. In this study we investigate thermal properties of the solid state for a series of specially synthesized PZIs including: poly(sulfobetaine acrylate), PSBA; poly(sulfobetaine methacrylate), PSBMA; and poly(ethyl sulfobetaine acrylate), PESBA. These PZIs can dissolve molar ratios of salt which increases their electrical conductivity. We added LiCl to PZIs at 1-20 wt.% and studied the impact of added salt on water uptake, degradation temperature, solid state heat capacity, and the glass transition temperature, T_g. Thermal gravimetry shows the percentage of bound water ranges from 5-18 wt.%, and increases as LiCl content increases. Using differential scanning calorimetry, we perform cyclic heating and cooling studies where each successive heat cycle reaches a higher maximum temperature. This procedure allows systematic removal of bound water and observation of the plasticization effect on T_g. Results show that addition of LiCl reduces the glass transition of dry PSBA (i.e., containing no bound water) from about 200°C to 150°C for the PSBA/LiCl blend at a 1:1 molar ratio.   

Phase Behavior and Flory-Huggins Interaction Parameter of Polyethylene-based Multiblock Copolymers with Tethered Lithium Sulfonates

We investigated the temperature-dependent phase behavior and Flory-Huggins interaction parameter of polyethylene-based multiblock copolymer with tethered lithium sulfonates, which are single-ion conducting polymers. The polymer contains short polar blocks with a single lithium sulfonate group precisely separated by polyethylene blocks of 12 carbons (PES12Li). At room temperature, PES12Li exhibits layered morphologies with semicrystalline polyethylene backbones. Upon heating, X-ray scattering observes a transition into hexagonally perforated lamella morphology ~70°C. Above the melting temperature of the PES12Li (~110°C), PES12Li exhibits an order-to-disorder transition (ODT) as indicated by the maximum scattering intensity (I_m) at first-order peak (q*). With increasing temperatures, disordered PES12Li melts exhibits increasing I_m^-1 and q*, consistent with typical polymer melt. The random phase approximation is applied at 150 - 170°C to obtain a Flory-Huggins interaction parameter for PES12Li. Understanding phase behavior and measuring the interaction parameter of polyethylene-based multiblock copolymers will facilitate the design of single-ion conducting polymers with controlled morphology and enhanced ionic conductivities.   

Giant spontaneous polarization for enhanced ferroelectric property of biaxially oriented poly(vinylidene fluoride) by mobile oriented amorphous fractions

Poly(vinylidene fluoride) exhibit distinctive ferroelectric property; however, their spontaneous polarization (P_s =60-105 mC/m²) is still inferior to those (>200 mC/m²) of the ceramic counterparts. Here we report an giant P_s (140 mC/m²) for a highly poled biaxially oriented PVDF (BOPVDF) film, which contains a pure β crystalline phase. Given the crystallinity of 0.52, the P_s for the β phase (P_s,β) is 279 mC/m², if a simple two-phase model of semicrystalline polymers is assumed. This high P_s,β is invalid, because the theoretical limit of P_s,β is 185 mC/m² by density functional theory. To explain such a high P_s for the poled BOPVDF, a third component in the amorphous phase must participate in the ferroelectric switching to contribute to the P_s. Namely, an oriented amorphous fraction (OAF) links between the lamellar crystal and the mobile amorphous fraction. The OAF content was determined to be ~0.28, more than 50% of the amorphous phase. The fundamental knowledge obtained from this study will provide a solid foundation for the future development of PVDF-based wearable electronics and soft robotic applications.   

BigSMARTS: A Structurally-Based Line Notation for Macromolecule Search, Classification, and Reactions

Data-driven research, reaction retrosynthesis, and small-molecule property design are rapidly advancing due to the availability of open source toolkits like RDKit that enable search of deterministic small molecule graphs encoded in SMILES (Simplified Molecular-Input Line-Entry System) using the subgraph search syntax SMARTS (SMILES Arbitrary Target Specification). BigSMILES has extended SMILES to represent polymers as ensembles of molecular graphs, motivating a parallel extension of SMARTS to the macromolecular domain. This work discusses the significant expansion and rich complexity of searching polymers through the new search syntax BigSMARTS. BigSMARTS queries the elements of a molecular graph set containing one or more elements of a subgraph set and includes searches within the polymer’s building blocks (repeat units and end groups). BigSMARTS enables polymer reactions to be encoded and searched and coarse-graining searches to classify chain topologies (star polymers and block copolymers) that influence properties. With the development of new search tools based on RDKit to support BigSMARTS, polymer informatics will enjoy the benefits of search that have advanced small molecule design in the era of artificial intelligence.   

Backmapping coarse-grained macromolecules: an efficient and versatile machine-learning approach

Multiscale modeling of polymers exchanges information between coarse and fine representations of molecules to capture material properties over a wide range of spatial and temporal scales. Restoring details at a finer scale requires one to generate information following embedded physics and statistics of the models at two different levels of description. In this work, we present an image-based approach for structural backmapping from coarse-grained to atomistic models with cis-1,4 polyisoprene melts as an illustrative example. Through machine learning, we train conditional generative adversarial networks on the correspondence between configurations at the levels considered. The trained model is subsequently applied to provide predictions of atomistic structures from the input coarse-grained configurations. The effect of different data representation schemes on training and prediction quality is examined. Our proposed backmapping approach shows remarkable efficiency and transferability over different molecular weights in the melt based on training sets constructed from oligomeric compounds. We anticipate that this versatile backmapping approach can be readily extended to other complex systems to provide high-fidelity initial configurations with minimal human intervention.   

Finding Needles in Haystacks: Deciphering a Structural Signature of Glass Dynamics by Machine Learning

The complex, disordered structure of glasses makes it challenging to elucidate how their atomic structure control their dynamics. Here, based on molecular dynamics simulations, we adopt machine learning (ML) to interrogate whether a structural signature governing the dynamics of atoms in a glass can be found. We find that the dynamics of glasses is encoded in their static structures. These results establish machine learning as a promising pathway to “find needles in Haystacks,” that is, to pinpoint important structural patterns in large complex datasets generated by atomistic simulations.   

A Hierarchical Model for Polymer Data

Information in polymer literature is often provided through a combination of text, tables, and illustrations dispersed within multiple documents, making it challenging to collate data across sources using automatic tools. To make polymer data more interoperable and reusable, we propose a model for reporting polymer data. In the model, data are grouped according to the synthetic pathway they belong to. Each synthetic pathway is encoded as a graph interconnecting multiple species. Within the graph, polymers are described by a topological-chemical-physical characterization hierarchy. The first two levels establish the molecular structure of a polymer with line notation and chemical characterizations such as tacticity. On top of that, physical characterizations such as rheology are encoded to provide information beyond single molecules. Furthermore, within each level of characterization, data is organized in another hierarchy: at the bottom level lies raw data such as GPC traces, which can be extracted into intermediate data such as the molecular weight distribution and further distilled into attributes such as the dispersity. Together, the data model provides a template that reveals inherent relationships between distinct data entries, thereby supporting data analytics downstream.   

Multiscale theory, mechanisms, and designs for giant flexoelectricity of elastomers

Soft materials which can undergo large deformations and exhibit a strong coupling between strain-gradient and polarization would have wide ranging applications in nanotechnologies, wearable sensors and electronics, soft robotics, and energy harvesting. This coupling--namely, the flexoelectric effect--is a universal phenomenon. While flexoelectricity is well understood for crystalline materials, the molecular-scale theoretical underpinnings of flexoelectricity in elastomers is lacking. In this talk, using statistical mechanics and network theory, we develop a multiscale constitutive model of elastomers consisting of polar monomers. This not only allows us to explain the flexoelectric effect across scales; it also agrees with a poorly understood, but experimentally validated, mechanism for achieving giant flexoelectricity in elastomers: combining stretching with bending. We conclude by discussing how polymer network architectures may be designed to tune the flexoelectric effect in different directions relative to the strain-gradient. This work contributes to our theoretical understanding and has important implications towards achieving high-fidelity soft sensors, energy harvesters, and soft robots with many degrees-of-freedom mechanisms.   

Simulations of Lignin-Polysaccharide Complexes in the Secondary Cell Wall of Plants for Biofuel Production

We use all-atom molecular dynamics simulations to understand the conformational and dynamical properties of the secondary cell wall components plants. To this end, we specifically model the lignin, hemicelluloses (xylan compounds) and other carbohydrate molecules for Alamo switchgrass plants (Panicium virgatum). First, the influence of hemicellulose in the absence of cellulose on the conformational properties of lignin melt is probed. Subsequently, using hydrated lignin-xylan complex, we analyze the spatial dynamics of water near the lignin-hemicellulose complexes and the structural reorientation of the monolignol and p-hydroxycinnamate subunits of lignin molecules. Finally, an all-atom model, comprising the lignin-carbohydrate complexes with inputs from solid-state NMR experiments, is simulated to understand the organization of the secondary cell wall structure in switchgrass plants. These models will then be used in conjunction with solvents such as tetrahydrofuran (THF) to understand the efficacy of these organic solvents for reducing the recalcitrance of lignin-polysaccharide matrix, thereby leading to effective plant degradation.   

Learning Composition-Transferable Coarse-Grained Models: Designing External Potential Ensembles to Maximize Thermodynamic Information

Simulation of complex, heterogeneous molecular systems requires models that are thermodynamically accurate and transferable across composition. However, current bottom-up strategies for parametrizing coarse-grained (CG) models from all-atom simulations often poorly reproduce thermodynamic properties. Current remedies have largely focused on increasing the complexity of coarse-grained Hamiltonians and interaction potentials. Here, we pursue an orthogonal approach that instead seeks to design better coarse-graining ensembles, i.e., the state conditions under which bottom-up coarse graining is performed. We introduce a quantitative metric for the quality (or informativeness) of a given ensemble, based on the Fisher information metric. Moreover, we highlight a physical basis for the Fisher information in terms of variances of important structural variables. Using these ideas, we use the Fisher information to optimize externally applied potentials to improve sampling of composition fluctuations and variations. With this approach, we show that even very simple coarse-grained interaction potentials can be optimized to quantitatively reproduce activity coefficients of a methanol-water binary mixture across the entire composition range.   

A Kirchhoff-like theory for the mechanics of magnetic rods

Magneto-rheological elastomers (MREs) are functional materials that can undergo deformations when subject to external magnetic fields. MREs consist of hard magnetic particles dispersed into a nonmagnetic elastomeric (soft) matrix. There have been recent advances in the theoretical description of MREs using the framework of (3D) continuum mechanics. Reduced-order structural theories have also been developed for magnetic linear beams and elastica, based on the planar (2D) deformation of the centerline. In this talk, we derive an effective theory for rods made of MRE undergoing 3D geometrically nonlinear deformations. Our theory is based on the procedure of dimensional reduction of the 3D magneto-elastic energy functional of the MRE into a 1D (centerline and Kirchhoff-like) description, which encompasses the previous 2D theories under appropriate limits. We demonstrate the accuracy of our theory by performing precision-model experiments to explore a set of specific problems involving the buckling behavior of MRE rods. These experiments are also used to test the range of validity of our theory.   

Computational and Experimental Identifications of Hierarchical Peptoid Self-Assembly Pathways

Rational design of self-assembling nanomaterials is predicated on the fundamental understandings of their atomic-level assembly pathways. Here, we conduct an integrated computational and experimental investigation of the hierarchical self-assembly pathways of short amphiphilic peptoids. Peptoids are a class of highly tailorable synthetic peptidomics polymers, which can be engineered to assemble into various hierarchical nanostructures including spheres, helices, tubes, and sheets. For a particular peptoid design, we resolve the critical stages in the peptoid assembly pathway using molecular dynamics calculations that we corroborate by experimental measurements. Our results support an assembly mechanism by which monomers first assemble into disordered cylindrical aggregates that self-orders into a helix, and that multiple helices aggregate and unravel into crystalline sheets. This new understanding of the hierarchical peptoid assembly pathways provides guidance for the future rational design of peptoid-based nanomaterials.   

A simulation study on nonlinear mechanical responses of glassy polymer nanofibers

The confined polymer glasses, such as glassy polymer fibers, exhibit unique glassy behaviors that differ from bulk polymer glasses. We perform molecular dynamics simulations and study nonlinear mechanical responses of glassy polymer nanofibers under uniaxial deformation. We investigate not only nonlinear mechanical responses but also the dependence of mechanical properties on the strain rates of typical polymer glasses, which were observed experiments. We find from our simulations that the local modulus in the surface region of fibers is less than that in the core region of fibers. Also, the non-affine displacements in the surface region of glassy polymer fibers are greater than those in bulk polymer glasses. These results mean that the microscopic events during deformation are closely related to the mechanical responses of polymer glasses.   

Prediction and optimization of ionic conductivity in nanoparticle-based electrolytes using convolutional neural networks

We demonstrate the application of deep learning approaches for prediction and optimization of nanocomposite properties. Our recent study on nanoparticle-based electrolytes suggested the nanoparticle configuration in oligomeric solvent host to be a key design parameter influencing ionic conductivity. Navigating the vast structural search space for identifying optimal nanoparticle microstructures at a fixed nanoparticle loading, is however limited by the computational cost of the accompanying simulations. To overcome such challenges, in this work, we develop a convolutional neural network (CNN) model for quantitative prediction of ionic conductivities. The model is trained using selected mesoscale simulation results. The structure-property linkages established using the CNN model exhibit better predictive capability compared to that using deep learning models based on physics-inspired approaches. By integrating the trained CNN models with a topology optimization algorithm, we demonstrate accelerated morphological space search to identify nanoparticle networks with enhanced ionic conductivities.   

Effects of Ionic Clusters on Dynamics of Sulfonated Polystyrene Blends: Computational Insight

Ionizable polymers have an immense potential to enhance a broad range of technologies from clean energy to water purification and biotechnology, where their structure and dynamics determine their function. Here we follow the formation of ionic-non-ionic interfaces in blends of short 20-mers of polystyrene (PS) and polystyrene sulfonates (SPS) using molecular dynamics simulations with the overarching goal of correlating the degree of phase segregation induced by the ionic groups and their dynamics. Blends of 1:1 made by merging dilute implicit solvent solutions and compressing the systems to melt densities were followed for times up to 1000 ns and the static and dynamic structure factors were extracted and correlated with the distribution of ionic clusters. We find that for these low Mw, local phase segregation dominates the structure. Even though small clusters are formed and slow the motion of the molecules, both the PS and PSS chains remain dynamic.   

Associating Groups Versus Molecular Architecture: Molecular Dynamics Insight Towards Blending Ionizable Linear-Star Polymers

Associating groups to a polymer backbone has dramatic effects on the mobility and viscoelastic response. The associating group form assemblies whose lifetime depends on the strength of their interaction. Here, using molecular dynamics simulations, we probe the effects of strength of the interactions of associating groups on linear and star polymer melts and their blends. The polymer chains are described by a bead-spring model and the associating groups are incorporated in the form of associating beads on either the linear or star polymers or both with an interaction strength between associating beads that is varied from 1-20 k_BT. With increasing associating strength, the polymers associate into clusters of increasing size, independent of the polymer topography. These cluster act as crosslinkers which slow the chain mobility. Blends of chains with and without associating groups globally phase segregate even for relatively weak interaction between the associating groups.   

Molecular Understanding for Large Deformations of Soft Bottlebrush Polymer Networks

Networks formed by crosslinking bottlebrush polymers are a class of soft materials with stiffnesses matching that of ‘watery’ hydrogels and biological tissues but contain no solvents. Because of their extreme softness, bottlebrush polymer networks are often subject to large deformations. However, it is poorly understood how molecular architecture determines the extensibility of the networks. Using a combination of experimental and theoretical approaches, we discover that the shear fracture strain, γ_s, of the network equals the ratio of the contour length L_max to the end-to-end distance R of the bottlebrush between two neighboring crosslinks: γ_s=R/L_max-1.This relation suggests two regimes: (1) for stiff bottlebrush polymers, γ_s is inversely proportional to the network shear modulus G, γ_s∼G^-1, which represents a previously unrecognized regime; (2) for flexible bottlebrush polymers, γ_s∼G^-2 , which recovers the behavior of conventional networks. Our findings provide a new molecular understanding of the nonlinear mechanics for soft bottlebrush polymer networks.   

In/Out Swappable Polymer Brush Model of DNA Kinetoplast

Kinetoplasts are complexes made up of cyclic DNA typically observed to form curved spherical caps in mitochondria. We model this topologically entangled ring network as an underlying entangled ring-membrane with extraneous rings that can arrange themselves to protrude on either side of the membrane. We model these rings as polymers grafted to a semi-rigid substrate in a strong-stretching limit for various solvent concentrations. Each grafted polymer is free to arrange itself to extend from either side of the membrane (a toy model for the actual knotted topology of each DNA ring). We show that this model predicts a spontaneous curvature driven by chain entropy and stabilized by the underlying stiffness of the ring-mat. Apart from modelling this biological system, this “swappable-graft” polymer system could result in interesting textures in engineered 2D polymers.   

Single-molecule elasticity of bottlebrush polymers

The elasticity of highly branched polymers characterized by a dense grafting of
side chains, known as bottlebrush polymers, is central to understanding their
physical properties in a wide variety of applications such as soft elastomers,
solution processing, and confinement-induced stretching. Unlike linear polymer chains where the molecular
origin of this extension is well understood, it remains a challenge to connect
the bottlebrush architecture to force-extension behavior. We study single
bottlebrush polymers subjected to a constant pulling force using molecular
simulations and determine force-extension curves as a function of side-chain
length. To understand bottlebrush elasticity at the
single-molecule level, we compare with a parameterized wormlike cylinder
implicit side chain model. We demonstrate the emergence of two distinct modes
of bottlebrush stretching; at low forces, we show that both linear and
non-linear regimes correspond to stretching the overall cylindrical shape of the
molecule, and at high forces, there is specific extension of internal degrees of
molecular freedom corresponding to the bottlebrush backbone. This two-regime
molecular picture provides insight valuable to the molecular design of
bottlebrush materials.   

Shapes and Spatial Organization of Self-Avoiding Semiflexible Ring Polymers on Solid Substrates

Using coarse-grained molecular dynamics simulations, the conformational behavior and spatial organization of self-avoiding semiflexible ring polymers, that are fully adsorbed on solid substrates, are investigated as a function of their rigidity and surface density. Single polymer conformations and their spatial arrangements depend strongly on the ratio, ζ=l_p/L (l_p is the persistence length and L is the polymer contour length) and the polymers surface density, σ. Regadless of σ, the anisotropy of the polymers dependes non monotonically on the polymer rigidity, in agreement with earlier theories and expriments. For ζ >1 and σ around the overlap density, σ ^*, the polymers self-assemble into a triangular lattice. For σ > σ ^* and ζ >1, the polymers elongation leads to their self-assembly into structure with local nematic or smectic order.   

Modeling Bottlebrushes with Fluorinated Sidechains: Focus on Clusters Formation, Migration, and Restructuring at Interfaces

Molecular bottlebrushes are becoming a material of choice for various applications, largely because their structural characteristics and properties can be tailored in a straightforward manner by regulating their architectural parameters along with the chemical composition. Our experimental studies show that conventional thermoplastics such as PMMA, Nylon 6, and PET can exhibit extremely high water and oil repellency provided that only a small volume fraction of bottlebrush additives with fluorinated polyfluoropolyether (PFPE) sidechains is added to polymer matrices during their fabrication (Wei, Liying, et al. ACS Appl. Mater. Interfaces 12.34 (2020): 38626-38637). We develop a coarse-grained model of this system using dissipative particle dynamics (DPD) approach. The bottlebrush architecture and affinities between different moieties are chosen based on our concurrent experimental studies. We focus on the migration of the bottlebrush clusters onto the interfaces. We characterize the spreading of the bottlebrush cluster at the interface by calculating the shape anisotropy of this cluster and evolution of a number of contacts between different moieties. Our findings are in good agreement with the corresponding experimental results.   

How Does Monomer Structure Affect the Interfacial Dynamics of Supported Ultrathin Polymer Films?

We utilize chemically specific but coarse-grained models of poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) to explore the influence of monomer architecture on the dynamics of supported thin polymer films based on molecular dynamics simulations. We contrast differences in the molecular packing and mobility gradients in these materials near the substrate and “free” interface regions. The dynamics near the substrate are more sensitive to monomer structure, and are enhanced with increasing polymer–substrate interaction strength, ε. PMMA is relatively stiff compared to PEO and has a side group, and the dynamics of PMMA near the substrate are slowed significantly in comparison. Substrate interactions lead to a notable difference in local fragility due to a higher cohesive interaction strength of PMMA in this region. Our data also reveal the inadequacy of the these coarse-grained models to reproduce the experimentally known differences in the fragility. However, this technical shortcoming is not expected to alter our qualitative conclusions regarding the comparative effect of substrate interactions on relatively flexible polymers such as PEO versus a relatively stiff polymer such as PMMA.   

Nanogel degradation promotes interfacial spreading

Microgels and nanogels are being used extensively in applications ranging from drug delivery to soft emulsifiers. In many of these applications, degradation of the polymer network plays an important role. We model first order degradation kinetics in nanogels using our recently developed Dissipative Particle Dynamics (DPD) based approach. We first simulate equilibration of nanogels in a surrounding liquid phase. The equilibrated nanogels attain an equilibrium swelling polymer volume fraction and radius of gyration. We then place nanogels in a vicinity of a liquid-liquid interface and characterize adsorption of nanogels onto the interface. We characterize spreading of nanogels onto the interface by measuring properties such as shape anisotropy and extent of spreading of the nanogel particle. The extent of spreading over the interface is governed by a competition between the nanogel’s inherent elasticity and the interfacial energy. Introducing stimul-controlled degradation in the adsorbed nanogels causes an enhancement in spreading behavior. We further compare the degradation behavior at the interface to degradation of the nanogels suspended in the liquid phase.   

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

Conformations of DNA inside the phage are crucial in the ejection dynamics. The equilibrium conformation of DNA inside the viral capsid is determined by the shape of the capsid. As there are diverse shapes of viral capsid in nature, one should consider capsid-shapes to understand ejection dynamics. In this study, we investigate the effects of the capsid-shape on the conformation of a polymer and its ejection rate by Langevin Dynamics simulations. We perform ejection simulations of three capsid-shapes; 1) sphere 2) cube and 3) cuboid. From our results, a polymer is ejected faster in case of the sphere compared to other shapes of capsids. Polymer chain ejects 2 and 1.5 times faster from the sphere than the cube and cuboid, respectively. Collective rotational motion of a polymer chain is available inside the sphere confinement, whereas only threading motion is available inside the cube or cuboid confinement. The collective rotational motion increase the mobility of the polymer chain, thus accelerating the ejection process.   

Predicting Molecular Design Features for Charge Transport in Radical Polymers

Conducting polymers based on open-shell radical moieties exhibit potentially advantageous processing, stability, and optical attributes compared with conventional doped conjugated polymers. However, reported radical conductors have been based almost exclusively on (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), which raises fundamental questions regarding the ultimate limits of charge transport in these materials. Herein, we have performed a density functional theory (DFT) study of the charge transfer characteristics of a broad range of p-type, n-type, and ambipolar open-shell chemistries. We have determined that, far from being representative, TEMPO exhibits anomalously high reorganization energies, due to strong spin localization. This, in turn, limits charge transfer in TEMPO compared with more delocalized species. By comprehensively mapping the dependence of charge transfer on radical-radical orientation, we also have identified a large mismatch between the conformations that are favored by intermolecular interactions and the conformations that maximize charge transport. These results suggest that significant opportunities exist to exploit directing physical interactions at the molecular level such that charge transport is promoted in radical polymer conductors.   

Relation of Photoabsorption and Excitation Energy Transfer in Fluorinated Non-Fullerene Type Organic Thin Film Solar Cells

The π-conjugated system is important as a semiconductor material in organic electronics such as organic solar cells (OSCs). 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 [1]. In this study, we theoretically investigate the photovoltaic performance of fluorinated acceptor. We performed density functional theory (DFT) calculations with ωB97XD/6-31G(d) using Gaussian16. At first, we compared NTz-T_eh-FA and FNTz-T_eh-FA fluorinated to NTz which are non-fullerene acceptors. Consequently, fluorination of the central π-conjugated system part in non-fullerene acceptor improved photoabsorption because of electron-attracting. In addition, it is expected that it provides high charge-transfer in the complex with the fluorinated acceptor. Next, we calculated the donor-acceptor excitation energy transfer (EET) with DFT. we used PCPD-TBT for donor molecule. Comparing fluorination with non-fluorination, it increased EET coupling. Therefore, it is expected that J_SC [mA/cm²] will be increased. It provides the key to improve power conversion efficiency in OSCs.
[1] S. Chatterjee, Y. Ie, et al., NPG Asia Mater. 10, 1016-1028 (2018).   

Donor-acceptor properties of cyanoacrylated oligothiophenes

Hybrid organic-inorganic setups are promising to achieve high power conversion efficiency for solar cells, including direct bonding molecule-substrate or dye-sensitized SCs. Recent works [1] propose sensitizing through cyanoacrylic termination of the organic moiety. We present a theoretical study of cyanoacrylated thiophene TC short oligomers, focusing on electronic properties. We adopt ab-initio many-body GW on top of hybrid DFT+HF [2,3] calculations, for 1 to 3 thiophene units, clean T and TC. Regarding the TC oligomers, we find that the highest occupied orbital H is more localized on the T-part, and the Ionization Potential is deeper by just 0.4 eV for 3TC relative to clean 3T, while the lowest unoccupied L is more localized on the C-part, bringing the first excitation H-L down (at GW level) by 1.5 eV. The next excited state relates to the L+1 which is again localized on the T-part, with excitation energy H-(L+1) very similar to the H-L of the 3T. Our results are consistent with [1] and point to good photo-conversion properties.
[1] F. Pandolfi et al., Org. Biomol. Chem. 17, 3018 (2019); [2] M. Pinheiro Jr et al. Phys. Rev. B, 92, 195134 (2015); [3] V. Blum et al., Comput. Phys. Commun. 180, 2175 (2009).   

Phase behaviors of a mixture containing charged polymers with or without conformational asymmetry

Through the combination of a molecular equation-of-state model based on liquid state theory and a self-consistent field theory, the phase behaviors of a mixture containing weakly charged polymers is investigated. The free energy of the system in the bulk state consists of that from the corresponding charged hard sphere chain system and nonbonded dispersion interactions. Then, a self-consistent field theory is formulated through the incorporation of the excess free energy into the Hamiltonian. The system is allowed to have different monomer sizes or in other words conformational asymmetry. It is shown that the conventional unfavorable energetics between components, excluded volume along with conformational asymmetry, and the effective shielding of charged monomers come into play to affect the inhomogeneity in the mixtures. The responses of the systems, in particular, thermo-responsivity and baro-responsivity are to be discussed.   

Conformations and Dynamics of Polymer Chains in Cis and Trans Poly(butadiene)/Silica Nanocomposites through Atomistic Simulations: From the Unentangled to the Entangled Regime

We present a detailed investigation of the
conformations and the dynamics of cis and trans poly(butadiene) (PB) chains in PB/spherical-silica nanocomposites, through long time atomistic molecular dynamics simulations at 413 K, well above T_g.
PB chains of different lengths, covering the range from the unentangled to the mildly entangled regime are analyzed.
The systems contain 30 wt % (≈ 12 vol %) silica NPs of diameter ≈ 4 nm.
The nanocomposites are studied through analyzing: (i) interfacial packing and the dimensions of the PB chains; (ii) statistics of the train, bridge, loop, and tail conformations and the coupling between segmental orientational dynamics and chain conformations; (iii) the orientational and translational dynamics of the polymer chains, and the desorption kinetics of chains and segments.
The dimensions of PB chains, excluding a small fraction of chains that wrap
around the NP, are not modified.
The segmental and terminal dynamics of PB chains slow down in the nanocomposites. At long times, bridge segments, exhibit a slow orientational decorrelation.
The correlation times for the desorption of segments and chains, are much larger
than the segmental and end-to-end-vector correlation times, respectively.   

Molecular Dynamics in liquid-like Polyethylenimine-based Nanoparticle Organic Hybrid Materials

We have employed broadband dielectric spectroscopy (BDS), rheology, and nuclear magnetic resonance spectroscopy (NMR) to study molecular dynamics in nanoparticle organic hybrid materials (NOHMs) comprising 20 wt% silica nanoparticles ionically-bonded to a polyethylenimine canopy. By comparing the neat polymer – used as a canopy – to the derivative NOHMs, we find that timescales characterizing segmental dynamics in the NOHM are identical to those for the neat polymer as determined by BDS. Rheology, however reveals slower dynamics for the NOHMs. Detailed analysis of the carbon-spin lattice relaxation times yields mechanistic insights into localized and collective dynamics, in quantitative agreement with dielectric results. Interestingly, the NOHMs retain liquid-like characteristics unlike conventional polymer nanocomposites but exhibit higher viscosity due to additional contributions from tethered polymer chains. These findings demonstrate the potential of achieving unique and desired material properties via NOHMs by an informed choice of the constituent materials.   

Modeling Pairs of Polymer-Grafted Nanoparticles (PGNs) in Solution

By grafting polymer chains onto nanoparticles (creating PGNs), one can precisely control interparticle interactions. PGNs are typically synthesized and processed in solution before use in applications such as flexible electronics where a precise spacing of inorganic particles in a robust and flexible matrix is desirable. Understanding their solution properties is crucial to control their structure during deposition and drying. We use coarse-grained molecular dynamics (MD) simulations to study the chain conformations and effective interactions between two PGNs in implicit solvent. Specifically, we use a Kremer-Grest type of model for graft chains and spherical nanoparticles ten times the monomer size. Nanoparticle interactions are of an integrated form as though they are composed of a uniform melt density of monomers. Monomers interact via mixed Lennard-Jones potentials; the repulsive part is kept constant while an attractive part is added with adjustable strength to consider various solvent strengths. As expected, in good solvent, we find that higher graft density yields more extended chains and a larger effective repulsion between PGNs. At lower solvent strength, PGN-PGN interactions become attractive; details of chain conformations and effect of graft density will be discussed.   

Structural insight into the interface effect in ferroelectric polymer nanocomposites

The interfaces in complex oxides and semiconductors have been successfully tailored to introduce emergent phenomena and novel functionalities. However, the interfacial approach has not been rationally utilized in electroactive polymer nanocomposites because the origin of the physical, chemical and electrical interaction between nano-fillers and polymer matrix remains essentially unknown. In this work, we present, for the first time, the direct structural analysis on the interfaces and a chemical mapping of the interfacial coupling in the ferroelectric polymer composites at the molecular level by using atomic force microscope infrared spectroscopy (AFM-IR) in combination with phase field simulations and first principles calculations [Y. Liu et al, Adv. Mater. 2020, in press. https://doi.org/10.1002/adma.202005431]. This work reveals a series of unique features of the filler-matrix interfaces, including the local stabilization of all-trans conformation and highly polar and inhomogeneous interfacial regions, which are lacking in the current understanding. The discovery of the size-driven enhancement of local all-trans chain conformation in this work addresses the long-standing controversy surrounding the dependence of the dielectric constants of the composites on the filler size.   

Spectroscopic Investigations on Polyethylene Oxide Based Nanofibers

Polyethylene Oxide (PEO) nanofibers, were obtained by force spinning using a Fiberio Cyclone L-1000 M equipment operating at spinning rates between 2,500 and 12,500 rotations/minute. PEO nanofibers were obtained from solutions of various concentrations in water or chloroform. Nanocomposites of PEO have been obtained by loading the PEO matrix with various amounts of C60 nanoparticles (average diameter 2 nm) and Single-Walled Carbon Nanotubes (SWCNTs). The research is focused on an improved understanding of nanocomposites, as C60 is soluble in chloroform but not soluble in water.
Spectroscopic investigations by Raman spectroscopy (Renishaw InVia confocal microscope) operating at both 785 and 532 nm will be analyzed. The research aims at a better understanding of the residual stress in polymer and polymer nanocomposites nanofibers, with emphasis on stress transfer in PEO-C60 and PEO-SWCNTs nanocomposites, where the Radial Breathing Mode in SWCNTs and C60 is typically related to the stress-transfer. Complementary data obtained by FTIR (Bruker Tensor), and Wide-Angle X-Ray Spectrometry (Bruker Discovery 8) will be reported.   

Polymer Infiltrated Nanoporous Metals: A New Class of Composite Material

Most research on polymer composites has focused on adding discrete inorganic nano-fillers to a polymer matrix in order to impart properties not found in polymers alone. However, properties from ion conductivity to mechanical reinforcement would be greatly improved if the composite exhibited an interconnected network of inorganic and polymer phases. Here, we fabricate bicontinuous composite materials by infiltrating polymer into nanoporous gold (NPG) films. Polystyrene (PS) and poly(2-vinylpyridine) (P2VP) films are infiltrated into the 40nm pores via capillary forces by thermal annealing above the polymer glass transition. The two polymer chains, which have different affinities for the gold scaffold, exhibit slower segmental dynamics with varying strengths inside the confined pores as measured through Tg. The more attractive P2VP shows a 20°C increase in Tg relative to that PS, which shows only a of 6°C increase at a comparable molecular weight. The infiltrated polymer, in turn, stabilizes the gold nanopores against collapse from temporal aging. The broad tunability of these polymer/metal hybrids represents a unique template for designing functional network composite structures from flexible electronics to fuel cell membranes.   

Unusual Protein Adsorption Phenomena on Ultrathin Homopolymer Films

We recently 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 this protein adsorption, photon counting spectrofluorometry along with the fluorescence-labeled proteins was utilized. Therefore, 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.   

Mechanical reinforcement of polymer nanocomposites at large deformation: new insights from small-angle neutron scattering and rheology

Incorporation of nanoparticles (NPs) into polymer matrices can significantly improve the mechanical performance of polymer nanocomposites (PNCs). Despite the wide recognition of the nano-reinforcement effect in PNCs, the molecular origin of this phenomenon remains largely elusive. Although molecular overstraining or strain amplification has often been invoked to explain the modulus enhancement of PNCs, a rigorous examination of the presence of molecular overstraining is still missing. In this contribution, we quantify the structural anisotropy of the PNCs at deformation through small-angle neutron scattering (SANS). While molecular deformation of the matrix polymer dominates the stress of PNCs, quantitative analyses of SANS spectra reveal no enhanced structural anisotropy in the PNCs compared with the pristine polymers under the same deformation conditions. These detailed mechanisms of the lack of molecular overstraining will be discussed in the talk.   

Molecular Dynamics Simulation of Organically Functionalised Nanoparticles Applicable to Polymer Nanocomposite Systems

We present a molecular simulation study a series of organically functionalised polyhedral oligomeric silsesquioxanes (POSS) systems and use these to elucidate the thermomechanical capabilities of POSS-containing materials. Since they inherently comprise both organic and inorganic components, POSS systems can exhibit a wide range of possible behaviours through variation of the geometry of their central inorganic cores. In simulation, it is also straightforward to systematically explore i) the range of ligands appended (either uniformally or asymmetrically to the central cores), ii) mixing ratios of various POSS forms and iii) their influence on polymer matrices.

These systems are amenable to simulation via atomistic MD methods, with gross system-wide behaviours (most obviously, the glass transition temperature of pure POSS systems) proving sensitive to a range of model inputs, such as geometry and choice of force-field. We also identify the key molecular degree of freedom which freezes in at Tg, thereby providing important insight for synthetic control of this key property. Finally, we present initial steps towards more ambitious systems of functionalised POSS cubes incorporated into polymeric melts.   

The Effects of Twin Screw Extrusion Screw Design and Screw Speed on Filler Dispersion in Polystyrene

The properties of polymer nanocomposites can be improved by greater filler dispersion. Filler dispersion in highly viscous polymers is affected by the kinetics of mixing and chemical affinity. It is challenging to quantify nano-dispersion due to the small-size features averaged over large volumes. In order to quantify dispersion, X-ray scattering data was measured as a function of filler concentration and used to calculate the pseudo-second order virial coefficient, A2. In a true thermodynamic sense, A2 is a measure of miscibility. For polymer nanocomposites, a larger A2 indicates a better dispersion. This nano-scale measure of dispersion is compared with macroscopy measures using microscopy. Nanocomposites of carbon black filled polystyrene mixed in a single and twin-screw extruder (following Schadler et al.) and in a Brabender mixer were compared. The effect of screw design and screw speed were explored for the twin screw extruder.   

Dissipative Particle Dynamics (DPD) Simulation to understand the Nanoparticle Dispersion and Aggregation behavior in Polymer Nanocomposites

Polymer nanocomposites, i.e. polymer matrix with nanoparticles of variable amounts and types, possess a broad range of applications. In particular, 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 as well as the processing history. In this research, we perform Dissipative Particle Dynamics (DPD) simulation of these blends, varying polymer-polymer interaction energy to study the dispersion and aggregation mechanism of fillers. Our results demonstrate the role of concentration on the clustering of fillers and methods to quantify the filler percolation threshold and mesh size. 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 explored. The simulation results are also validated against small-angle x-ray scattering data.   

Control of Self-Assembly in Hierarchical Organic Nanoparticles

Semi-dilute nanoparticles tend to cluster in order to reduce surface area. Clustering can be controlled using surfactants. In this work a polyethelene oxide (PEO) based surfactant, Triton X-100®, is used to control clustering of an organic pigment. PEO displays a lower critical solution temperature (LCST) at 66 °C. It has been found that reduced miscibility in the vicinity of the LCST can be used to control clustering. For small clusters significant aggregation occurs while for sufficiently large clusters aggregation does not occur. In this way a thermally controlled hierarchical structure can be produced. The thermally tuned hierarchical structure can be locked in using chemically modified surfactant.   

Percolation, dispersion and structure-conductivity relationships in carbon black nanocomposites

The dielectric and the mechanical frequency spectra for carbon black nanocomposites is tied to their complex multi-hierarchical structure. The structure is influenced by interfacial chemistry and accumulated strain. These factors can be quantified using the second virial coefficient. The effective interaction between filler and polymer leads to local clusters and a global network structure. In the linear viscoelastic regime, this local network dictates the high-frequency dynamic response. These clusters percolate on the macroscopic scale mitigated by the accumulated strain in the mixing process and impact the electrical conductivity as well as the gel-like dynamic response at low frequencies. The impact of the multi-hierarchical network structure on the dielectric and mechanical spectra will be described.   

Ternary nanocomposites of styrene-butadiene rubber (SBR), polybutadiene, and carbon black

Ternary nanocomposites offer a challenge to understand dispersion and properties. Filler particles can uniformly disperse if the matrix is homogeneous, act as a Pickering emulsifying agent and coalesce at the interface between phases or locate preferentially to one of two immiscible phases. For a block copolymer (SBR) component the situation can be further complicated by partial microphase separation. The latter situation is found to exist in the materials studied. A detailed justification for the proposed resulting structure, and quantified dispersion is presented based on thermal analysis, microscopy, and X-ray scattering.   

Silane coupling agent and dispersion in silica nanocomposites

Dispersion of nano-silica in polybutadiene and SBR is of interest to the elastomer industry. Often, silane coupling agents are used for compatibilization. This is done by melt blending untreated precipitated silica, a coupling agent, and elastomer in a Brabender mixer. The coupling agent contains sulfur that can react with vinyl groups in the elastomers as well as ethoxy groups that can react with surface hydroxyl groups on silica. We have quantified the extent of these two reactions using IR spectroscopy, and quantified the dispersion using microscopy and small-angle x-ray scattering (SAXS). From SAXS we obtain the second virial coefficient which is used as a measure of dispersion.   

Dispersion and dynamic response for in-flame and chemically modified fumed silica nanocomposites

Surface modified silica fillers in polymer matrixes can display enhanced dispersion due to improved filler/polymer interactions which in turn impacts its dynamic response. Surface modification is usually achieved by chemical grafting of elastomer compatible moieties and silane coupling agents. It is also possible to deposit carbon directly during pyrogenic synthesis. In this study, in-flame and chemically modified pyrogenic silica fillers were dispersed in styrene-butadiene (SBR) rubber to explore their differences in nanocomposites. The impact of the surface carbon content on the extent of dispersion and rheological properties were explored. X-ray scattering results indicated that the in-flame and chemically modified fillers were well dispersed on the nanoscale in the SBR polymer matrix as inferred from the pseudo-second viral coefficient A_2. In previous work, we have demonstrated that pyrogenic silica displays correlations in similar nanocomposites due to the presence of silanol groups on the surface. Some of the in-flame coated silica fillers had sufficient surface carbon to mitigate the charge repulsion due to the silanol moieties. The efficacy of these carbon coated silica nanocomposites at exhibiting a balance between rolling resistance and wet grip was evaluated   

Exploring Multifunctional Potentials of Carbon Nanotube-Epoxy-Glass Fibers Composites: Mechanical and Microwave Absorption Properties

In this work, we explored the multifunctional potentials of multi-walled carbon nanotube-epoxy-glass fibers composites. We aimed to enhance the mechanical properties of these microwave absorption composites. We investigated the tensile strength, morphologies, dielectric permittivity, and electromagnetic (EM) wave absorption properties (in a frequency range from 1 to 26 GHz) of the composites. The tensile strength of the composites was enhanced to about 400 MPa, which is comparable to that of commercial Aluminum alloy 6061 (about 300 MPa). The mass density of the MWCNT-Epoxy-GF composites was found to be around 1.6 g/cm³, while Aluminum alloy 6061 has a mass density ~ 2.7 g/cm³. Further, with the increased loadings of MWCNTs, the composites show excellent EM wave absorption performance and high reflection loss. This suggests that the composites may have the multifunctional potential as microwave absorption materials and also as structural components.<gdiv></gdiv><gdiv></gdiv><gdiv></gdiv><gdiv></gdiv>   

Rheological Properties of Bare and Grafted Nanoparticle in Crosslinked Polymer Networks

In this study, we aim to explore the dynamics of polymer nanocomposite networks in crosslinked states through linear rheology experiments. Particle size, loading and crosslinking degrees will be varied in poly(methyl methacrylate) (PMMA) network systems to understand the topological effects of chains and particles on rheological responses of networks. Dispersion and rheological behavior of PMMA-grafted particles in poly(methyl acrylate) (PMA) composite networks will be presented. Interfacial mixing of low and high glass-transition temperature polymers determines how these networks deform. The model system enables us to investigate how bare and polymer-grafted particle diffuse into entangled and unentangled crosslinking networks to further understand their mechanical properties.   

Structural control of photo-curable polymer/silica NP nanocomposite by photo-polymerization induced phase separation under a confined film geometry

Polymerization-induced phase separation of nanoparticle additives in photo-initiated polymerization of monomers can serve as an effective strategy for fabricating novel 3D printed nanocomposite structures in stereolithography. For better dispersion of nanoparticles in the photo-curable monomers, the surface modification holds a key in controlling their compatibility with the monomers, and silica nanoparticles are ideally suited for its wide applicability of silica surface chemistry using silanes or polymer adsorption. We have compared the phase separation morphology difference of bare-silica and surface-functionalized silica nanoparticles in photo-curable (meth)acrylate monomers. Upon losing the compatibility of nanoparticles with the formation of highly crosslinked polymer networks, the repeated layer-by-layer photopolymerization for the polymerization-induced phase separation of silica nanoparticles allowed us to sequester the nanoparticles in less cross-linked regions. Optical and electron microscopy techniques are used to investigate the morphology, while the in-situ light transmission measurements have been utilized to have the real-time monitoring of the photopolymerization-induced phase separation.   

Size and Surface Functionality Effects of Silica Nanoparticles for the Photopolymerization-Induced Phase Separation in Confined Film Geometry

Polymerization-induced phase separation of nanoparticle additives in photo-initiated polymerization of monomers can serve as an effective strategy for fabricating novel 3D printed nanocomposite structures in stereolithography (SLA). However, the dispersion of nanoparticles in the photo-curable monomers could be challenging for the surface dominant interactions of nanoparticles with monomers. Nevertheless, viscous SLA monomers would be favorable to maintain the nanoparticle suspension in the photo-curable resin, once the nanoparticle dispersion is successfully achieved via sonication, for example. Herein, we have compared the dispersion of silica nanoparticle with different diameter and surface functionality and examined its impacts on the morphological development via the photopolymerization-induced phase separation. Optical and electron microscopy techniques are used to investigate the morphology, while the in-situ light transmission measurements have been utilized to have the real time monitoring of the photopolymerization-induced phase separation upon the continuous irradiation of UV LED lights onto the reactive phase separation mixture.   

Mutually Reinforced Polymer-Graphene Bilayer Membrane for Energy-Efficient Acoustic Transduction

We present a lightweight, flexible, transparent, and conductive bilayer composite of polyimide and single-layer graphene suspended on the centimeter scale with an unprecedentedly high aspect-ratio of 1e5. The coupling of the two components leads to mutual reinforcement and creates an ultra-strong membrane that supports 30,000 times its own weight. Upon electromechanical actuation, the membrane pushes a massive amount of air and generates high-quality acoustic sound. The energy efficiency is ~10-100 times better than state-of-the-art electrodynamic speakers. The bilayer membrane’s combined properties of electrical conductivity, mechanical strength, optical transparency, thermal stability, and chemical resistance will promote applications in electronics, mechanics, and optics.
  

Excited state dynamics in cross-linked covalent hybrids of graphene and diketopyrrolopyrrole oligomers revealed via ultrafast pump-probe and 2D spectroscopy

Cross-linked covalent hybrids of graphene and diketopyrrolopyrrole (TDPP) oligomers were studied via ultrafast pump-probe and 2D electronic spectroscopy with sub 30 femtosecond time resolution. Comparisons between polymerizing (c-EXG-TDPP) and non-polymerizing (EXG-TDPP) hybrids show that charge/energy transfer occurs faster than 50 ps in EXG-TDPP, while the c-EXG-TDPP system quenches within 18 ps. The transient response of c-EXG-TDPP is completely quenched (>99%); full quenching occurs in EXG-TDPP after 1.39 ns. Photocurrent studies of c-EXG-TDPP films show IPCE > 2%, indicating ultrafast charge transfer between the TDPP moeity and graphene.   

Manipulating ionic conductivity in insulating thermoplastics by use of commercial additives: altering the fluid response to applied electric fields, motivated by electrospinning applications

The goal of this project is to develop a better understanding of how ionic conductivity in viscous melts can be manipulated with commercial additives and how such changes influence fiber size in the electrospinning process. Our aim is to alter the polymer melt conductivity (in the range 10^-12 – 10^-6 S/cm) with different types and concentrations of additives and understand the resultant changes in jet size and fiber diameter. Changes in mat porosity, decreased fiber size, increased jet density, and transition from electrospinning to electrospraying will be discussed.   

Simulation and theoretical study of immiscible binary fluids with particles

In recent studies, the addition of inorganic materials has been attempted to improve the physical properties of polymer blends. To achieve superior physical properties, it is important to control the dispersion of the inorganic materials. The present study investigates the behavior of small particles added to immiscible binary fluids using simulation and theory.
In the present simulation, the dynamics of the fluids is described by Cahn-Hilliard and Navier-Stokes equations. The particle-fluid interaction is described using the Smoothed Profile Method [1]. Using the simulation, the authors investigate the dispersion of particles in the fluids, changing the wetting interaction between fluids and particles. The present theory is based on the theory of Ginzburg et al. [2]. The present authors extend the theory of Ginzburg so that it can treat the particles being trapped at the fluid-fluid interface. By comparing the simulation and theoretical results, the authors obtain insight into the particle dynamics during the phase separation process.

[1] Y. Nakayama and R. Yamamoto, Phys. Rev. E, 71, 036707 (2005).
[2] V. V. Ginzburg, et al., Phys. Rev. E, 63, 4352 (1999).   

Physically motivated feature engineering for classification of fluorescent DNA-stabilized silver clusters

DNA-stabilized silver clusters (Ag_N-DNAs) are fluorescent nanomaterials whose optical properties are encoded by DNA sequence. Due to the nucleobase-specific interactions of silver atoms with DNA [1], DNA sequence can tune the cluster sizes of Ag_N-DNAs from about 10 to 20 atoms per cluster, corresponding to a wide range of fluorescence colors spanning the visible to near infrared spectrum. Recently, machine learning approaches have been used to learn how DNA sequence relates to Ag_N-DNA fluorescence emission wavelength, allowing predictive design of new Ag_N-DNAs with bright fluorescence in desired wavelength ranges [2]. In order to improve the classification accuracy of these methods, we have developed physically motivated features based on the first crystallographic structures reported for Ag_N-DNAs. By better capturing the occurrences of certain non-adjacent nucleobase motifs, the accuracy of assigning a color class to an input DNA sequence is increased by up to 18%. This work contributes to the development of biomolecules as templates for ultrafine metallic nanostructures.

[1] S. M. Swasey, L. E. Leal, O. Lopez-Acevedo, J. Pavlovich, and E. G. Gwinn, Sci. Rep. 5, 10163 (2015).
[2] S. M. Copp, S. M. Swasey, A. Gorovits, P. Bogdanov, and E. G. Gwinn, Chem. Mater. 32, 430 (2020).   

Interplay between photopolymerization and phase separation kinetics in photopolymerization-induced phase separation for stereolithography

Photopolymerization-induced phase separation can serve as a useful strategy for fabricating 3D printed multiscale polymer structures in stereolithography (SLA), when the phase separation of segregation agents is triggered by the polymerization to convert the low molecular weight monomers into polymer networks. Several factors control the structure and dynamics of polymerization-induced phase separation for SLA 3D printing, such as resin composition, molecular weight of polymers as a phase separation agent, and UV curing kinetics of the photo-curable monomers. Real-time turbidity measurements have been carried out to measure UV light transmittance through a photo-curable sample of predetermined thickness have been employed as a main method to monitor the phase separation of poly(ethylene glycol) with different molecular weights in (meth)acrylate monomer mixtures. UV LED intensity in particular serves as a useful experimental handle to tune and control the polymerization kinetics in the phase separation process and resulting network morphology within the confined space of resin films gap, which is analogous to the slice thickness in SLA.   

Shape-Transforming Block Copolymer Particles Driven by Photoisomerizable Surfactants

Shape-transformable particles by light are useful for developing active, programmable smart materials. In this work, we report block copolymer (BCP) particles with reversible shape-changing property activated by wavelength-selective light irradiation. We design the spiropyran-based surfactants, which can be isomerized between ring-opening and ring-closing structure by UV or visible light irradiation, respectively. This photoisomerization of the spiropyran surfactant effectively tunes the wetting behavior of BCP chains. In the UV light irradiation, spherical BCP particles with hydrophilic outermost layer are generated. By contrast, surfactants of ring-closed form under visible light provide a neutral surrounding to the BCP particles, producing oblate or prolate ellipsoids. We also demonstrate the reversibility of these shape transformations of the BCP particles by sequential UV or visible light irradiation.   

Shape memory elastomers from reactive monomer/polymer blends

Shape memory polymers (SMPs) can memorize a programmed ‘temporary shape’ and return to their initial shape under an external stimulus. Most SMPs contain two structure-spanning, solid networks; a permanent elastic network that is strained during programing to drive shape recovery; and a temporary network that fixes the programmed shape. An interesting route to fabricate SMPs is by blending an elastomer and a crystalline small molecule, where the elastomer forms the permanent network, and the small molecule crystal forms the temporary network. However, a drawback of this approach is the blooming and expulsion of the small molecule during programming and recovery. To address this issue, blends of polybutadiene and octadecyl acrylate (ODA) have been investigated, where the ODA can either be grafted to the PB through an ene-reaction or undergo simultaneous polymerization, grafting and crosslinking with the PB using a peroxide free radical initiator. It will be demonstrated that the side-chain crystallization of the ODA produces shape memory polymers with high shape fixity and recovery ratios (> 95%) with processing compatible with molding operations. The structure-property relationships between the physical parameters and shape memory properties are discussed.   

Photopolymerization-induced phase separation of PEO, PPO and PEO-PPO-PEO in photo-curable monomers in confined film geometry

Polymerization-induced phase separation in photo-initiated polymerization can serve as a useful strategy for fabricating 3D printed porous structures in stereolithography. While polymer can remain miscible with photocurable monomers, the phase separation will be eventually triggered by the polymerization to convert the low molecular weight monomers into polymer networks. We have compared the difference in the morphological development of polymerization-induced phase separation when different types of polyethers are used as phase separation additives in the photocurable (meth)acrylate monomers. In particular, poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO) and PEO-b-PPO-b-PEO are examined as the polyethers. This series of polymers have been chosen in the experimental design because of their differential incompatibility with (meth)acrylate monomer systems, as well as their easily removable properties using alcohol to create porous structural polymer networks. Optical and electron microscopy techniques are used to investigate the morphology, while the real-time light transmission measurements have been utilized for the in-situ monitoring of the photopolymerization-induced phase separation.   

Effects of Environmental Temperature and Humidity on the Geometry and Strength of Polycarbonate Specimens Prepared by Fused Filament Fabrication

While it is known that the printing quality of fused filament fabrication is heavily affected by environmental temperature and humidity, there is little understanding of the relations between environmental conditions, geometry, and the mechanical properties of printed parts. We investigated those relations using polycarbonate. Infrared imaging analysis showed up to a 5.4 ^○C/mm thermal gradient when printing in an open-chamber with a heated build plate. Micro-CT scans showed an up to 11.7% porosity from polymer water content absorbed from environmental moisture. Meanwhile, tensile tests showed a mechanical performance loss associated with those defects, but, surprisingly, the transverse direction ductility increased at a higher porosity. Finally, the experimental results were combined with analytical and parametrical studies to elucidate quantitative relations between environmental conditions and printing quality. We envision that our studies can provide quantitative guidelines for the estimation of printing quality based on environmental conditions and help users to obtain desired printing results by considering environmental conditions.

Ref. Materials 2020, 13, 4414.   

Two-Photon Direct Laser Writing of Shape-Memory Polymer Networks

Shape-memory polymers have been widely popularized and studied since their debut decades ago and have recently been explored for their uses in soft robotics, adhesion, and medicine. Our lab is evaluating photoinitiator and resin systems for high-resolution, direct laser writing of shape-memory polymer networks. Traditional photoinitators generate free radicals which often catalyze undesirable side-reactions and can be quenched by oxygen, dampening reaction kinetics and reducing the resolution of the structure with unpredictable chemical network formation. Glass-forming thiol-acrylate and thiol-ene resins are examined that, when polymerized, exhibit thermally triggered shape-memory in the 40-50 °C range. Networks are formed using a light sensitive free-radical or photobase generator at either 390 nm or at 780 nm with a femtosecond pulsed laser, and the resulting chemical composition and thermomechanical properties are evaluated. Results offer pathways to significantly improve the resolution of shape-memory features beneath the micron scale, opening new avenues for topographical modulation of surfaces.   

Pressure-Dependence of Water Dynamics in Concentrated Aqueous Poly(N-isopropylacrylamide) Solutions with a Methanol Co-solvent

The application of high pressure has a substantial effect on the phase behavior of the thermoresponsive polymer poly(N-isopropyl acrylamide) (PNIPAM). In a water-methanol mixture the one phase region is hugely expanded along the pressure axis in comparison with purely aqueous solutions. The water dynamics of a 25 wt % PNIPAM 80:20 v/v H₂O/CD₃OD solution are probed over wide temperature ranges around the respective cloud points at both 0.1 and 200 MPa. Quasi-elastic neutron scattering reveals the relative fractions and relaxation rates of bulk and hydration water during the reversal of co-nonsolvency at high pressure. At atmospheric pressure, the susceptibility spectra are in agreement with preferential adsorption of methanol on the chains far below the cloud point temperature, while bound water is released at the transition. On the other hand, at 200 MPa the polymer chains are more hydrated on the cost of methanol while dehydration sets in gradually at the cloud point. The pressure dependence of the relaxation time of the bulk water diffusive mode reflects the release of methanol from the polymer chains altering the effective solvent composition.   

Incorporating Interlayer Crosslinks in Fused Filament Fabrication: Insights into Post Deposition Reactions

3D printed objects exhibit low interlayer adhesion and defects that result in anisotropic mechanicalproperties. In this presentation, we will describe the introduction of covalent bonds between filaments to enhance interlayer adhesion by the reaction of multi-amines with oxygenated functional groups on the recently deposited filament. Interfacial fracture energy evaluation and infrared (IR) spectroscopy indicate the successful formation of interlayer bonds and strengthening of interlayer adhesion with a variety of multi-amine materials. More importantly, the spectroscopy results also reveal the relative reaction rates of each multi-amine with the oxidized filament. The results show that crosslinking reactions occur rapidly at elevated temperatures immediately following filament deposition. Data also indicates that amine reactivity is not the prevailing factor in the observed interfacial strength improvement, as crosslinker aromaticity also plays an essential role. Therefore, these results offer important foundational information that can foster the use of similar protocols in other extrusion additive processes and materials.   

3D Printing of Conducting Polymers

3D printing can bring exciting new opportunities to the field of organic electronics by defying the current 2D form factor, creating electronics that conform to the body contour of each unique individual, and realizing electronics that can fit in irregularly-shaped void spaces. Direct ink write (DIW) is the most versatile 3D printing method for conjugated polymers due to their solution processability. However, current DIW-printed conjugated polymers suffer from inferior electrical properties (e.g. conductivity one order of magnitude lower than spin-coated counterparts) because of their sensitivity to processing conditions. Here, we elucidate the effect of this new processing method on the fundamental structure-property relationships of a benchmark conducting polymer, PEDOT:PSS. New insights from these studies subsequently guide us to create tailored material formulation and processing conditions for this extrusion printing mechanism, which led to PEDOT:PSS conductivity comparable to their conventionally 2D processed thin films. Finally, we demonstrate 3D freeform electronics and functional prosthetics using multi-material DIW printing of PEDOT:PSS and materials with different electrical and mechanical properties.   

Interplay of Photo-Polymerization and Phase Separation Kinetics in 3D Printing Using PEG & Methacrylated PEG

Photo-polymerization-induced phase separation would serve as a useful strategy to tailor the physical properties of 3D printed porous materials. The interplay between photopolymerization kinetics and phase separation can be studied using a multiscale method to show the extent of phase separation and its overall effect on the material once 3D printed. UV transmittance experiments are performed as a main characterization method using a custom-built light transmission apparatus. Additional characterizations such as polarized optical microscopy and scanning electron microscopy have been used to understand the morphology of polymer films. In particular, vegetable oil-based monomers are considered as a base of photocurable resin as they are sustainable and recyclable. Polyethylene glycol (PEG) and PEG methylether methacrylate (PEGMEM) are used as polymer additives. PEG-containing resins show a drastic increase in turbidity upon photopolymerization, while the PEGMEM-based resins do not. The grafting of PEGMEM during the photopolymerization serves as a surfactant for the system, thus reducing phase separation effects and altering the physical properties of the material.   

Tuning Block Polymer Design to Enable for PFAS Remediation

Perfluorinated alkyl substances (PFAS) pose a significant threat to water supplies due to their high toxicity and robust stability. Current remediation methods are typically limited by their cost, their ability to only capture or decompose the PFAS compounds, the volume of water they can treat, and the time required to process said volume. Here, we demonstrate a new type of material composed of an organic membrane functionalized with β-cyclodextrin moieties and doped with electrochemical oxidative catalyst (i.e. a composite based upon Ti₄O₇ and carbon nanotubes) to be capable of providing in-place, high-throughput degradation of PFAS. This is accomplished through the hydroxyl radicals produced by the electrocatalyst and PFAS high affinity to β-cyclodextrin, and the utilization of the surface-segregation and vapor-induced phase separation (SVIPS) casting technique which induces dense pore distributions across the membrane in conjunction with high analyte capacity due to the self-assembly of functional groups on the pore walls and increased surface area that arises from the nanostructured morphology. This lacey nanostructure thus shifts the mode of PFAS adsorption to the β-cyclodextrin from usually being diffusion controlled to a faster convective mass transport.   

Rapid Vertical Ordering of Lamellar Block Copolymer Films by Dynamic Thermal Gradient Annealing for Ion Conduction Membranes

To achieve the full technological potential of block copolymers (BCPs) for use as electrolytes, filtration membranes, or in nanolithography, rapid ordering of BCPs with vertically oriented nanostructures on unmodified substrates is desirable. In this work, we demonstrate the rapid ordering of lamellar BCPs (< 40 s) by utilizing the effect of thermal gradient based Cold Zone Annealing (CZA) technique. The evaporation fronts during film casting results in poorly ordered yet vertically oriented BCP morphology. We demonstrate that CZA, by driving in-plane defect annihilation in such vertically oriented as cast BCP nanostructures, resulted in perpendicularly oriented and well-ordered morphology on a variety of substrates, at short time scales. The kinetics of lamellar grain size (x) evolution was observed to be much faster in CZA (x ~ t^0.26) as compared to oven annealing (x ~ t^0.15). Subsequent post-annealing integration of Ionic liquid (IL) was shown to selectively swell well-ordered vertically oriented lamellar domain by 100%, having potential use for mechanically robust ion-conducting channels in BCP membranes and electrolytes.   

Vertically oriented nanoporous block copolymer membranes for oil/water separation and filtration

The separation of oil from water and filtration of aqueous solutions and dispersions are critical issues in the processing of waste and contaminated water treatment. Membrane-based technology has been proven as an effective method for the separation of oil from water. In this research, novel vertical nanopores membrane, via oriented cylindrical block copolymer (BCP) films, suitable for oil/water filtration has been designed, fabricated and tested. We used a ~100 nm thick model poly(styrene-block-methymethacrylate) (PS-b-PMMA) BCP as the active top nanofiltration layer, processed using a roll-to-roll (R2R) method of cold zone annealing (CZA) to obtain vertical orientation, followed by ultraviolet (UV) irradiation selective etch of PMMA cylinders to form vertically oriented nanopores as a novel feature compared to meandering nanopores in other reported BCP systems. The cylindrical nanochannels are hydrophilic, and have a uniform pore size (~23 nm), a narrow pore size distribution and a high nanopore density (~420 per sq. micron). The bottom supporting layer is a conventional microporous polyethersulfone (PES) membrane. The created asymmetric membrane is demonstrated to be effective for oil/water extraction.   

Towards the packed nanocylinders with square arrays using block copolymers

The bottom-up approach using self-assembling molecules for the fabrication of nanopatterned layers in semiconductor manufacturing processes has been of great interest both from relevant industries and academia. Among various materials and subsequent nanopatterns, the packed nanocylinders with square arrays is the most desirable one. It is well known that the hexagonally packed array is by far the strongest due to the packing effects. Using the self-consistent field theory and also theoretical crystallography, we search for the possible route towards the nanocylinders with square packing from just common diblock copolymer systems and how to overcome the hexagonal arrays.   

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

Co-continuous polymer nanostructures formed by both equilibrium and kinetic routes have been widely studied due to the attractive combination of material properties they offer in many contexts. Our group has recently shown randomly end-linked crosslinked networks (RECNs) can yield co-continuous nanostructures over wide composition windows of 30 - 45 wt. % depending on the strand parameters. Previously, the use of RECNs based on polystyrene (PS) has been focused, which can be converted to interconnected nanoporous PS by selectively etching the second microphase, polylactic acid (PLA). However, the brittle nature of the PS phase makes it challenging to apply these materials as membranes. To overcome this, we employ telechelic polysulfone (PSU) to generate co-continuous PSU/PLA RECNs and interconnected nanoporous PSU. Uniaxial tensile testing on PSU/PLA and PS/PLA at 80 °C was performed, above the glass transition temperature of PLA, to isolate the effect on mechanical properties of shifting from PS to PSU. Co-continuous PSU/PLA displays five times greater stretchability and three times higher Young’s modulus than co-continuous PS/PLA, which gives rise to a dramatic improvement in the properties of the resulting nanoporous membranes.   

Size and Stiffness Reduction of Phytoglycogen Nanoparticles Through Acid Hydrolysis

Phytoglycogen occurs naturally in the form of compact, 44 nm diameter glucose-based nanoparticles in the kernels of sweet corn. Its highly branched, dendritic structure leads to interesting and useful properties that make the particles ideal as unique additives in personal care, nutrition and biomedical formulations. The properties of phytoglycogen nanoparticles can be altered through chemical modifications such as acid hydrolysis, which not only reduces their diameter but also alters their internal structure, producing significant changes to the interactions between particles in solution. As the acid hydrolyzed particles are packed towards their glass transition volume fraction, the dependence of the zero-shear viscosity on the effective volume fraction abruptly changes from behaviour well-described by the Vogel-Fulcher-Tammann equation to more Arrhenius-like behaviour, with the transition marked by a pronounced kink in the data. This result is consistent with a reduction in stiffness for acid hydrolyzed phytoglycogen nanoparticles with a corresponding reduction in their fragility index.   

Controlling Solution Behavior of a Novel Peptide-based Polymer

Novel polymer materials are synthesized using N-terminal modified peptide coiled-coils via thiol-maleimide ‘click’ reaction. One example is peptide rigid rods, formed by conjugation of anti-parallel, tetrameric coiled-coils (bundlemers). Concentrated solutions of the rod chains display significant birefringence, thus showcasing the presence of lyotropic liquid crystal phases. In this work, we further explore ways to control liquid crystal behavior of rigid rods with new computational designs of peptide sequences. Various degrees of alignment of rigid rods in their liquid crystal phases can be achieved by manipulation of their net charge in solution. To understand the charge interaction of rigid rods, experiments were conducted using different sequences (net charge from -24 to +28 per bundlemer at pH 7) at the same pH as well as the same sequence at different solution pH and ionic strength. We also studied how these different solution conditions can affect semiflexible chains, which were synthesized with bundlemers connected to tetra-functional organic linkers. Inter- and intra-molecular electrostatic interactions of semiflexible chains are expected to change with its net charge, thus affecting its chain conformation in solution. Preliminary TEM and cryoTEM data will be presented.   

Polymer Brushes through Programmable Polymer Crystal Templates

Polymer brushes are polymer chains which are chemically or physically tethered to an exterior surface. Planar polymer brushes have been primarily deposited through two processes: grafting-to and grafting-from. Recently, our group developed an alternative grafting-to approach which utilizes polymer single crystals as a template for polymerization, which contain chains with an end-functionalized moiety for coupling to a surface. The planar polymer single crystals can then be deposited onto a functionalized surface and immobilized, yielding a polymer brush layer matching the geometry of the parent crystal. This affords unique control over the polymer brush deposition process as dependent brush morphology can be manipulated during the solution-based formation of the host crystals, leading to a versatile method to deposit brushes with various compositions and architectures. In this presentation, we demonstrate the synthesis and characterization of patterned and gradient polymer brushes of homopolymers and copolymers. The morphologies and methods developed in this study provide a robust template to deposit complex brush architectures through programmable solution-based methods.   

Determination of β Phase Crystalline Fraction in Electrospun Poly(vinylidene fluoride) Fibers

The β-crystallographic phase of poly(vinylidene fluoride), PVDF, is often used in fabricating piezoelectric and pyroelectric materials. However, the specific equilibrium heat of fusion of β-PVDF has not yet been definitively determined. The difficulty in this measurement arises from the necessity of obtaining solely beta phase crystals, as well as quantifying the overall degree of crystallinity. Here, we investigate the crystalline fraction of β-PVDF in fibers prepared by electrospinning from solutions of 12.5 wt% and 15.0 wt%. Scanning electron microscopy images show that the fiber diameters range from 0.2 µm to 10 µm. This preparation produces dominantly beta phase crystals by using the simultaneous application of mechanical and electrical forces to stretch the polymer fibers thus favoring the all-trans chain conformation. The crystalline fraction of a specific crystallographic phase was determined by wide-angle X-ray scattering and Fourier-transform infrared spectroscopy peak deconvolution. Melting enthalpy of the crystals was determined by the differential scanning calorimetry. A preliminary estimate of the equilibrium heat of fusion of beta-PVDF will be presented.   

The Early Stages of Polymer Crystallization: Nucleus-Induced Nucleation and Interfacial Structuring

Fully realizing the potential of advanced polymeric materials is predicated on controlling polymer crystallization. However, a comprehensive understanding of primary polymer crystal nucleation remains elusive. Therefore, we have directly quantified the shape and interfacial properties of embryonic polymer crystals (i.e., nuclei) at the molecular level during polymer crystal nucleation in entangled polyethylene melts under non-flow conditions. Through coarse-grain molecular simulations, we find that one nucleus can enhance the formation of additional nearby, distinct nuclei in a manner consistent with the lamellar stacking of semi-crystalline polymeric materials. Furthermore, the nucleus-melt interface spans several nanometers, and there is a decoupling of polymer properties in the interfacial region such that nuclei reside in nematic droplets. Through these insights, we are able to reconcile disparate previous results concerning polymer crystallization under flow and non-flow conditions from both experimental and computational studies, building bridges between regimes that have been considered distinct historically. Our work affords a new nanoscopic perspective on polymer crystallization.   

Combining shear and magnetic fields to tune the structure and properties of aqueous polymeric solutions

External stimuli such as electric and magnetic fields are commonly used to impart long-range ordering in diamagnetic block copolymers (BCPs) with anisotropic microstructures via phase alignment. However, we have recently shown that the rheology and microstructure of aqueous solutions of spherical BCP micelles can also be altered with low-intensity magnetic fields (0.1-0.5 T), via a mechanism alternative to alignment. Beyond a critical magnetization time, these low viscosity (~0.01 Pa.s) polymer solutions transform into semisolid gels with 3-6 orders increase in dynamic moduli, at temperatures far below the onset of thermal phase transitions. Small-angle X-ray scattering reveals that the field induces ordered packings of spherical micelles that lead to the observed changes in flow behavior. The transition kinetics, pathway, and ultimate phase formed can be altered by combining flow and magnetic fields or by varying the magnetic flux density, temperature, polymer concentration and salinity. This research demonstrates a facile new way to control the flow properties of polymer solutions by low-intensity magnetic fields.   

Sustainable Thermoplastic Elastomers with Ionic Interactions

Thermoplastic elastomers (TPEs) are widely used in electronics, clothing, adhesives and automotive components due to their high processability and flexibility. ABA triblock copolymers, in which A represents glassy end-blocks and B a rubbery midblock, are commercially available TPEs. commercial TPEs are derived from petroleum whose manufacturing and disposal have undesired environmental impacts, motivating the development of TPEs from sustainable sources. However, polymers with bulky constituents, such as the long alkyl side-chains of vegetable oil-derived polymers, exhibit poor mechanical performance due to lack of entanglements in the rubbery matrix. Transient networks were incorporated into the midblock through either hydrogen bonding or ionic interactions to improve mechanical performance. ABA triblock copolymers were synthesized with poly(n-butyl acrylate – ran – acrylic acid) or poly(lauryl methacrylate – ran – methacrylic acid) copolymer midblocks and poly(methyl methacrylate) endblocks. Characterization of these systems indicates that relaxation time of the rubbery midblock is the controlling parameter for enhancement of mechanical properties.   

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.   

Recycling Polyethylene terephthalate (PET) in milder and greener conditions by inserting the 2,5-dihydroxyterephthalate repeating unit.

Recycling of Polyethylene terephthalate (PET) has attracted extensive attention in recent years due to the growing utilization of single-use plastic products and subsequent accumulation in the environment. Physical recycling of PET is used industrially; however, only so much recycled material can be utilized and the number of times plastic is recycled has a significant impact on the performance. In this study, the insertion of diethyl 2,5-dihydroxyterephthalate into the backbone of PET during polymerization allows for selective and facile degradation of the produced material. By use of a 1% ZnCl2 aqueous solution, the metal ions coordinate with the functional groups ( hydroxy and ester) and allow for selective hydrolytic deconstruction of the polymer. Heating the polymer suspended in the salt solution to 180-200 °C for 2 hours allows for nearly complete (98%) deconstruction of the polymer. Following the deconstruction, the monomers (terephthalic acid and 2,5-dihydroxyterephthalic acid) and the bis(2-Hydroxyethyl) terephthalate dimer were reclaimed. This methodology demonstrates a facile, cost-effective procedure for recovering virgin monomeric units that could open the door to more efficiently recycled plastics and significantly less environmental waste.   

Pathway dependent self-assembly of positively charged computationally designed coiled-coil peptide 'bundlemer' chains

A series of 29 amino-acids peptides were computationally designed to be self-assembled into tetrameric, anti-parallel coiled coil peptide bundles, or ‘bundlemer’, with a desired net charge. Here, peptides with +4 charge are synthesized with the N-termini of constituent peptides modified with cysteine or maleimide functionality. These two peptides are self-assembled into 2x4 nm size peptide bundlemers and subsequently conjugated by Thiol-Michael reaction to form peptide bundle chains. However, the peptides chains’ length is only controlled by stoichiometry leading to high dispersity in length. Thus, another pathway is proposed to control chain length and dispersity. Peptides are first dissolved and reacted in an organic solvent and then self-assembled with different water addition pathways, including titration and direct quenching. Transmission electron microscopy (TEM) was used to investigate the length distribution of peptide rods. Small-angle neutron scattering (SANS) was utilized to characterize the size and structure of the bundlemer chains. Polarized optical microscopy (POM) was applied to study the liquid crystalline behavior of conjugated peptide bundle chains. The effect of salt on the solution behavior of the positively charged coiled coil chains also will be discussed.   

Uniformity of Partial Deuteration in Narrow-distribution High-density Polyethylene Prepared by Saturation of Polycyclopentene over Homogeneous Catalysts

Isotopic labeling - replacement of hydrogen (H) with deuterium (D) - is often required for
small-angle neutron scattering, which allows measurement of polymer chain dimensions in bulk and thermodynamic interactions in blends. It is highly desirable to achieve a uniform distribution of D at both the intra- and inter-chain levels, such that scattering contrast is evenly imparted along and between chains. We describe narrowly-distributed and uniformly partially-deuterated polyethylenes (DPE), prepared by ring-opening metathesis polymerization of cyclopentene followed by catalytic deuteration with either carbonylchlorohydridotris(triphenylphosphine)ruthenium(II) or Wilkinson’s catalyst. While density measurements, as well as FT-IR and ¹³C NMR spectroscopies, all indicate approximately 20% deuteration in both DPEs, as expected from deuterium addition across the double bonds in the polycyclopentene precursor, ¹³C NMR spectra reveal that Wilkinson’s catalyst produces more regular labeling (83% vicinal D) than the Ru-based catalyst (39%). Further analysis of isotopic shifts and multiplicity gives insights into the mechanisms of isomerization and the H/D exchange reactions that occur concurrently with deuteration using these catalysts.   

Terahertz time-domain spectroscopic study on boson peak of Li0.25Na0.25K0.25Cs0.25PO3 glass

Disordered materials exhibit universal dynamics in the terahertz (THz) region, that is, the so-called boson peak. The boson peak observed universally in the THz region is clearly characterized by making a plot which is constructed by the vibrational density of states divided by the squared frequency. In the case of infrared spectrum, the boson peak can be characterized by α(ν)/ν², where α(ν) is the absorption coefficient and ν is infrared frequency. In this study, we performed terahertz time-domain spectroscopy (THz-TDS) 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. By comparing the obtained THz-TDS spectra of this sample with results of low-frequency Raman scattering and low-temperature specific heat measurement, we estimated an infrared light-vibration coupling coefficient C_IR(ν) in the THz region. According to C_IR(ν) and a theoretical model, we evaluated the uncorrelated charge of this glass.   

Effect of post-processing treatments on completeness of cure and mechanical properties of FormLabs Clear and High-Temperature printed acrylic resin

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.   

The Effect of Water Sorption and High Temperature Aging on the Calorimetric Signature of the Aging of an Epoxy Glass

The cure of DGEBA with diethanolamine results in an epoxy that is susceptible to complex aging signatures. We explore the calorimetric signatures resulting from aging under a variety of conditions. Aging at room temperature (~Tg-50°C) and ambient humidity results in distinct features in the DSC thermogram. A broad “post-Tg shoulder” is seen extending from Tg (75°C) to 175°C and centered at ~150°C. A decrease in Cp is seen between 175°C and 200°C, indicating additional chemical aging. The first heating ramp erases the post-Tg shoulder, but the change in Cp is still observed on the second heating ramp. Isothermal holds at 200°C remove the high-T decrease in Cp and increase the Tg. Samples soaked in water at ~20°C show the gradual evolution of the post-Tg shoulder indicating that this epoxy is sufficiently hydroscopic that aging under typical laboratory conditions will yield a “shoulder” that will significantly affect the DSC thermogram and complicate the analysis of physical aging.   

Effect of Physical Aging on the Yield Stress of the filled Epoxy System DGEBA/DEA/GMB

To test the effect of physical aging on the D32-4500 glass micro-balloon (GMB) filled, diethanolamine (DEA) cured diglycidal ether of bisphenol-A (DGEBA) system, DGEBA/DEA/GMB, samples were uniaxially compressed after spending between 0 to 10,000 hours in an oven at a constant temperature. These compression tests were performed at 5 temperatures, ranging from 55°C to 75°C all of which were the same temperature that the sample was aged at. The goal of these tests was to monitor the yield stress of the material and how it varies with time and temperature. The data obtained was then refined using a Weibull analysis. Additionally, tests were performed on an unfilled 828DEA system where the samples were either aged normally or aged under stress for up to 24 hours. It is expected in both the filled and unfilled cases that the yield stress will increase logarithmically with aging time until the glass transition temperature is reached.   

Boson peak analysis of sodium carboxymethyl starch glass by terahertz time-domain spectroscopy

For glass forming materials, universal excitation called boson peak (BP) is observed in the terahertz region. On the other hand, in polymer glass, there is fractal dynamics, so-called fracton, which is expected to appear above the BP frequency as a result of self-similarity of monomer molecules and anomalous diffusion process. In this study, we performed terahertz time-domain spectroscopy on microwave-treated sodium carboxymethyl starch (CM-Starch), to detect the BP and fracton. The BP of the CM-Starch appeared around 1.2 THz at lowest temperature in the α(ν)/ν² plot, where α(ν) is the absorption coefficient. The peak in the α(ν)/ν² plot disappeared due to the excess wing at high temperature. Therefore, we propose new determination of the BP frequency by inflection point of the extinction coefficient. In addition, the exponential behavior of the α(ν) above the BP frequency suggested the existence of a fracton, and the fractal dimension was estimated to be 2.17.   

Terahertz spectroscopic study on polymethyl methacrylate: Boson peak and fracton

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 the boson peak frequency as a result of self-similarity of monomer unit. In this study, we performed terahertz time-domain spectroscopy on polymethyl methacrylate to detect the boson peak and fracton. Obtained spectra are compared with results of low-frequency Raman scattering and low-temperature specific heat measurement. We also determined both the infrared light vibration and Raman coupling constants to investigate how the fractal and fracton dimensions appear in both coupling constants.   

Waiting time dependence of aging

Many non-equilibrium systems show a delayed response to a sudden perturbation and the response is represented by a relaxation function ψ(t). I first define a temporal decay constant of the relaxation by λ(t)=–∂lnψ(t)/∂t . Writing it as λ(t_w)=–Cψ(t_w)/t_w +δ(t_w) for the observation at t_w , I classify the aging into two types: Type I when δ(t_w)=0 and type II when δ(t_w)≠0. Exploiting the free-energy-landscape (FEL) theory [1], I show that (1) the relaxation of the FEL manifests itself as type II aging, (2) the temporal relaxation time can be either increasing or decreasing function of t_w and (3) the relaxation time of the FEL can be deduced from t_w dependence of λ(t_w) .
As an example, I investigate aging phenomena of dielectric relaxation by a two-level model and show that relaxation functions of the polarization and of the linear susceptibility depend on t_w when the FEL has a delayed response. I also study the intermediate scattering function for a random walk model and show that the delayed response of the FEL gives rise to type II aging.
[1] T. Odagaki, J. Phys. Soc. Jpn. 86, 082001(2017).   

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 due to the adhesion forces being higher than the cohesion 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.   

Creating surface-anchored gradient networks using UV and thermally-active small molecular gelators

We present a versatile one-pot synthesis method for creating surface-anchored gradient networks using 4-(Azidosulfonyl) phenethyl trimethoxysilane (4-ASPTMS). The sulfonyl azide (SAz) group of 4-ASPTMS is UV (≤ 254 nm) and thermally active (≥ 100°C). It thus enables to vary the crosslink density in two opposite directions by activating the SAz groups independently via UV or temperature. We deposit a thin layer (~200 nm) of a mixture comprising ~90% precursor polymer and ~10% of 4-ASPTMS on a silicon wafer. Upon UV irradiation or annealing the layer, SAz releases nitrogen by forming singlet and triplet nitrenes that concurrently reacts with any C-H bond in the vicinity (via C-H insertion crosslinking reaction mechanism), forming the sulfonamide crosslinks. Condensation among trimethoxy groups in bulk connects the 4-ASPTMS units and completes the crosslinking. 4-ASPTMS near the substrate reacts with surface-bound -OH motifs and enables the network's covalent attachment to the substrate. Using such a simple process, we demonstrate the generation of surface-anchored gradient networks exhibiting crosslink density (or stiffness) gradients in orthogonal directions.   

Probing the glass transition and dynamic structural relaxation of polyzwitterions using fast scanning calorimetry

Polyzwitterions are polymers with side groups that contain a covalently linked anion and cation. Depending upon side chain structure, this can lead to high glass transition temperatures, occurring near the onset of degradation. In this study we are using fast scanning calorimetry to investigate how different chemical structures in the side-group alter the glass transition (T_g) of polyzwitterions. A series of six sulfobetaine polyzwitterions were synthesized, each with a unique chemical modification of the side chain. T_g could not be observed prior to degradation in four of the polymers using conventional slow scan DSC. Using fast scanning calorimetry, degradation was avoided by heating and cooling at 2000 K/s which allowed the first measurement of T_g in these materials. We found that all six polymers have a T_g of 200 ^oC or greater depending on the specific side groups. Measurement of the fictive temperature (T_f) on heating at 2000 K/s after cooling samples at different rates allowed determination of the dynamic fragility. We found that the polyzwitterions are moderately strong polymer glass formers, indicating the significance of the unique ionic structure of these polymers to their structural relaxation.   

Studying the Current Modulation of Bundled DNA Nanostructures in Nanopores

Experimental studies on the current modulation of bundled DNA structures revealed a non-monotonic dependency between the salt concentration of vanishing modulation and the size of the DNA bundle. Noteworthy, this crossover salt concentrations significantly deviate from the concentration for a single DNA molecule. We present results for simulation models on different levels of detail that show consistent results but deviate from the experimental findings.   

Quantitative Analysis of Isotopic Blends by Infrared Nanospectroscopy

Atomic-force microscopy coupled with infrared spectroscopy (AFM-IR) deciphers surface morphology by mapping physical topography as a function of relative chemical composition. While AFM-IR analysis provides information on the domain size and distribution of specific chemical moieties, it remains unclear if nanoscale phase separation obtained by AFM-IR can be compared to phase separation acquired by scattering-based techniques. Additionally, AFM-IR is severely limited by the lack of quantitative compositional information inherent to the detected signal. Herein, we resolve nanoscale phase separation in isotope polymer blends and compare the result to data acquired by small-angle neutron scattering and resonant soft X-ray scattering. Fundamental FTIR analysis is also applied to obtain quantitative blend composition of isotopic, polymer blends. Quantitative composition information is calculated by the construction of a calibration curve and applied to spectra acquired across phase-separated domains in each polymer blend. Demonstratively, combining fundamental FTIR analysis with AFM-IR is a robust technique and compliments scattering-based measurements for probing nanoscale morphology and composition.   

Interfacial Assembly of Bundlemer Brushes

Bundlemers are computationally designed peptides that self-assemble in water into coiled coils, essentially monodisperse nanoparticles with tunable surface chemistry. Leveraging this tunability we have demonstrated bundlemer 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 bundlemers as macromonomers to create stiff extremely high aspect ratio, hybrid covalent-supramolecular polymers through click chemistry between the neighboring bundlemer ends. Through incorporation of substrate binding terminal residues and careful selection of bundlemer functionalization we can design bundlemers to assemble with preferred orientation on a variety of surfaces and fine tune their adsorption kinetics. We can then develop bundlemer brushes through either a grafting to approach coupling the aforementioned high aspect ratio hybrid covalent-supramolecular polymers directly to a substrate through functionalized termini or a grafting from approach using a template layer of bundlemers as reaction sites for subsequent bundlemer coupling or assembly.   

Evaporation Induced Crystallization of Poly(L-lactide acid)-b-Poly(ethylene glycol) at Liquid-Liquid Interface

The self-assembly nature of amphiphilic block copolymer (BCP) has been widely studied to create complex structures with controlled size, shape, and distribution. To date, most efforts have been devoted to the study of self-assembly of BCPs in bulk or in solutions, in which the structure are determined primarily by polymer molecular weight, volume fraction, and the Flory-Huggins parameter. Liquid-liquid interface provides additional control for crystallization and self-assembly as the asymmetric interface is vital to the structure formation. In this presentation, we report the evaporation induced assembly behavior of a series of amphiphilic BCP Poly(L-lactide acid)-b-Poly(ethylene glycol) (PLLA-b-PEO) at liquid/liquid interface. Our work showed that the crystallization pathway and self-assembly behavior of the BCP are dramatically different from solution self-assembly and crystallization. Effects of the block length, polymer concentration and assembly kinetics have been systematically investigated. Our work demonstrated that liquid/liquid interface provides an important control for BCP structure formation and crystallization.   

Ultrahigh-χ Block Copolymer Materials with Versatile Etch Selectivity for Sub-10 nm Pattern Transfer

Studies of block copolymer (BCP) materials and their phase separation in bulk and thin-film have exploded over the last decades due to the wide range of accessible morphologies and feature sizes. The basic BCP self-assembly principles have enabled the community to control the domain size and target the smallest sizes possible using BCPs with high interaction parameter. Accessing sub-5 nm feature size is not a challenge anymore. Transferring the BCP features to a substrate over a large area with high fidelity presents additional challenges, especially at the 10 nm length scale. In this work, the highly polar poly(3-hydroxystyrene) (P3HS) is incorporated with poly(dimethylsiloxane) (PDMS). We explore both P3HS/PDMS-based diblocks and triblocks. The BCPs show various well-ordered structures with the smallest lameallar domain of 7.4 nm. Mean-field theory analysis of the temperature-dependent correlation-hole scattering gives χ_(T) = 33.491/T + 0.3126. Thin-film self-assembly is studied and the domains are aligned vertically by solvent annealing. These new BCPs not only exhibit high interaction parameter, but also present high etch contrast and versatility to facilitate pattern transfer.   

Structure of Irreversibly Adsorbed Star Polymer Layers M. Gizem Kirevliyasi1, Bulent Akgun1 1Department of Chemistry, Bogazici University, Bebek 34342, Istanbul, Turkey

Viscosity, glass-transition temperature, and diffusivity of polymer chains in thin films deviate from bulk due to the presence of adsorbed polymer chains on the substrate. The structure of adsorbed layers 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 architecture on the structure of the adsorbed layer is not clear.

Well-defined linear, 4-arm and 8-arm star polystyrene (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 AFM. 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. Star polymers formed thicker adsorbed layers on Si-H compared to SiO_x surfaces. Adsorbed layer thickness is found to be a function of initial film thickness up to 100 nm. Annealing temperature has an impact on the equilibrium adsorbed layer thickness.   

Large Tg shift in hybrid Bragg stacks through interfacial slowdown

Studies of glass transition under confinement frequently employ supported polymer thin films, which are known to exhibit different transition temperature T_g close to and far from the interface. Various techniques can selectively probe interfaces, however, often at the expense of samples designs very specific to a single experiment. Here, we show how to translate results on confined thin film T_g to a 'nacre-mimetic' clay/polymer Bragg stack, where polymer molecular layer number is precisely tunable. Exceptional lattice coherence multiplies signal manifold, allowing for interface studies with both standard T_g and broadband dynamic measurement. For the monolayer, we not only observe a dramatic increase of T_g (~ 100 K), but also use X-ray photon correlation spectroscopy (XPCS) to probe platelet dynamics originating from interfacial slowdown. This is confirmed from the bilayer, which comprises both “bulk-like” and interface contributions, as manifested in two distinct T_g processes. Since platelet dynamics of mono- and bilayers are similar, while segmental dynamics of the latter are found to be much faster, we conclude that XPCS is sensitive to the clay/polymer interface. Thus, large T_g shifts can be engineered and studied once lattice spacing approaches interfacial layer dimensions.   

Thermal Investigations on Polyethylene Oxide Nanofibers

Solutions of polyethylene oxide (PEO) - deionized water of concentrations between 3 and 10 % wt, have been prepared and homogenized by stirring. These solutions were centrifugally spun in the air, at room temperature by using a Fiberio Cyclone L-1000M equipment at various spinning rates between 2,500 to 12,000 rotations/minute. The nanofibers were allowed to dry at room temperature for 24 h. Thermogravimetric measurements were performed to control water’s evaporation and the thermal stability of the nanofibers by using a TG 209 F3 Tarsus-Netzsch instrument operating under the nitrogen atmosphere, in the temperature range 50 to 1000 ^oC. Measurements with different heating rates from 1 ^oC/min up to 50 ^oC/min have been performed on both nanofibers and pristine polymer powder. The goal was to determine if the diameter or the surface area of these nanofibers affects their thermal stability.
PEO nanofibers have been investigated by Differential Scanning Calorimetry (DSC), by using a Netzsch DSC 214 instrument. The research focused on the effect of PEO confinement in nanofibers on the glass transition. DSC data on both PEO powders and nanofibers have been recorded at various heating rates between 1 oC/min up to 50 ^oC/min.   

Manipulating Polymer Blend Composition Facilitates a Low-Cost, High-Fidelity Sensor for Indoor CO2 Monitoring

Carbon dioxide (CO₂) has been linked to many deleterious health effects and has thus created a need for an indoor small-scale sensor with suitable low-cost fabrication materials for adequate monitoring of CO₂. Here, a resonant mass sensor is treated with a low-cost and solution-processable polymer blend of poly(ethylene oxide) (PEO) and poly(ethyleneimine) (PEI) which facilitates a surface morphology alteration upon blending which promotes selective and significantly enhanced detection of CO₂ gas. Based on SEM, AFM, XRD, and DMA characterization techniques, we observed PEO when incorporated into PEI allowed for a physical polymer disruption of interchain amine interactions which facilitated the interaction between intrachain amines and CO₂. Due to these changes in surface morphology upon blending in PEO, there is less amorphous uniformity in the PEI and better stiffening of the overall polymer composition which allows for better diffusion of CO₂ into the material. This alteration is important for establishing fundamental models for polymer-based gas adsorption mechanisms and provides potential design rules for promoting gas adsorption capacity, selectivity against other interfering gases, and expanding detection limits for sensor applications.   

Functionalized Polymeric Coatings for Selective Ion Removal

Desalination processes such as reverse osmosis (RO) units are energy efficient, however they lack the capabilities to selectively remove specific ionic contaminants from their feed streams. Electrosorption separation processes are able to promote selective ion removal, but ion selectivities in current processes are limited. Here, we will develop ion selective materials with functionalities to promote ion selectivity and incorporate them into electrosorption ion removal processes. Specifically, we will study ion selective layers made of polymers functionalized with chelating groups such as sulfonic, iminodiacetic, and phosphonic acid that can target the selective removal of divalent scale forming ions such as calcium and barium. Additionally, we will synthesize and study covalent organic frameworks (COFs) with controlled pore sizes and functionalities to target the selective removal of scalants. The impact of this work will be multifold, creating a system with the capabilities to purify water for drinking water purposes while also working to efficiently clean industrial waste streams for environmental sustainability.   

Effects of Glass Transition Temperature and Homopolymer Additives on Ion Transport in Model Salt-Doped Block Copolymers

Salt-doped block copolymers (BCP) are promising as solid electrolytes because they simultaneously provide ionic conductivity and mechanical strength due to the combination of two distinct polymer microphases. The motion of ions within the BCP electrolyte is correlated with chain segmental motion and glass transition temperature (Tg). Recent work has shown that incorporating a relatively high molecular weight (MW) homopolymer can lead to a relatively mobile homopolymer-rich region within the conducting microphase, resulting in a higher overall ion conductivity. We use coarse-grained molecular dynamics simulations to understand these effects and guide further experimental study. We adjust the Tg difference between phases by varying the monomer interaction parameters and masses. We analyze the distribution of homopolymers with different MWs in the conductive domain at different Tg. We also calculate the ion velocity distribution for these systems and show that the dynamic behavior of ions depends on their location within the conductive domain.   

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.   

Through-plane Structural Analysis of Engineered Nafion Surfaces

The thin-film structures of Nafion, a model perfluorinated ionomer, impact advances in proton-exchange membrane fuel cells (PEMFC). Engineered Nafion surfaces were developed by Dowd et al. to alter the surface composition and wettability of Nafion^[a]. Using neutron reflectometry (NR), we probe the through-plane structure of Nafion thin-films. Buried layers of phase-separated lamellae were observed in Nafion thin-films at Au, Pt, and SiO₂ interfaces by Dura et al.^[b] and DeCaluwe et al^[c]. We developed and applied titanium nitride (TiN) substrates to minimize the neutron scattering contrast to interface structures thus optimizing the sensitivity of NR to the presence and structure of the engineered Nafion surface. Unprocessed Nafion thin-films were studied via NR in dry (0% RH) and humidified (92% RH) conditions with H₂O and D₂O vapor. Future research will measure the engineered Nafion thin-films to independently determine water uptake differences and possible surface layers after modification. We intend to expand the work to other substrates that may be sensitive to a modified Nafion surface, such as titanium dioxide (TiO₂) and sputtered carbon (C).

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

Effect of Interfaces on the Segmental Dynamics of Polymer Electrolyte in Lithium Ion Batteries

A polymer electrolyte consisting of a polymer host such as poly(ethylene oxide) (PEO) and a lithium salt may be used in solid-state lithium ion batteries as the electrolyte layer between the battery electrodes or as the catholyte in the battery cathode. In a solid-state battery, the polymer electrolyte may face many interfaces, as ceramic fillers being added to the electrolyte layer and as a catholyte being blended with the active cathode material. The segmental mobility of the polymer electrolyte determines its ion transport rate. In this work we use quasi-elastic neutron scattering (QENS) to examine the presence of interfaces on the segmental mobility of polymer electrolytes. Two examples are investigated. In the first example the segmental dynamics of a composite electrolyte consisting of a polymer electrolyte and ceramic fillers is discussed. In the second example the effect of the cathode active material on the segmental dynamics of the polymer electrolyte in a composite cathode is investigated. These results shed light on strategies to synthesize interfaces for optimized polymer electrolyte performance in a lithium ion battery.   

Planar Assembly ‘Puzzlemer’ Particles: A Coarse Grained Model of Geometrically Frustrated Self-Assembly

Geometrically frustrated assemblies (GFAs) are attractive as they can exhibit self-limiting assembly into equilibrium structures characterized by a finite dimension. Here, we describe a new coarse grained model of a simple GFA building block that we call the “puzzlemer” particle. The taper shape and interactions of this particle promote a locally planar crystal packing that is frustrated by a preferred curvature of the lattice rows. Our central aim is to understand how particle properties control the intra-assembly mechanics, and how these in turn dictate the growth of stresses in the assembly with increased size. In particular, we focus on understanding the relative stiffness to inter-particle bending vs. stretching. This ratio tells us something about how the assembly accommodates frustrated packing: i.e. is it energetically favorable for the monomers to spread out, or flatten their rows? We determine an effective length scale from the ratio, which we hypothesize to be related to the maximal size of self-limiting assembly. We test the predicted connection between pair-wise particle mechanics and self-limiting assembly thermodynamics via numerical studies of the energy landscapes of puzzlemers for variable particle interactions and shape misfits.   

Correlating Geometric Features with Stress Distributions in Pressurized Anisotropic Aortas to Study the Mechanics of Aortic Diseases

Aortic dissections originate with a tear in the inner layer of the aortic wall, which comprises its integrity and creates a mechanically unstable system prone to fracture. Such fractures are often seen to propagate from the descending aorta to the ascending aorta. The mechanism by which this occurs is unclear but is strongly influenced by the aorta’s geometric complexity (composed of a hyperbolic section and a cylindrical section) and anisotropic fiber-reinforced composition. As such, the ability to link stress information from finite element analysis (FEA) with purely geometric information can help improve understanding of dissection transitions. Here, we seek to develop this linkage by studying a patient model with a dissection propagation from the descending to ascending aorta. We performed FEA to model the aorta as a pressurized curved shell made of an anisotropic elastic material (using the Ogden-Gasser-Holzapfel constitutive model). From this, we can calculate both the curvature tensor and the stress tensor and correlate them. In particular, we concentrate on the hyperbolic lesser aortic arch, where double curvature strongly influences the stress field. Further establishing this link between biomechanical stress and geometry may help improve treatment for these diseases.   

Characterization of the counter-ion cloud of pNipam microgel via SANS

Microgels as soft colloids are mesoscopic particles suspended in a solvent. In contrast to hard colloids, the phase behavior of soft, deformable particles is not well understood. Although pNipam is uncharged, pNipam microgels contain charged groups because of particle synthesis. The counterions are expected to be mostly located at the periphery of the microgels. A fraction of the counterions if free sets the osmotic pressure of the microgel suspension and causes the spontaneous microgel deswelling reported in previous work.

Using SANS, we find pNIPAM microgels to deswell at a true volume fraction well below random close packing. This suggests that microgels are effectively larger due to their counterion cloud. To obtain the form factor of the ion cloud, we compare SANS measurement of pNIPAM suspension with Ammonia and Sodium ion clouds at the same concentration. We find a signal reminiscent of the counter-ion cloud, but it is at the detection limit of SANS. While more measurements are on the way, the analysis of our current data indicates the cloud could present at the fuzzy shell with width at around 50nm.   

Mesoscale Polymer Ribbons as Hierarchical Building Blocks

Mesoscale polymer ribbons (MSPs) are unique high aspect ratio structures formed by controlled evaporative self-assembly. Due to their 3D helical conformation upon release in solution and their sensitivity to environmental changes, MSPs offer a novel system of building blocks to construct hierarchical assemblies. Control of synthetic materials from building blocks spanning hundreds of nanometers to tens of microns presents an opportunity to impart anisotropy and enhanced mechanical performance, though current methods of mesoscale control are limited. We harness interfacial forces to control the assembly of long, flexible MSPs in aqueous media.Our data quantifies the changes in inter-ribbon entanglement and aggregation as a function of ion concentration in aqueous environments and at the perfluorodecalin-water interface. Using light microscopy, we study the evolution of ribbon conformation and inter-ribbon interactions as a function of time. This work elucidates the interfacial interactions and assembly phenomena in multi-ribbon mesoscale structures, underpinning the MSP as an emerging platform in soft, bioinspired hierarchical assemblies.   

Active contact forces drive non-equilibrium fluctuations in membrane vesicles

We analyze the non-equilibrium shape fluctuations of giant unilamellar vesicles encapsulating motile bacteria. Owing to bacteria--membrane collisions, we experimentally observe a significant increase in the magnitude of membrane fluctuations at low wave numbers, compared to the well-known thermal fluctuation spectrum. We interrogate these results by numerically simulating membrane height fluctuations via a modified Langevin equation, which includes bacteria--membrane contact forces. Taking advantage of the length and time scale separation of these contact forces and thermal noise, we further corroborate our results with an approximate theoretical solution to the dynamical membrane equations. Our theory and simulations demonstrate excellent agreement with non-equilibrium fluctuations observed in experiments. Moreover, our theory reveals that the fluctuation--dissipation theorem is not broken by the bacteria; rather, membrane fluctuations can be decomposed into thermal and active components.   

Surface patterning droplets: A new route to hierarchical self-assembly

In this study, we present a strategy for controlling the spatial distribution of binary nanoparticle mixtures on a liquid droplet interface. Molecular dynamics simulations reveal that mixtures of dipolar and non-dipolar particles assemble into dynamically frustrated amorphous structures at the interface. A time-dependent magnetic field is used to overcome kinetic barriers, forming configurations resembling the thermodynamic ground state. These features are encapsulated in a dynamic phase diagram with field strength (relative to dipole strength) and frequency (relative to nanoparticle relaxation time) acting as state-controlling variables. The orienting field biases dipolar particles near the equator while non-dipolar particles are localized at the poles which are along the axis of the field. This patterning is reversible, as changing field direction will redirect the poles on the droplet to align with the field. Finally, we discuss droplet ferromagnetic properties determined by micro-magnetic simulations.   

Light sheet microscopy for fast volumetric imaging of colloidal fluids under shear

Light sheet fluorescence microscopy (LSFM) is emerging as a popular volumetric imaging method to study living cells, tissues, and whole organisms. It minimizes potential photodamage to the sample by confining the excitation light to the focal plane. However, most implementations of LSFM require two objective lenses placed orthogonally to one another. Such a setup precludes the use of high numerical aperture lenses and of many common sample mounting methods, such as between a glass slide and coverslip. Here, we adopt a single objective LSFM design. A single objective is used to both illuminate and image our sample. Downstream remote focusing lenses allow us to image the plane coincident with the illumination plane. With this configuration we are able to use a high numerical aperture objective lens (NA > 1.0) and acquire volumetric imaging data at several Hz. We show that this configuration and other LSFM setups are well suited to imaging colloidal samples. Particularly, we show that LSFM can be used to image colloidal and other soft matter samples while those samples are mechanically perturbed.   

Boson peak behavior of sodosilicate glasses probed by terahertz time-domain spectroscopy

Disordered solids are characterized by a lack of long-range structural order, which influences their vibrational dynamics. The appearance of an excess of vibrational modes over the Debye level at the terahertz frequencies is called the boson peak (BP). However, the origin of the BP is still an unsolved problem. For detecting and interpreting this behavior, we find that the BP also appears in the spectrum of α(ν)/ν² where α(ν) is the absorption coefficient. In this study, we successfully detect the BP behavior of sodosilcate glasses (xNa₂O●(1-x)SiO₂, x = 0.2, 0.25, and 0.33) by terahertz time-domain spectroscopy. Finally, we observe that the BP frequency shifts to a higher value as the Na₂O concentration increases and the shape of the BP becomes broad and blurry as the temperature increases.   

Light-switchable deposits from evaporating droplets containing motile microalgae

Deposits from evaporating droplets have shown to take a variety of shapes, depending on the physicochemical properties of both solute and solvent. Classically, the evaporation of droplets of colloidal suspensions leads to the so-called coffee ring effect, a nuisance in many technological applications. Here, we investigate deposits from evaporating droplets of living motile microalgae (Chlamydomonas reinhardtii) and take advantage of their light-sensitivity via phototaxis to control the final pattern: adjusting the incident angle and wavelength of the light makes it possible to force or completely suppress the formation of algal 'coffee rings', and even to control their spatial structure.   

Colloidal gels displaying both flow-aligned and vorticity-aligned flocs under shear

To connect the structural and flow properties of colloidal gels we experimentally probe colloid-polymer mixtures on a rheometer with an optical microscopy attachment. We study mixtures of a polymer and temperature-responsive hydrogel particles (pNIPAM) which undergo fluid-fluid phase separation or gelation at room temperature depending on concentrations. At higher temperatures (above about 32 °C) the hydrogel particles form a gel. We observe these mixtures as they are sheared in a parallel-plate geometry with an optically transparent base plate and a small gap thickness. At room temperature, we observe flow-aligned domains with shear-rate-dependent dimensions. Above 32 °C, the stronger interparticle attraction leads to vorticity-aligned flocs. We can cycle between flow-aligned and vorticity-aligned domains by varying the temperature by just a couple degrees. This allows us to investigate the formation of these shear-induced domains and to connect their spatial characteristics with the bulk sample flow properties.   

Stokesian Dynamics Simulations of a New Microswimmer Model in Various Settings: Daily Life at the Small Scale

We introduce a new theoretical microswimmer model which in principle also admits of physical construction. The swimmer is implemented computationally within the framework of our constraint Stokesian Dynamics (SD) approach in the particular form of the HSHAKE algorithm[1],
as recently extended to allow for the presence of a linear flow[2]. We apply this microswimmer model in SD simulations to explore various aspects of the physics of swimming in the low Reynolds number regime. First, we study the swimmer motion in unbounded quiescent fluid, then we examine the effect of inserting a nearby infinite plane wall, specifically as it pertains to swimmer retardation and possible capture by the hydrodynamic effect of the wall, and
finally we investigate the result of applying a shear flow to a wall-captured swimmer. Our findings highlight the often counterintuitive aspects of swimming at the microscale. Lastly, we stress that the approach adopted to implement computationally our conceptual microswimmer is applicable in general to any other theoretical microswimmer.

[1] R. Kutteh, J. Chem. Phys., 2003, 119, 9280; R. Kutteh, Phys. Rev. E, 2004, 69, 011406.
[2] See other talk “Stokesian Dynamics of Arbitrary-Shape Passive and Active Particles in Linear Flow: Constraint and Rigid Body Approaches”   

Microscopic Picture of Calcium-Assisted Lipid Demixing and Membrane Remodeling Using Multiscale Simulations

The specificity of anionic phospholipids–Ca²⁺ ion interaction and lipid demixing has been established as a key mechanism in several cellular signaling processes. The mechanism and implications of this Ca²⁺-assisted demixing have not been elucidated from a microscopic point of view. Here, we present an overview of atomic interactions between Ca²⁺ and lipids that can drive non ideal mixing of lipids in a model bilayer composed of zwitterionic (phosphatidylcholine - PC) and anionic (phosphatidylserine - PS) lipids with computer simulations at multiple resolutions. Lipid nanodomain formation and growth were driven by Ca²⁺-enabled lipid bridging of the charged PS headgroups, which were favored against inter-PS dipole interactions. Consistent with several experimental studies of Ca²⁺-associated membrane sculpting, our analyses also suggest modifications in local membrane curvature and cross-leaflet couplings as a response to such lateral heterogeneity. In addition, reverse mapping to a complementary atomistic description revealed structural insights in the presence of anionic nanodomains, at timescales not accessed by previous computational studies. This work bridges information across multiple scales to reveal a mechanistic picture of Ca²⁺ ion’s impact on membrane biophysics.   

Clustering and Pair Dynamics of Induced-Charge Electrophoresis Driven Janus Particles

Active colloids far from equilibrium can interact to generate remarkable collective behaviors. Among different propulsion methods, induced-charge electrophoresis (ICEP) holds a unique advantage of fine control over direction and energy flux. Herein, we report the pair dynamics and clustering of ICEP-driven Janus particles. At moderate densities, clusters are smaller than those in known diffusiophoresis experiments, suggesting there to be interactions suppressing cluster growth. In the dilute regime, the mean pair lifetime is an order smaller than the mean time between collisions, which corroborates an absence of clustering. The pair correlation function yields a prominent peak outside of hard-sphere range, suggesting interparticle repulsion. In simulations of active Brownian particles with long-range repulsive forces, we find particles parameterized with the pair dynamics produce consistently smaller clusters when compared with a hard-disk model. We rationalize that repulsive hydrodynamic forces which slows down particles over a distance are the primary role players. Our results indicate long-range interactions as a key control parameter for the cluster phase of active colloids.   

Crumpled Matter for Roughness Tolerant Adhesion

Effectively adhering to rough surfaces can be a challenge. Strong adhesion typically relies on maximizing three factors: chemical interactions, contact area, and stiffness. On a rough surface balancing the compliance and contact area is especially challenging as low compliance adhesives are unable to achieve high contact area as the adhesive is unable to deform around obstacles. High compliance adhesives are able to deform around surface obstacles creating a high contact area but fail at lower peak forces due to the decreased stiffness. In the experiments described here, a crumpled, inextensible, and sticky sheet is used to create a rigid, but high contact adhesive bond. Specifically, sheets of various controlled polymer materials were crumpled and force-displacement curves were measured with two parallel glass plates. Next, the crumple was tested against a controlled obstacle added and compared to the data from the flat glass walls. We show that there is no significant difference between the adhesion of the crumple on smooth or “rough” surfaces.   

Crumpled Kirigami

Crushing a paper into an approximately spherical shape will create a crumpled ball made up of complex structural features such as long-range structures (folds, bends, and ridges) as well as short-range structures (D-cones). One of the most surprising things about crumpled matter is its high rigidity and light weight. Previous studies have suggested different origins of the stiffness through models based on the dominance of different structural features. In this study, we attempt to experimentally narrow down possible origins by topologically impairing long range structures in the sheet. Simple experiments using two parallel walls, a force sensor and a confocal microscope for observation, show that ‘topologically impaired’ sheets have compressive behavior which is indistinguishable from pristine sheets. We discuss several materials but are forced to conclude that short range structures (d-cones) must dominate long range structures (bends, folds or ridges).   

Not Participating

Nematic liquid crystal droplets are being investigated as a means to create self-assembled shells of quantum dot nanoparticles.1 A rich variety of possible structures is suggested both by experiments using polarized optical microscopy and by numerical modeling, but it is challenging to compare the two. Although some liquid crystal droplets display the well-known ‘Maltese cross’, the structure of the nematic director and topological defects near the self-assembling shell breaks rotational symmetry in a manner not yet well understood. Here we explore one way to characterize the director fields under spherical confinement using Jones matrix calculations. Starting with simulated director fields, we compute the predicted intensity of transmitted light that a nematic director field would show under polarized optical microscopy. This approach will enable us to compare predicted director fields with experimental observations of various assembly outcomes, providing a link between experimental data and theoretical models for controlled self-assembly in liquid crystal droplets.   

Harnessing fractal cuts to design lattice metamaterials for energy dissipation

This study explores a new group of lattice metamaterials with fractal cuts, demonstrating a noticeable energy dissipation capability via sliding friction and viscoelastic damping. Here, lattice metamaterials with three types of cuts were designed and manufactured by 3D Polyjet printing technology. Experimental results show that the structural ductility can be enhanced sharply by increasing fractal levels while keeping their shape recoverability. Though energy absorption and dissipation obtained from bending tests are significantly different, their loss factors are very close, which could be attributed to the hybrid effect of friction and viscoelasticity. Guided by finite element simulations, we further demonstrate that the sliding friction plays a critical role. Results suggest that bending depth has neglectable effect on the bending stiffness and loss factors while there exists a linear scaling law between bending stiffness and sample thickness. Our study opens a new avenue to create mechanical metamaterials with exceptional energy dissipation, finding applications in many industrial sectors such as defense, energy and transportation.   

Differential Dynamic Microscopy of Active Actin-Microtubule Networks

The cytoskeleton is a non-equilibrium network of protein filaments, including semiflexible actin and rigid microtubules, as well as associated motor proteins that can dynamically rearrange the filaments. Here, we create co-entangled networks of microtubules and actin that exhibit active contractile dynamics driven by myosin II motors. We use multi-spectral confocal fluorescence microscopy and differential dynamic microscopy to determine the time-varying fluctuation decay times and contraction rates of the networks. We vary the concentrations of actin, microtubules and myosin to tune the dynamics from slow, organized contraction to disordered vortical flow. We also examine the impact of sample age on the non-equilibrium dynamics. Our findings shed light onto how the active composite cytoskeleton can finely tune its dynamics by varying the interactions between its constituents.   

Decoding the Structural Origin of Creep in Colloidal Gels by Machine Learning

When subjected to a sustained load, jammed colloidal calcium-silicate-hydrate (C-S-H) gels (the binding phase of concrete) tend to exhibit delayed viscoplastic creep deformations. However, the structural mechanism of creep in C-S-H gels (and driving force thereof) remains only partially understood. This partially arises from the fact, due to the long timescale of creep, its physical modeling has remained challenging. Here, based on a mesoscale model of C-S-H gels, we present an accelerated simulation method (based on stress perturbations and overaging) to model creep deformations in C-S-H [1]. Based on these simulations, we adopt Support Vector Machine (SVM, a supervised machine learning classification algorithm) to decode the structural mechanism of creep in C-S-H gels [2]. This allows us to "find Needles in haystacks," that is, to pinpoint the key structural features enabling grain reorganizations and creep deformations within the gel.

[1] Liu, H., Dong, S., Tang, L., Krishnan, N. A., Masoero, E., Sant, G., & Bauchy, M., Journal of Colloid and Interface Science, 542, 339-346 (2019).
[2] Liu, H., Fu, Z., Yang, K., Xu, X., & Bauchy, M., Journal of Non-Crystalline Solids: X, 4, 100036 (2019).   

The stochastic dynamics and energetics of a microswimmer

Optical tweezers have been employed widely to explore the physical properties of matter. In our experiments, we implement a calibration technique known as the Photon-Momentum Method which allows the direct measurement of forces exerted by a microswimmer without prior knowledge of its refractive index and size. We trap Chlamydomonas reinhardtii using optical tweezers and measure the stochastic forces generated. We study the force dynamics of the swimmer using tools from statistical mechanics to quantify violation of the fluctuation-dissipation theorem and estimate the energy dissipation rate of the non-thermal forces generated by the microswimmer. We find that the average power dissipated by the swimmer to be 5 fW with a maximum power dissipated to be 10 - 15 fW. Interestingly, this power dissipation estimated from optical trapping measurements is in close agreement with fluid dynamics studies of viscous dissipation. Our results suggest that the stochastic energetics of a microswimmer as measured from force fluctuations via optical tweezers is compatible with the fluid dynamics approach. We anticipate that these tools can be extended to dense suspensions of self-propelled particles/swimmers.   

A step toward Darwinian Evolution in Artificial Self-Replicating Tiles

Artificial self-replication and exponential growth holds the promise of gaining a better understanding of fundamental processes in nature but also of evolving new materials and devices with useful properties. A system of DNA origami dimers has been shown to exhibit exponential growth and selection. Here we introduce mutation and growth advantage to study the possibility of Darwinian like evolution. We seed and grow one dimer species, AB, from A and B monomers that doubles in each cycle. A similar species from C and D monomers can replicate at a controlled growth rate of 2 or 4 per cycle but is unseeded. Introducing a small mutation rate so that AB parents infrequently template CD offspring we show experimentally that the CD species can take over the system in ~ 6 generations in an advantageous environment. The introduction of the possibility of mutations into an artificial self-replicating system opens the door to the use of evolution in materials design.   

Long-range structure in simulations of glassy water exhibits evidence of metastable criticality

Supercooled and glassy water exhibits many interesting and unusual properties; two examples include polyamorphism (multiple glassy states) and the theorized liquid-liquid critical point (LLCP) and associated liquid-liquid transition under metastable supercooled conditions. Despite the natural analogy between the low-density and high-density forms of amorphous ice and the corresponding forms of supercooled liquid water, the exact relationship between the amorphous ices and the supercooled liquid is incompletely understood. We performed molecular dynamics simulations of isobaric glass formation at various pressures using the TIP4P/2005 and mW water models, as well as the Kob-Andersen mixture. We observed that systems that possess an LLCP (TIP4P/2005) exhibit increased long-range density inhomogeneities in glasses formed near the liquid-liquid critical pressure, which persist even as the system is cooled to low temperatures. This behavior is absent at pressures far away from the critical pressure, as well as in systems that do not exhibit an LLCP (mW, Kob-Andersen). These findings suggest a possible structural metric to detect whether a system exhibits an LLCP, and shed additional light on the relationship between polyamorphism and metastable criticality in water.   

Mesomorphic Ceramics via Self-Assembly and Sintering of Metal Oxide Nanorods

Mesomorphic ceramics are defined as solid-state systems with intermediate order between isotropic and crystalline. A new class of mesorphic ceramics has been synthesized via lyotropic self-assembly of titanium dioxide, TiO₂, nanorods followed by sintering. A lyotropic dispersion of ligand-capped anatase nanorods forms a polydomain nematic phase and, when thermally treated, nanorods fuse together into low aspect ratio grains that preserve crystallographic orientation within textured domains. Lyotropic suspensions can be sheared and subsequently sintered to form mesomorphic ceramic monodomain with uniaxial orientation across millimeter dimensions. The resutling monodomains exhibit high optical transparency and a nearly constant birefringence. Distinct from liquid-crystal templating, this new approach yields superstructures of nanoparticles with relative ease and at lower costs. The present study opens a pathway toward robust, ceramic-based solid films for diverse applications.   

Interfacial strength measurement of soft hydrogels with needle insertion and pressurization

Biological interfaces are critical for maintaining the structure and performing functions; however, current knowledge of interfacial strength is limited, especially for soft tissues. Here, we introduce a method of measuring interfacial strength by needle insertion and pressurization to provide quantitative insight into the criteria of interfacial damage initiation. We demonstrate this method and develop a quantitative analysis by measuring the strength of ultra-soft polyacrylamide (PAM) hydrogel interface. In order to make the PAM self-interface, plastic sheet separators are inserted before crosslinking and removed when the hydrogel is fully cured. The residual stress on the interface is tuned by the thickness of the separators. The critical pressure is observed to open the PAM self-interface and this critical pressure scales with the thickness of the separator and the size of the needle. We propose a quantitative relationship between the critical pressure and the energy release rate for the PAM self-interface. This pressurization method of adhesive strength measurement can provide information about internal interfacial properties at desired locations, making it helpful for in vivo measurements of biological tissues.   

Disordered elastic systems theory as a framework to study collective cell migration

Solving interface dynamics and statics in realistic systems beyond the elastic approximation is still a largely open theoretical/analytical problem. In this work, we propose to address this problem by analyzing a Ginzburg-Landau model, where interfaces with overhangs may be studied. We make the connection between the Ginzburg-Landau model and the elastic Hamiltonian, and propose a new observable to probe the validity of the elastic theory as a function of "defects". We approach the problem numerically and analytically, and our simulations, in addition to making contact with experiments, also allow us to test and provide insight to develop new analytical approaches to this so far intractable problem. We apply these tools to unravel properties of migrating cells-fronts, by treating its boundaries as interfaces moving in a disordered landscape. In particular, we analyze the roughness, defined as the height-height correlations in space of the interface. For experiments done on extended moving fronts of epithelial rat cells under different sets of conditions, we show how by using this framework it is possible to distinguish short-range correlations (intra-cell) and long-range correlations (inter-cell), which depend on the internal mechanisms dominating the cell colony dynamics.   

Not Participating

Climbing robots have applications like search and rescue, surveillance, and inspection. To execute such tasks and successfully navigate their complex environments, inclined, vertical, and inverted climbing is required. To address this challenge, we have developed Microelectroadhesive Treaded Robot (μETR), an origami-based, lightweight, small sized autonomous system. μETR’s key innovations include a seamless electroadhesive tread design utilizing screen printing and custom laser micromaching that adheres it to conductive surfaces, in addition to consisting of a drive alignment system, a tensioner/voltage transferer, crowned wheels, and a rigid chassis. Preliminary experiments suggest that the robot can climb 30 degree slopes, tug loads up to 7 grams, and statically adhere to angles up to approximately 70 degrees. We are currently integrating a custom, lightweight powerboard to make it untethered, as well as demonstrate vertical and inverted climbing on conductive surfaces. In the near future, an interdigitated tread design will be explored to experiment with non-conductive surfaces.   

Binding, Unbinding and Aggregation of Crescent-Shaped Nanoparticles on Tubular Lipid Membranes

The binding of crescent-shaped nanoparticles (NPs) on nanoscale tubular lipid membranes is investigated through systematic coarse-grained molecular dynamics simulations of an implicit-solvent model. We consider NPs that adhere to the outer surface of the tubule through their concave side. The binding/unbinding transition is highly first-order, with the binding energy, E_b, being higher than that of the unbinding energy, E_u. The energy barrier between the bound and unbound states increases with increasing the NP's arclength L_np or curvature mismatch μ=R_c/R_np, where R_c and R_np are the radii of curvature of the tubule and the NP, respectively. Moreover, the binding and unbinding threshold energies increase with increasing L_np or μ. NPs lie perpendicularly to the tubule's axis for μ > 1. However, for μ smaller than a specic arclength-dependent mismatch μ*<1 , the NPs are tilted with respect to the tubule's axis, with the tilt angle that increases with decreasing μ. We also investigated theaggregation of the NPs on the tubule as a function of L_np and μ, and found that the particles self-assemble into chains for high L_np and μ>1, while for μ^*<μ<1, the NPS are distributed uniformely. The NPs also aggregate into chains for μ<μ^*.   

Universal Stokesian Dynamics based procedure for determining the relationship between the effective and geometric aspect ratio of a particle

Jeffery's theory[1] of the orbital motion of a spheroid in unbounded shear flow, as extended by Bretherton[2] to any axisymmetric particle with fore-aft symmetry (APFAS), permits straightforward rapid dynamical calculation for either one such particle or a dilute suspension thereof. Although this approach eliminates the need for costly numerical methods, it requires substituting the effective aspect ratio (EAR) of the APFAS for the geometric aspect ratio (GAR) of the spheroid in the Jeffery formalism. The EAR of any APFAS is a function of its GAR determined by the APFAS shape but unknown in advance. To overcome this obstacle, we exploit our rigid body Stokesian Dynamics approach[3], as recently extended to allow for the presence of a linear flow[4], to provide a universal computational procedure for obtaining empirically the functional dependence of the EAR on the GAR for an APFAS of any shape. We illustrate this procedure for a rod and note its usefulness for other interesting shapes.

[1] G. B. Jeffery, Proc. R. Soc. Lond. A 102, 161 (1922).
[2] F. P. Bretherton, J. Fluid Mech. 14, 284 (1962).
[3] R. Kutteh, J. Chem. Phys. 132, 174107 (2010).
[4] See other talk "Stokesian Dynamics of Arbitrary-Shape Passive and Active Particles in Linear Flow: Constraint and Rigid Body Approaches"   

The leaking elastic capacitor as a model for active matter

The leaking elastic capacitor (LEC) is a non-conservative dynamical system that combines electrical and mechanical degrees of freedom. We show analytically that a LEC connected to an external voltage source can become dynamically unstable (Hopf bifurcation) due to positive feedback between the mechanical separation of the plates and their electrical charging. Numerical simulation of the fully non-linear system allow us to identify regimes in which the LEC exhibits limit cycles (regular self-oscillations) or strange attractors (chaotic behavior). This thermodynamically irreversible process generates a non-conservative force that can be used to pump current or to sustain waves propagating along flexible plates. The LEC therefore acts as an engine, cyclically performing work at the expense of the constant voltage source. We argue that this model can offer a qualitatively new and more realistic description of some important properties of active matter in condensed-matter physics, chemistry, and biology.

This work has been submitted to PRE. The full manuscript is available at arXiv:2010.05534   

Unusual Ionic Effects in Liquid Crystal Materials

Display and non-display applications of thermotropic liquid crystals rely on the reorientation of liquid crystal molecules by the applied electric field. This reorientation can be affected by ions typically present in liquid crystals in small quantities (in the simplest case, through a well-known screening effect when the electric field due to the ions can cancel out the applied electric field). Therefore, an understanding of ion-related effects in liquid crystals is critical for the development of advanced liquid crystal devices. Important information about ions in thermotropic liquid crystals can be obtained by measuring their DC electrical conductivity. An interpretation of the DC electrical conductivity data requires reasonable approximations. As a rule, a single type of dominant ions or two symmetric ions of the same mobility are assumed. In this presentation, the effects of several types of ions (characterized by different values of their mobility) on the measured DC electrical conductivity of liquid crystals are discussed. The substrates of the liquid crystal cell and their important role in the ionic processes is also considered.   

Flower-inspired tunable hierarchical wrinkles for broad-angle structural color

The cuticle on the epidermal cells of some flower petals forms nano-ridges. The hierarchical grating structures that selectively show broad-angle iridescence can enhance the foraging efficiency of pollinators. Although efforts have been devoted to mimicking this unique broad-angle structural color, the intrinsic tunability offered by natural systems to control such a broadened spectrum is still absent in synthetic models. In this work, we present a biomimetic design, based on hierarchical wrinkling of thin film multilayers, to precisely control the broad-angle diffraction. The wrinkling morphology programs the diffraction pattern: the small wrinkles control the diffraction angles, and the large wrinkles broaden the observable range of the diffraction. The hierarchical wrinkling system only occurs within a limited range of material properties and conditions. The control of the expression of structural color resembles the flowers' tunability of structural color in different regions. We anticipate that our findings will play a foundational role in the rational design of wrinkling photonics with broad-angle iridescence.   

Microscopic Difference of Hydrogen Double-minimum Potential Well Detected by Hydroxyl Group in Hydrogen-bonded System

We investigate the microscopic structure of hydrogen double-well potentials in a hydrogen-bonded ferroelectric system exposed to radioactive particles of hydrogen-ion beams. The hydrogen-bonded system is ubiquitous, forming the base of organic-inorganic materials and the double-helix structure of DNA inside biological materials. In order to determine the difference of microscopic environments, an atomic-scale level analysis of solid-state ¹H high-resolution nuclear magnetic resonance (NMR) spectra was performed. The hydrogen environments of inorganic systems represent the Morse potentials and wave function of the eigen state and eigen-state energy derived from the Schrödinger equation. The wave functions for the real space of the localized hydrogen derived from the approximated solutions in view of the atomic scale by using quantum mechanics are manifested by a difference in the chargedensity distribution.   

The Effects of Handedness Distribution on the Anisotropy Chiral Mechanical Metamaterial

Chiral mechanical metamaterial has broad applications in designing stretchable electronics, soft robotics, and smart and responsive materials and devices. The chiral geometry often introduces anisotropy which is not well understood due to the lack of an appropriate constitutive model. In addition, because of the overall handedness, chiral mechanical metamaterials show a unique property that under uniaxial stress/deformation, a shear deformation/stress will be generated simultaneously. Few efforts were made on understanding this unique property of normal/shear coupling, therefore, an integrated analytical-numerical method is developed to systematically quantify the overall mechanical properties of chiral mechanical metamaterials via a monoclinic constitutive model. By varying the distribution of local handedness in the chiral cells, a family of chiral mechanical metamaterials with hybrid local handedness are designed. The overall mechanical properties of them are quantified and compared via finite element simulations together with the monoclinic model. We will show that by programming the handedness in each chiral cell, the overall mechanical properties including stiffness, Poisson’s ratio, the normal/shear coupling effects, and the anisotropy can be tuned in a wide range.   

Inelastic collision between oscillatory robots drive collective synchronization

Movement in biology and robotics arises from oscillatory motions of appendages and bodies. When groups of organisms and robots move in close proximity they may collide with each other yet the influence of collisions on collective oscillatory dynamics are unknown. We study a representative colliding oscillatory system: pairwise interactions between two robots oscillating as phase oscillators. We vary the separation distance between robots to study how proximity affects their phase dynamics. We observe three behaviors in colliding oscillators: 1) in-phase synchronization at close separation, 2) compatible oscillations in which a stationary phase mismatch persists when at intermediate separation, and 3) anti-phase synchronization which leads to repeated high-impact collisions at large separation. We develop a phase-oscillator model and calculate the stability of these behaviors dependent on the instantaneous phase difference. To understand how contact interactions influence oscillatory dynamics of larger groups of animals or robots we study an oscillator lattice with contact interactions as a function of density. At high density the group synchronizes with long range phase correlation however at lower densities the system enters an asynchronous state with continuous collisions.   

Arrangement of Spherical Nanoparticles on Nanoscale Liposomes

The understanding of the interaction between nanoparticles (NPs) and lipid membranes is important to the development of safe and effective nanomaterials for many applications. The adhesion of a spherical NP on a lipid membrane leads to its wrapping by the membrane, and deformations of the membrane leading to effective attractive interactions between the NPs and their self-assembly on the membrane¹. Here, we present results based on coarse-grained molecular dynamics simulations of an implicit solvent model in conjunction with the Weighted Histogram Analysis Method, of the arrangement of two NPs on liposomes. The finite size of the liposome leads to preferred configurations of the NPs that depend on the adhesion strength and the ratio between the liposome's diameter and that of the NP. We show that depending on these parameters, the NPs prefer to be apart at specific locations on the liposome, dimerize into in-plane or out-of-plane dimers, or are endocytosed. Our results agree, to some extent, with those of Bahrami et al.², which are based on a Monte Carlo minimization of the system's energy.
[1] E.J. Spangler et al., Soft Matter 14, 5019 (2018)
[2] A.H. Bahrami et al., Phys. Rev. Lett. 109, 188102 (2012)   

Collective response in phototaxis of Chlamydomonas reinhardtii

Many photosynthetic microorganisms exhibit phototaxis behavior wherein the organism moves towards or away from the light gradient to inhabit in an optimal light condition for adequate health and growth. Phototaxis also plays a crucial role in the aquatic ecosystem by affecting phytoplankton mass transfer through diel vertical migration and algal bloom and has significant application in bioreactor, microbiopropellers and artificial microswimmers technologies. Here we present a quantitative measurement of phototaxis in Chlamydomas reinhardtii cells, a unicellular biflagellate microalgae, using high spatio-temporal video microscopy. We find that at a given light intensity, phototactic efficiency as a function of cell concentration is non-monotonic and has a re-entrant nature. We demonstrate that in the high-density collective regime, enhancement in phototactic efficiency with the cell concentration is due to the decrease in the linear speed of the cell. Furthermore, we also show that the re-entrant nature of phototactic efficiency is well captured by density-dependent aligning torque on active Brownian particle.   

Relationship among Phase Behavior, Micellar Structure and Thin Film Drainage in Aqueous Surfactant Solutions

Sodium naphthenates (NaNs) are petrochemical anionic surfactants that stabilize undesired petroleum emulsions and foams and pose significant environmental challenges when released into water bodies. Relatively little is known about the phase behavior and self-assembly (including micelle formation) as well as thin film drainage kinetics of aqueous NaN self-assemblies, impeding the development of strategies for NaN sequestration and petroleum foams and emulsions destabilization.
In this presentation, we will discuss experimental methodologies to elucidate surfactant phase behavior and self-assembly structure through a combination of small-angle X-ray scattering and thin film stratification studies. First, we will explicate the influence of surfactant and salt concentrations on micellar structure evolution and interparticle interactions of SDS solutions as a model system. These trends will be compared with nanoscopic topography of draining SDS foam films, elucidating the striking similarity between intermicellar separations in bulk solutions and stratification step sizes in thin films. Next, phase behavior and micelle structure of aqueous NaN self-assemblies will be discussed, again highlighting the role of intermicellar correlations in dictating drainage of thin liquid films.   

Active Matters in Structured Liquids

The motion of active matter is the basic form of locomotion in biology, a vital ingredient in many functions of cells, and an essential design challenge in nanorobotics. Here, we integrated active matter into structured liquids to harness its motions to perform work on liquid interfaces. The structured liquids, produced by interfacial jamming of nanoparticle-surfactants (NPSs), are reconfigurable and therefore provide an ideal platform for generating active energy-consuming systems. The liquid shape will evolve and respond to external stimuli when the interfacial tension is sufficiently low. 3D active fluids consisting of microtubules and kinesins are directly attached to water-oil interfaces. The ultralow interfacial tension of the structured liquid is achieved by proper selection of NPSs. The interaction between active matter and liquid interfaces is observed in real time by confocal microscopy. This strategy would provide a route to a new class of biomimetic, reconfigurable, and responsive materials, delivering mechanical responses unlike those of conventional materials.   

Not Participating

This study considers the impact of a series of macroscopic defects on a solid surface on an advancing contact line (CL) to understand collective effects on interface shape, depinning force (F_d) and energy (E_d).
Macroscopic defects were produced on stainless steel (s/s) in the form of regular holes and cocoa butter (cb) patches and by depositing cb patches on glass slides. Samples were submerged into DI water and EtOH aqueous solutions. The defects varied in radii (r) (0.27-1.44mm) as well as number. The distance between defects (d) varied in the range 0.9-2.7mm The contact angles of the liquids on s/s ranged from 50-95°, 10-30° on glass and 70-110° on cb.
The depinning depth (h_d) and the position of the CL (h_m) depend on d. Cumulative effects of the holes lead to greater h_d and h_m. Cb defects showed lower h_d and h_m, which can be explained by the ability of the four-phase CL to move on the surface of the defects, as opposed to the complete pinning observed at the base of the holes. This justifies also the lower F_d and E_d of cb defects compared to holes of same size. A linear dependence of the F_d was observed with n r (n is the number of defects), while E_d was proportional to n r^2, but lowering γ decreased h_d , F_d and E_d in a non-linear way.   

Liquid Foams as Template for Macroporus Hydrogels Synthesis

Foam templates are produced by a simple rapid gas dispersion into an aqueous monomer solution at various dispersion concentrations. The templates are corsslinked at room temperature to obtain interconnected polydispersed solid foams. The foams are analyzed for their stability by measuring coarsening rates and drainage times to quantify and compare the effect of the volume of gas dispersion. The pseudoplastic flow behavior of foamswas fitted using Herschel-Bulkley model to calculate the required foaming energy. The benefits of using a foam templating method in comparison to emulsion templating method are the increased starting material efficiency and elimination of the removal step of the dispersed phase as in emulsion templating. Another challenge in emulsion-templating is to make porous polymers with pore size in the range of 50-200 µm for applications such as tissue engineering, which can be resolved in foam-templating method. We investigate the morphological characteristics and mechanical properties of obtained porous polymers from foam- and emulsion-templating methods.   

Properties of the density of shear transformations in driven amorphous solids

The strain load Δγ that triggers consecutive avalanches is a key observable in the slow deformation of amorphous solids. Its temporally averaged value〈Δγ〉displays a non-trivial system-size dependence that constitutes one of the distinguishing features of the yielding transition. Details of this dependence are not yet fully understood. We address this problem by means of theoretical analysis and simulations of eleastoplastic models for amorphous solids. An accurate determination
of the size dependence of〈Δγ〉leads to a precise evaluation of the steady-state distribution of local distances to instability x. We find that the usually assumed form P(x) ∼ x^θ (with θ being the so-called pseudo-gap exponent) is not accurate at low x and that in general P(x) tends to a system-size-dependent finite limit as x → 0. We work out the consequences of this finite-size dependence standing on exact results for random-walks and disclosing an alternative interpretation of the mechanical noise felt by a reference site. We test our predictions in two- and three-dimensional elastoplastic models, showing the crucial influence of the saturation of P (x) at small x on the size dependence of〈Δγ〉and related scalings.

Ref:
E E Ferrero & E A Jagla
arXiv:2009.08519   

Measuring biomembrane undulations at sub-µm lengthscales using single gold nanorods

We measure the orientation of gold nanorods bound to a lipid membrane to probe its undulation dynamics on sub-µm lengthscales. Using high-speed polarimetric microscopy we measure the time dependent mean-squared displacement of the out-of-plane rotation of the nanorod. Fitting the results to a theoretical model allows us to determine various mechanical properties of giant unilamellar vesicles including their bending rigidity, surface tension, and the inter-monolayer friction coefficient. Probing membrane fluctuation in angular space rather than height space makes our approach sensitive to the short wave-length undulations which previously had only been accessible via measurement techniques like small-angle X-ray scattering and neutron spin echo spectroscopy on model membranes. We validated our approach by comparison to a simulation of the motion of a nanorod on a model membrane, using a Monte Carlo realization of the membrane at different states of tension. Motion of single GNRs on the plasma membrane of cultured cells reveals its complex physics. Our findings for the time dependent motion of the plasma membrane normal vector reveal sub-µm wavelength Brownian undulations expected for a lipid membrane coupled to a cell cortex undergoing active undulations at longer time-scales.   

Experimental and theoretical developments in understanding yielding

Gels exhibit yielding behavior are used in many applications, from spreadable foods and cosmetics to direct write three-dimensional printing inks. Their key design feature is the ability to transition behaviorally from solid to fluid under sufficient load or deformation. Despite their widespread applications, little is known about the dynamics of yielding in real processes, as the nonequilibrium nature of the transition impedes understanding. We demonstrate an iteratively punctuated rheological protocol that combines strain-controlled oscillatory shear with stress-controlled recovery tests. This technique provides an experimental decomposition of recoverable and unrecoverable strains, allowing for solidlike and fluid-like contributions to a yield stress material’s behavior to be separated in a time-resolved manner. Using this protocol, we investigate the overshoot in loss modulus seen in materials that yield. We show that this phenomenon is caused by the transition from primarily solid-like, viscoelastic dissipation in the linear regime to primarily fluid-like, plastic flow at larger amplitudes. We further show the development of a simple model based on these measurements that shows yielding is a smooth and continuous transition that may be viscoelastic in nature.   

Simple Micro-machine fueled by RNA

We are interested in making an artificial motor that is driven autonomously by DNA/RNA hybridization and RNA hydrolysis. As in molecular motors we use mechanical advantage to transduce nanoscopic motion to motion on a micro-scale, in our case from 24 nm to 2.4 um. The basic design is a hinge with a level arm. RNA linkers in the solution hybridize with two DNA single strands on each side of the hinge to close the arm. RNase H enzymes cleave the RNA in the RNA-DNA duplexes to open and reset the hinge. With both RNA and RNase H enzymes in the solution, the hinge can achieve cyclic opening and closing fueled by RNA, with rates that can be tuned by changing the concentrations of RNA and RNase H. This hinge mechanism can be also be used to activate other designed structures and machines.   

Buckling, Crumpling, and Tumbling of Semiflexible Sheets in Simple Shear Flow

As 2D materials such as graphene, transition metal dichalcogenides, and 2D polymers become more prevalent, solution processing and colloidal-state properties are being exploited to create advanced and functional materials. However, our understanding of the fundamental behavior of 2D sheets and membranes in fluid flow is still lacking. In this work, we perform numerical simulations of athermal semiflexible sheets with hydrodynamic interactions in shear flow. For sheets initially oriented in the flow-gradient plane, we find buckling instabilities of different mode numbers that vary with bending stiffness and can be understood with a quasi-static model of elasticity. For different initial orientations, chaotic tumbling trajectories are observed.   

Characterization, stability, and application of domain walls in flexible mechanical metamaterials

Domain walls, commonly occurring at the interface of different phases in solid-state materials, have recently been harnessed at the structural scale to enable additional modes of functionality. Here, we combine experimental, numerical, and theoretical tools to investigate the domain walls emerging upon uniaxial compression in a mechanical metamaterial based on the rotating-squares mechanism. We first show that these interfaces can be generated and controlled by carefully arranging a few phase-inducing defects. We establish an analytical model to capture the evolution of the domain walls as a function of the applied deformation. We then employ this model as a guideline to realize interfaces of complex shapes. Finally, we show that the engineered domain walls modify the global response of the metamaterial and can be
effectively exploited to tune its stiffness as well as to guide the propagation of elastic waves.   

Harnessing topological vector solitons for locomotion

The propagation of topological solitons in mechanical metamaterials has attracted significant attention not only for its rich physics, but also for the wide range of potential applications, including unidirectional wave propagation, mechanical logical computation, and shape morphing. In this work, we exploit the vectorial nature of topological solitons supported by metamaterials based on a rotating squares mechanism to build crawling robots. We first use a combination of experimental measurements and analysis to characterize the topological solitons. We then show that by harnessing their rotational component to modify the friction between the metamaterial and the underlying substrate, locomotion becomes possible. Unlike previously proposed crawling robots that require complex input control of multiple actuators, a simple, slow input signal suffices to make our system crawl. All features required for locomotion are embedded into the architecture of the metamaterial and sequentially activated by the topological solitons.   

The Shape of Aerosol-OT Reverse Micelles and the Impact of Force Field

Aerosol-OT (AOT) reverse micelles are ideal systems for the study of nanoconfinement with molecular dynamics (MD) simulations often serving as a useful way to gain molecular level understanding of the system. In the present study, we examine the impact of the force field on the behavior of the simulated AOT reverse micelles and benchmark the results. We simulated w₀=5 reverse micelles using the CHARMM force field and three variations on the OPLS-AA force field. We benchmark the system against experimental values for the dipole moment, the relaxation constant of the complex permittivity, and the quasi-elastic neutron scattering spectrum. We evaluate the eccentricity and convexity as well as local curvature of the reverse micelle to assess how the force field impacts various values. We find that the force field has a large impact on the eccentricity of the micelle despite all force fields tested recreating the experimental dipole moment well. Our eccentricity results also report a surprisingly high abundance of oblate ellipsoidal shaped micelles not reported in previous literature. These results highlight the importance of testing the force fields for accuracy in order to obtain correct molecular level insight into the system.   

A photo-driven self-excited hydrogel oscillator

When a photo-responsive hydrogel cantilever vibrates under a fixed light source, a periodic photo moment is imposed on the cantilever by the ever-switching light incidence between the top and bottom surfaces. The photo moment is induced by a diffusion driven inhomogeneous distribution of water concentration through the cantilever’s thickness. Combining theory and experiments, we find that when the water’s diffusion time scale is comparable to the cantilever’s inertia time scale, net energy can be pumped into the cantilever to not only maintain a self-excited vibration by overcoming the damping, but also to increase the oscillation amplitude. Scaling analysis is conducted to understand the effect of material and geometric parameters on the self-excited oscillation, and phase diagrams for amplitude-increasing and decreasing oscillation is constructed. A mass-spring model is studied as a simplified epitome to understand the features observed in the hydrogel oscillator.   

Molecular-Sized Outward-Swinging Gate: Entropy Decrease in an Isolated System?

We investigate the concept of molecular-sized outward-swinging gate, which allows for entropy decrease in an isolated system. In one direction, the gate can be pushed open by individual gas molecules; in the other direction, it tends to be pushed close, so that the gas permeability is asymmetric. It is experimentally realized by a nanoporous membrane one-sidedly surface-grafted with bendable organic chains, across which gas spontaneously and repeatedly flows from the low-pressure side to the high-pressure side. The system follows the basic principle of the second law of thermodynamics, as entropy remains maximized. However, because of the local nonchaoticity and the path irreversibility of the gate, entropy reaches a nonequilibrium maximum, and may shift to and from the equilibrium maximum without an energetic penalty. This process is passive, fundamentally different from Maxwell’s demon and its variants.   

From bulk descriptions to emergent interfaces

Controlling interfaces is highly relevant from a technological point of view. However, their rich and complex behavior makes them very difficult to describe theoretically, and hence to predict. We establish a procedure to connect two levels of descriptions of interfaces: for a bulk description, we consider a two-dimensional Ginzburg-Landau model evolving with a Langevin equation. At this level of description, no assumptions need to be done over the interface, but analytical calculations are very difficult to handle. On a different level of description, we consider a one-dimensional elastic line model evolving according to the Edwards-Wilkinson equation, which only allows one to study continuous and univalued interfaces, but which was up to now one of the most successful tools to treat interfaces analytically. We propose a simple method to establish the connection between the bulk and the interface description. We probe the connection by numerical simulations, and our simulations, in addition to making contact with experiments, allow us to test and provide insight to develop new analytical approaches to treat interfaces.   

Vacuum Balloon – a 350-Year-Old Dream

The centuries-old idea of a lighter-than-air vacuum balloon has not materialized yet as such structure needs to be both light enough to float in the air and strong enough to withstand atmospheric pressure. We propose a design of a rigid spherical sandwich shell and demonstrate that it can satisfy these stringent conditions with commercially available materials, such as boron carbide ceramic and aluminum alloy honeycomb. A finite element analysis was employed to demonstrate that buckling can be prevented in such a structure. Other modes of failure were evaluated. Approaches to manufacturing are discussed briefly.   

Tempered Fractional Brownian Motion with Reflecting Walls

Fractional Brownian Motion (FBM) is a Gaussian stochastic process with long-range correlations and a paradigmatic model for anomalous diffusion. For FBM confined by reflecting boundaries, recent work [1] demonstrated unusual accumulation and depletion of particles close to the walls. In many applications of FBM to physics, chemistry, and beyond, the long-range correlations are cut off (tempered) beyond a certain time scale [2]. Here, we study the behavior of tempered FBM in the presence of reflecting walls. More specifically, we analyze the probability density of tempered FBM on a one-dimensional interval between two reflecting walls.
[1] A.H.O. Wada and T. Vojta, Phys. Rev. E 97, 020102 (2018)
[2] D. Molina-Garcia et al., New J. Phys. 20, 103027 (2018)   

Analytical Survival Analysis of the Ornstein-Uhlenbeck Process

We use asymptotic methods from the theory of differential equations to obtain an analytical expression for the survival probability of an Ornstein-Uhlenbeck process with a potential defined over a broad domain.
We form a uniformly continuous analytical solution covering the entire domain by asymptotically matching approximate solutions in an interior region, centered around the origin, to those in boundary layers, near the lateral boundaries of the domain. The analytic solution agrees extremely well with the numerical solution and
takes into account the non-negligible leakage of probability
that occurs at short times when the stochastic process begins close to one of the boundaries. Given the range of applications of Ornstein-Uhlenbeck processes, the analytic solution is of broad relevance across many fields of natural and engineering science.   

Topological Active Matter in Complex Environments

Flocking models with metric and topological interactions are supposed to exhibit very distinct features, as for instance the presence and absence, respectively, of moving polar bands. By using Voronoi and k-nearest neighbors (kNN) interaction rules, we show that topological models in a complex environment recover several features of metric models in homogeneous media. In particular, we find that order is long-ranged even in the presence of spatial heterogeneities, and that the environment facilitates an effective density-order coupling that allows the formation of traveling bands.   

Two-Dimensional Frustration Modeling

Frustration results from competing and random interactions among spins, atoms or characters generally. Computationally, we study frustrated systems like (spin) glasses, and amorphous solids. We define frustration of a character to be proportional to the square of the distance between the actual and assigned positions. This frustration model is similar to an Ising model and comparable to a set of harmonic oscillators.
The total time-dependent frustration is recorded, as characters move to relax and minimize their frustration in a two-dimensional lattice. Using Monte Carlo simulations, frustration is studied for different numbers of characters and assigned distances. Characters assigned to regular structures like Thomson Figures show exponential relaxation. For characters with random assignments, power relaxation is seen.
For characters where the assignments are to form a chain, we observe unexpected behavior. Initially, the frustration relaxes exponentially to zero. Yet, the frustration then jumps up, after which it relaxes to zero while the chain is formed. Our frustration modeling may enable a better understanding of glasses and glassy structures. A thriving glass technology could be a potential outcome.   

The dynamics of intermediate scale active matter

In the context of soft matter, active matter systems are often studied at the microscopic scale (e.g. colloidal particles) where thermal noise has a strong effect, but inertia is negligible. Contrarily, in the context of nonlinear dynamics, active matter systems are often studied at the meter scale, such as flocks of birds, where thermal noise is neglected but inertia dominates. We seek to study an intermediate scale where the dynamics of a centimeter-scale particle is governed by both noise and inertia. We use a vibrating motor to create self-propelled particles that crawl along a solid/dry surface. Our active particles can be described using statistical mechanics and exhibit common features of both larger and smaller active matter, namely scale-free Brownian-like motion and a directional persistence due to inertia and activity. Here we present our centimeter-scale active matter system, some interesting single-particle dynamics, and our observations on the density dependence of the collective dynamics. In our combined experimental and theoretical approach, we investigate single and multiparticle dynamics of intermediate-scale active matter.   

Thermalized buckling of clamped plates

Atomically thin free-standing 2D materials such as graphene offer new opportunities to revisit classic mechanical instabilities such as the Euler buckling transition. In such fragile materials, thermal fluctuations dominate and dramatically renormalize the elastic moduli of the membrane. By focusing on geometrically confined thin sheets, we are able to investigate the interplay between boundary conditions and thermal fluctuations in controlling buckling. Remarkably, clamped boundary conditions induce a novel long-ranged nonlinear interaction between the local tilt at distant points, which along with a spontaneously generated thermal tension leads to a renormalization group description of thermalized buckling as an unusual phase transition with a size dependent critical point.   

An exactly solvable ansatz for statistical mechanics models

We propose a family of "exactly solvable" probability distributions to approximate partition functions of two-dimensional statistical mechanics models. While these distributions lie strictly outside the mean-field framework, their free energies can be computed in a time that scales linearly with the system size. This construction is based on a simple but nontrivial solution to the marginal problem. We formulate two non-linear constraints on the set of locally consistent marginal probabilities that simultaneously (i) ensure the existence of a consistent global probability distribution and (ii) lead to an exact expression for the maximum global entropy.   

Dynamics of self-propelled particles at the air-water interface

We study the behavior of active (self-propelled) particles at the millimeter scale using camphor-infused discs that glide at the air-water interface. The particles are driven by variations in surface tension generated by the diffusion and evaporation of camphor and move within a circular boundary providing strong confinement. Long-ranged interactions provide a system to study the dynamics of strongly interacting particles under confinement. These interactions with the boundary and other particles give rise to complex dynamics that are affected by active non-thermal noise and inertial effects. We report interesting single-particle behavior and observations on the density dependence of multi-particle dynamics. Using our combined experimental and theoretical approach, we are able to explore the strongly interacting active particles in confinement and how their behavior depends on density.   

Estimating the energetics along a fluctuating trajectory of an active particle

Active matter is a specific type of matter with constituents that consume energy to exert mechanical forces. Such systems are intrinsically out of thermal equilibrium and have characteristics that are not well understood. In equilibrium systems, the thermal energy is dissipated as described by the fluctuation-dissipation theorem. Likewise, an active system must also dissipate the input energy to remain in a steady state; however, in contrast to thermal forces, active fluctuations are likely non-Gaussian, strongly dependent upon frequency, and with an amplitude proportional to the specific characteristics of the active system. On average, these characteristics are well-defined; however, their stochastic fluctuations with time are less understood. We seek to estimate the energetics of dissipation along a stochastic trajectory of an active particle. To this end, we study the inertial Langevin equation, since the commonly-used overdamped approximation yields diverging results for a particle's path energetics. We use a combined numerical and analytical approach to estimate fluctuating properties along a trajectory and then compare the results to experimental measurements.   

Modeling Dynamics of Pattern Formation and Restructuring in Constrained Hydrogel Membranes

Understanding dynamics of pattern formation in hydrogels is important for designing functional surfaces and interfaces for a number of applications. Herein, we focus on dynamically controlling patterns in three-dimensional thermoresponsive poly(N-isopropylacrylamide) (PNIPAAm) gel membranes with two clamped edges. The patterns are formed due to mechanical instability, which arises due to the constrained swelling of a polymer network undergoing extensive volume changes in response to external stimuli. We use three-dimensional gel Lattice Spring Model (3D gLSM) to simulate dynamics of the confined membranes and characterize the patterns formed. We perform a linear stability analysis using Foppl-von Karman equations for this geometry to predict patterns wavelength and critical stresses during the onset of instability. We then focus on dynamic restructuring between different patterns in response to variations in external temperature. Our results demonstrate that the wavelength, amplitude, and mode of patterns formed could be controlled dynamically by simply varying the rate of changing temperature. Our results show that a confined network exhibits bistability, which in turn is controlled by the rate of changing temperature.   

Controllability of Granular Packings

Granular packings are disordered, athermal systems common in our everyday lives. Following perturbation, their evolution depends on their mesoscale stress-distributing structure, or force-chain network. Yet, it remains unclear precisely how force-chain networks change under applied stress. Here, we tackle this gap in knowledge using network control theory (NCT). NCT is a branch of systems engineering and statistical physics developed to understand and control the activity of networked systems in technology, robotics, and many other contexts. NCT is a promising approach to study force-chain networks both conceptually and mathematically, because it considers the network of connectivity between units, modeling the nature of the system’s dynamics as being constrained by that connectivity. We use NCT to estimate the control energy needed for a packing to transition between contact states. Our preliminary results indicate that control energy increases with system size and jamming, providing physical intuition for characterizing force-chain architecture evolution. More broadly, our findings can inform design principles by determining how changing the physical features of a granular packing impacts the system's force-chain network architecture and stress behavior.   

The Application of Semantic Network Theory to Visual Art

Complexity science uses complex networks to algorithmically examine many-body systems containing varying types of interactions. Though commonly used in STEM applications, this form of modeling is effective when used to analyze the arts and humanities. This project applies complex network theory to visual arts in pursuit of a computational system that can determine the artistic movement any arbitrary 2D piece originates from. The system will interrogate the visual characteristics of pieces from the Renaissance period of the 1400s through to the Surrealist period of the 1950s. Following a semantic network blueprint, the system takes an input image, reduces that image into a collection of super pixels, builds a network, and performs network measurements. These measurements are recorded, and after all the images have been analyzed, the different artistic movements are distinguished from each other through a statistical analysis. By quantitatively distinguishing between art periods, the system will draw qualitative conclusions on the evolution of art, such as the motivation in stylistic change.   

Speed and dissipation in the paths to dynamic function

Physical systems that generate work and assemble into three-dimensional structures often accomplish these dynamic functions transiently and away from steady-state. To analyze these processes, we demonstrate a path-integral formalism for stochastic paths that occur in a fixed amount of time. We illustrate the theory with four models: a clock, a ratchet, a self-assembling tetramer, and a copier. Central to the theory is an analytical expression for path probabilities that we use to determine the speed, through the mean path occurrence time, the efficiency, through the entropy flow and energy dissipation, and the effectiveness of the dynamical functions, such as the work and assembly yield. Our results confirm a recent thermodynamic uncertainty relation and establish a method for characterizing the efficiency of functioning on finite timescales.   

Studies of the 2D Potts Model via Convolutional Neural Network

In this poster, employing a convolutional neural network (CNN), we investigate a new aspect of the phase transition for the 2D Potts models. First, we train the CNN by images of spin configurations labeled with their temperatures. Namely, the trained CNN can predict the temperature of the snapshot of an Ising spin configuration. Second, we examine whether the trained CNN by the Ising configuration can detect the phase transition of the 3- and 4-state Potts models. To this end, in the image of the Potts configurations, we divide the three (resp. four) types of spins 1,2,3 (resp. 1, 2, 3, 4) for the 3-state (resp. 4-state) Potts model into two parts, say {1,2} and 3 (resp. {1,2} and {3,4}), and then replace {1,2} with spin up and 3 (resp. {3,4}) with spin down. We call them the filtered Potts model. Of course, the geometric properties of the filtered Potts configuration at its critical point is completely different from the critical Ising configuration. Nevertheless, the CNN can recognize the phase transition of the filtered Potts model with high accuracy.   

Diffusion matrix and its application to the spin transport for the XXZ spin-1/2 chain in the gapless regime

A general formula of the diffusion matrix has been recently obtained by generalized hydrodynamics (GHD), and it was shown in [1] that the spin diffusion at infinite temperature of the gapped XXZ chain is non-zero at the Ising limit, while it diverges at the isotropic limit. Being inspired by their study, we applied the GHD formula to the XXZ chain in the gapless regime and obtained exactly the spin diffusion constant at infinite temperature in this regime.
As the result, we observe that the spin diffusion in the gapless regime exhibits a fractal structure.

[1] Diffusion in generalized hydrodynamics and quasiparticle scattering (J. D. Nardis, et al., 2019)   

A Numerical Exploration of Nonlinear Microscale Elastic Structures

Micro and nanoscale elastic structures at room temperature exhibit stochastic dynamics that are driven by Brownian motion. Measuring and interpreting the mechanical motion of these structures in the linear regime form the centerpiece of many important technologies. Typical elastic structures include cantilevers, doubly-clamped beams, and nanostrings for use as biomolecular sensors, spectrometers, thermometers, etc. With the advance of 3D printing technologies, it is now possible to build complex three-dimensional elastic structures at the micron and nanoscales. This provides access to a large parameter space where nonlinearities of the elastic structure can have a significant effect upon the dynamics. In this poster we explore the elastic properties of micron scale three-dimensional structures designed to exhibit behavior beyond that of a Hookean spring. Using finite element numerical simulations, we quantify several elastic objects of interest that have been tailored to probe their nonlinear response. Guided by simplified models, we discuss the expected stochastic dynamics that these devices would yield if excited by Brownian motion.   

Chaos in the quantum Duffing oscillator in the semiclassical regime under parametrized dissipation

We study the quantum dissipative Duffing oscillator across a range of system sizes and environmental couplings under varying semiclassical approximations. Using spatial (based on Kullback-Leibler distances between phase-space attractors) and temporal (Lyapunov exponent-based) complexity metrics, we isolate the effect of the environment on quantum-classical differences. Moreover, we quantify the system sizes where quantum dynamics cannot be simulated using semiclassical or noise-added classical approximations. Remarkably, we find that a parametrically invariant meta-attractor emerges at a specific length scale and noise-added classical models deviate strongly from quantum dynamics below this scale. Our findings also generalize the previous surprising result that classically regular orbits can have the greatest quantum-classical differences in the semiclassical regime. In particular, we show that the dynamical growth of quantum-classical differences is not determined by the degree of classical chaos.   

Fractals, Complexity Science, and the Visual Arts

This project strives to quantify ambiance in the visual arts with fractals, entropy, and complexity science techniques with the broad goal of bridging the gap between humanities and physics by finding meaningful mathematics to describe phenomena characteristic of the humanities and visual arts. The power of the arts is conveyed emotion and meaning as a construct from basic artistic features. Current research revolves around the use of neural networks to perform image analysis, which place an emphasis on the recognizable content of the image. This study focuses on quantifying artistic features such as color, hue, composition, and texture that are integral to the ambiance and meaning that a painting conveys. Relaying emotion in visual arts can be considered analogous to relaying information, which can be quantified by means of entropy. By experimenting with different microstate definitions, entropy calculations are adapted to include the color, composition, texture, and other defining features of paintings. Coupling entropy calculations with investigations of fractional dimensions creates unique methodology for mathematically quantifying the ambiance of a painting without forgoing the stylistic nuances that are the basis of the study and theory of art.   

Active Matter based on Varying Topologies of DNA

Active soft materials have been widely investigated in recent years. However, using polymer topology as a route to actively alter the viscoelastic properties of a material remains largely unexplored. Here, we confer non-equilibrium dynamics into concentrated DNA solutions by using enzymes to alter the topology of DNA molecules. Specifically, we incorporate restriction endonucleases into concentrated solutions of supercoiled DNA to slowly convert them to linear and relaxed circular topologies. We use particle-tracking microrheology to measure how the viscoelastic properties of the materials change over time during enzymatic activity. We use gel electrophoresis to map the viscoelastic properties to the distribution of topologies of the DNA molecules. We examine the effect of enzyme and DNA concentrations as well as DNA length on the active material properties. Our initial results show that during enzymatic conversion from supercoiled to linear topology, DNA solutions monotonically increase in viscosity. However, the time dependence of this trend is quite noisy due to large non-equilibrium fluctuations in the distribution of DNA topologies.   

Theoretical Investigation of the Physics ofv Chaos in Fourth and Higher Dimensions

The possible existence of spatial dimensions beyond the known 3-dimensional space is a question of great importance in Theoretical Physics. Our approach is to look for the signature of extra spatial dimensions by simulating the apparent 3-D chaotic dynamical phenomena that could result from the existence of the extra dimensions. In 1975, Li and Yorke formulated the concept of Dynamical Chaos, and gave a condition for it in scalar difference equations: the “period three implies chaos” result. Subsequently other researchers have generalized the Li and Yorke definition of chaos to difference equations in Rⁿ and formulated higher dimensional conditions ensuring its existence, specifically the “snap-back repeller” condition of Marotto and its counterpart for saddle points. In this paper, we utilize the Feynman Path Integral approach, implemented with computer codes in Python and Fortran programming languages, to simulate generalized chaos. We propose possible laboratory experiments that might help to check the validity of our theoretical results. The possible link of an extra spatial dimension to the existence of Dark Matter is also discussed.   

“Controlled” geometric frustration leveraging local bistabililty in structural systems

Geometric frustration arises when a lattice system cannot simultaneously minimize all of its local interaction energies due to constraints, thus leading to degenerate and multiple disordered ground state configurations. Harnessing the ensuing multiple phase states for practical applications is a difficult controls problem due to the strong tendency for disorder, thus limiting the utility of these systems. Here, we present theoretical and experimental analyses of an archetypical lattice system for achieving “controlled” geometric frustration. The lattice features locally bistable dome units which upon inversion introduce prestress in the system. The patterning constrains the minimization of the interaction energies between neighboring inverted units and leads to geometric frustration that uniquely manifests in the form of hierarchical multistability i.e. multiple global states emerge for a given local inversion pattern. We further show that any desired frustrated global state can be achieved on demand by controlling the history of dome inversion, thus allowing for a simplified controls problem. This analysis essentially serves as a blueprint for leveraging geometric frustration for enabling mechanical computing platforms and programmable metamaterials.   

Nonlinear magnetization of layered spin models

Our research centers on studying two-dimensional layered Ising and XY models in a magnetic field using Monte-Carlo simulation, implementing it using high-level, high-performance, dynamic programming, Julia. Nonlinear magnetization results will be presented.   

Scrambling and the Anti butterfly effect

Complex dynamics spread local quantum information over the entire system -- a process known as scrambling. If the dynamcs is reversed, the system will be unscrambled to the initial state. The composed process "scrambling + perturbation + unscrambling" can magnify the perturbation, exhibiting the butterfly effect. This process is known as the quantum twirling channel and lies in the core of various diagnostics for quantum chaos. In this talk, we show that, through the twirling channel, part of the information damaged by certain types of perturbations can be recovered -- the anti-butterfly effect.<gdiv></gdiv>   

Development of Native-Based Dissipative Particle Dynamics Framework for Modeling Lysozyme

Conjugating enzymes with copolymer bottlebrushes has been recently shown to increase structural stability and enhance activity of enzymes at high temperatures. Herein we develop a first native-based approach to simulate dynamics of lysozyme using dissipative particle dynamics (DPD) simulations. DPD is a mesoscale approach that utilizes soft repulsive interactions, thus permitting the use of a significantly larger time step between successive iterations. The native contact map obtained from the crystal structure and structural characterizations from our molecular dynamics (MD) simulations are used to optimize DPD parameters to model lysozyme. We focus on developing an algorithm to assign the interactions between the enzyme’s beads based on native contact pairs to achieve a closest match between the DPD and MD structural characteristics. Using our approach, all six right-handed helical segments in lysozyme are reproduced in DPD simulations and can be located on the time-averaged contact map calculated from DPD simulations. Radius of gyration is also closely matched in DPD simulations to that obtained from atomistic trajectory. Further development of this framework could allow one to model a variety of larger systems incorporating enzymes and polymer chains.   

A Universal Mechanism of Nanoscale Transport of Interfacial Liquids Near a Solid Surface

Properties and characteristics of interfacial fluids confined by a solid surface play critical roles in a variety of physical and chemical processes. One of the most intriguing features of a solid-liquid interface is that the reconfiguration of near-surface liquid molecules gives rise to an oscillatory profile of liquid properties. Molecular dynamics (MD) simulations have disclosed an in-plane ordering and a shifted glassy transition of liquid nanofilm confined by a solid surface. Here we report our analysis of the transport of simple liquids adjacent to a solid wall using MD simulations. The oscillatory distribution of interfacial liquids is shown to be governed by self-diffusion, surface-induced convection and shifted glass transition in a combined manner. In particular, we put forward an universal governing equation for the oscillatory density profiles of interfacial liquids, which can be derived either from the free energy analysis of a discrete molecular system, or from the Reynolds transport theorem under the continuum framework. The analytical solution to the governing equations is able to capture all prominent features of the patterned oscillations of interfacial liquids and bridges the gap between molecular dynamics of liquids and their macroscopic continuum behaviors.   

Capillary Bridge of binary immiscible fluids

When a liquid is placed between two rigid bodies, a bridge with minimized surface is formed. The shape of capillary bridges of single liquid between various shapes such as plates, cylinders, and spheres has been investigated theoretically and experimentally. Less attention has been paid to capillary bridges made of two or more mutually immiscible liquids between a pair of solids. This talk will describe our work on using surface evolver simulations to probe the static morphology of capillary bridges made of two immiscible fluids in two geometries: between parallel plates and between a pair of contacting spheres. A phase diagram of configurations for two liquid bridges is obtained upon varying the mutual interfacial tension of the two liquids and the solids, liquid volumes, and geometrical parameters like the inter-plate distance and radii of spheres. We show that capillary bridges between contacting spheres can become non-symmetric when the interfacial energy and volume of the bridge exceeds that of a catenoidal, symmetrical bridge between two spheres.   

A Dynamic Density Functional Theory approach to Pattern Formation

Density functional theory (DFT), in both its classical and quantum formulations, presents a framework for determining the ground-state density of a given material through the variational minimization of a free energy functional. Using this framework, relevant energetic contributions can be accounted for self-consistently, and both microscopic and macroscopic degrees of freedom can be connected naturally. While DFT is traditionally used to determine equilibrium properties, one possible extension to model non-equilibrium systems is through the use of dynamic DFT (DDFT). DDFT builds upon the continuity equation within the Stokes limit, which includes nonlocal effects that capture the contributions of many-body correlations. Here, we propose a phenomenological DDFT model for the purpose of describing the emergence of pattern formation, that maintains both conservation laws and proper scalings in the high-frequency limit. Simulation results are presented and compared to experiment and other models for pattern formation.   

Evaporation of Squeezed Droplets between Two Non-Parallel Hydrophobic Surfaces

When a droplet is squeezed by two non-parallel hydrophobic surfaces, both the constant contact angle (CCA) mode and constant contact radius (CCR) mode are observed during evaporation. However, the evaporating droplet between two non-parallel surfaces would laterally transport towards the cusp due to its asymmetric shape. At the beginning of CCA mode, the contact line motion of the droplet is asymmetric and only the receding contact line keeps moving whereas the advancing contact line is fixed. Once droplet volume is reduced to certain volume, both the advancing and receding contact lines begin to move while the whole droplet is self-propelled towards the cusp. Finally, the evaporating droplet will be pinned then rupture near the cusp, leaving two daughter droplets at the upper and lower surfaces, respectively. We use Surface Evolver to obtain the shapes and positions of squeezed droplets during the CCA mode. The evolution of Laplace pressure difference and adhesion force exerted on the droplet are calculated. The droplet mass scales as ~R³ during evaporation, while the length of contact line and surface scale as R and R², respectively. It results in the increase of droplet acceleration during the evaporation which is the origin of lateral transport of a smaller droplet.   

Tumor Cell Deformation in Shear Flow

Chemotherapy or radiotherapy, suffer from a lack of specific targeting and consequent off-target effects. Nanomedicine holds the potential to minimize severe adverse side effects of anti-cancer therapy. Separation and detection of circulating tumor cells (CTC) has an important role in early cancer diagnosis and prognosis. Tumor cells are formed as adhesion of activated thrombocytes (platelets) with erythrocyte (red blood cells) which in turn adhere to endothelial cells of the walls of the arteries or capillaries. A grown tumor under high shear rate breaks into a small portion of tumor forms circulating tumor cells in the blood stream. Red blood cells (RBCs) from healthy individuals regulate T-cell activity through modulating cytokine interactions, and that stored RBCs or RBCs from inflammatory cohorts are dysfunctional. As a treatment, CTC are separated by fractionation, reacted with nanoparticles and injected back into the blood stream to prevent new tumor forming in other parts. Shear stresses existing in the cancerous surroundings have a profound effect on cancer cell/nanoparticle interaction. Activation of nanoparticle systems due to altered cancer cell metabolism is discussed by considering pH-, enzymatic-, and concentration-dependent activation.   

Study of air pollutant precipitated particulates of Shiraz Urban region atmosphere

Airborne Particulates considered as a kind of air pollutant by different air pollution standards. Such particulates at western and southwestern parts of the Persian plateau’s atmosphere are known to be main source of air pollution. In spite of their small size (with mean diameter of 550 nm) such particulates would precipitate at lower altitude of atmosphere mainly due to their heavier mass in comparison to other invisible components of air (N₂ and O₂ gas molecules) at normal weather condition. Prototypes of such precipitated particulate samples from Shiraz Urban region investigated through PXRD structural analysis and PIXE method. In addition to that, physical parameters of the collected prototypes such as size and density distributions measured to be used modelling the precipitation process assuming classical Maxwell Boltzmann distribution of particulates within air. Our findings confirms the clay nature of pollutants with their phase composition. We plan to present our findings through a poster at APS march meeting.   

Non-linear shallow water dynamics with odd viscosity

In this work, we derive the Korteweg-de Vries (KdV) equation corresponding to the surface dynamics of a shallow depth (h) two-dimensional fluid with odd viscosity (ν_o) subject to gravity (g) in the long-wavelength weakly nonlinear limit. In the long-wavelength limit, the odd viscosity term plays the role of surface tension albeit with opposite signs for the right and left movers. We show that there exist two regimes with a sharp transition point within the applicability of the KdV dynamics, which we refer to as weak (|ν_o|< 1/6 (gh^3)^1/2) and strong (|ν_o|>1/6 (gh^3)^1/2) parity-breaking regimes. While the `weak' parity breaking regime results in minor qualitative differences in the soliton amplitude and velocity between the right and left movers, the `strong' parity breaking regime on the contrary results in solitons of depression (negative amplitude) in one of the chiral sectors.   

Control of Janus Droplets via Optothermally Induced Marangoni Forces

Micro-scale double emulsion droplets are a versatile material platform for dynamic optical components, that generate unique color effects, form liquid lenses, or enable rapid food-borne pathogen detection. Reliable in-situ control of the droplet morphology generally requires careful design of responsive surfactants and delicate tuning of the droplet’s chemical environment. Here, we present a simple alternative for droplet actuation: Using optically-induced thermal gradients, an interfacial tension differential is generated across the internal capillary interface of Janus droplets. The interfacial tension differential causes droplet-internal Marangoni flows and a net torque, resulting in a predictable and controllable reorientation of the droplets. The effect is quantitatively described with a simple model that balances gravitational and thermal torques. These optothermally-induced Marangoni dynamics, which require only small temperature gradients, represent a promising mechanism for controlling orientation and tilt of droplet-based micro-optical components and biologically-inspired artificial microswimmers.   

Toward Accurate Short-Range Physically-Derived Models for Charge in Molecular Dynamics Simulations

A smeared-charge model is presented pursuant to the inability of partial point charge models to capture short-range screening effects; the importance in molecules with deep potential wells among numerous molecules for which rough dynamical intuition fails is discussed.   

Identifying Conformational Changes of Membrane Transporter MHP1 By Molecular Dynamics Simulations

All-atom Molecular Dynamics (MD) simulation provides insight into individual atomic motions to predict the detail of the structural changes caused by the forward and backward conformational transitions. In this study, we implemented Umbrella Sampling (US) to characterize the conformational changes between both ligand-free outward-facing open and outward-facing occluded state of Mhp1. Mhp1 is a secondary active membrane transport protein that exploits sodium and obtains the potential energy to transport molecules across the membrane. The Mhp1 structure is formed by 14 Transmembrane-Spanning Helices (TMHs). The loop between TMHs 9 and 10 is extended enough to partially seal the substrate-site from the exterior to close the outward-facing cavity. Sequentially, choosing the Ψ dihedral angle of the residues on the loop region as the reaction coordinate of the US simulation, we measured the free energy difference between the open and occluded state of the Mhp1 protein. Our result shows that the free energy change is △G = 5.7±0.5 K_bT. We conclude that the transition pathway of the short helix TMH-10 along with the loop determines a consistent and reversible pathway for the conformational changes of outward-facing open and outward-facing occluded state of the ligand-free Mhp1 protein.   

Activation function dependence of the storage capacity of treelike neural networks

The expressive power of artificial neural networks crucially depends on the nonlinearity of their activation functions. Though a wide variety of nonlinear activation functions have been proposed for use in artificial neural networks, a detailed understanding of their role in determining the expressive power of a network has not emerged. Here, we study how activation functions affect the storage capacity of treelike two-layer networks. We relate the boundedness or divergence of the capacity in the infinite-width limit to the smoothness of the activation function, elucidating the relationship between previously studied special cases. Our results show that nonlinearity can both increase capacity and decrease the robustness of classification, and provide simple estimates for the capacity of networks with several commonly used activation functions.   

Green algae scatter off sharp viscosity gradients

It is quite common to find neighbouring regions of different viscosity in a variety of places in nature, such as biofilms or the female reproductive tract. For this reason, microorganisms can change the way they swim in the presence of steep viscosity changes to move away or towards regions that are more favourable to them. Recently, many efforts have been made in an attempt to develop a theoretical framework to predict where microswimmers would concentrate based on their flow fields (pushers/pullers), but no extensive experimental investigation has been carried out to confirm predictions so far. In order to investigate this phenomenon we use a microfluidics device which allows to generate two adjacent regions of significantly different viscosity and uniform concentration of microorganisms throughout the chamber.
We then investigate how the behaviour of Chlamydomonas Reinhardtii (puller) compare in the two regions, as well as how they approach the viscosity interface and cross it. Lastly, we measure how the concentration profile of the swimmers changes over time in the microfluidics device, leading to results not immediately obvious from previously published theoretical frameworks.   

Using Convolutional Neural Networks to Segment Images of Packed Epithelial Cell Layers for Inferring Cellular Forces

Cellular force inference is a powerful tool that provides insights into the growth and morphogenesis of epithelial cell layers. To infer intercellular tensions and pressures between cells, it is mandatory to first segment a microscope image of the packed epithelial cell layer with its cell borders marked, for example by using a fluorescent protein such as E-cadherin-GFP. Current procedures depend on guided watershed algorithms. Here we present, CS-Net, a novel convolutional neural network based on a modified U-Net structure to transform a raw microscope image to a mask of pixels defining cell boundaries and cell interiors. Our neural network has an accuracy of 90% with an AUC-ROC of 0.86. CS-Net is made accessible through an accessible web API, and it is designed to be used in conjunction with a Python-based cellular force inference tool.   

Pairwise approximation of interaction between perturbations on complex networks

Recent work has shown that the effects of drug combinations, such as antibiotics, can be predicted from the effects of the drugs in pairs. Unfortunately, little is known about why these pairwise approximations work so well. Interactions between drugs are typically not purely chemical, but instead arise from the ways that different drugs impact cell physiology. Therefore, drug interactions more closely resemble generalized perturbations to complex networks than the molecular-level behavior governing similar approximations in statistical physics. Here we investigate how combinations of perturbations impact behavior of three types of dynamical systems: coupled biochemical reactions, large-scale models of bacterial metabolism, and networks of coupled phase oscillators. We find that the “macroscopic” effects of combined perturbations—for example, the net flux through a reaction network—can often be predicted from the effects of pairwise perturbations, but pairwise effects are rarely predictable from single-perturbation effects. We also highlight exceptions to these trends and propose network-level properties that may facilitate these pairwise approximations. Our results show that pairwise approximations may help predict the effects of combined perturbations in a wide range of systems.   

Quantum sensing in a warm, noisy environment: Understanding spin-mediated effects in biological materials

Evidence suggests that cellular interactions with magnetic fields (MFs) might be more widespread and impactful for physiology than previously assumed. The leading model to explain MF biosensing is rooted in the fact that MFs can alter the products of light-dependent chemical reactions involving spin-correlated radical pairs (RP). These reactions have been observed e.g. in cryptochrome (CRY), a photoreceptor protein that has been linked with magnetosensitivity in multiple organisms. This research aims to explore multiple aspects of this RP quantum phenomena in CRY at the nanoscale. Our goal is to confirm both the RP mechanism and CRY’s ability to accurately sense a variety of magnetic field strengths and orientations. We will present advances in the building of a Lattice Light Sheet (LLS) microscope with magnetic excitation capabilities, with the advantage of low phototoxicity. In addition to developing the microscope, we aim to program automatic “coherent control” experimental sequences with ns time resolution to resolve these quantum spin effects. We will then study CRY-mediated effects in vivo and investigate the biophysical mechanisms of cellular interaction with MFs and thus open the possibility of mastering metabolic functions via the control of magnetic-responsive pathways.   

Inferring the plasticity of epithelial tissues from their geometry

Amorphous materials exhibit complex material properties with strongly nonlinear behaviors. At low stress they behave as plastic solids and start to yield above a yield stress. A key quantity controlling plasticity is the density P(x) of weak spots, where x is the additional stress required for local plastic failure. In the thermodynamic limit P(x)∼x^θ is singular at x=0 in the solid phase. Here we address the question if the density of weak spots and the flow properties of a material can be determined from the geometry of an amorphous structure alone. We show that vertex model of epithelial tissues exhibits the phenomenology of plastic amorphous systems. We then show that, in materials where the energy functional depends on topology, x is proportional to the length L of a bond that vanishes in a plastic event and P(x) is readily measurable from geometry alone. In the developing wing epithelia of the fruit fly, we find that P(L) exhibits a power law with exponents similar to those found in the vertex model in its solid phase. This suggests that this tissues exhibit plasticity and non-linear material properties emerging from collective cell behaviors. Our approach relating topology and energetics suggests a new route to outstanding questions associated with the yielding transition.   

Nonlinear Dendritic Integration Increases Alignment of Basal and Apical Input under Hebbian Plasticity

Experimental studies have shown that the integration of synaptic input and its effect on neural activity depends on the synapse's location within the postsynaptic dendritic structure. In L5 pyramidal neurons, temporally correlated input from basal and apical dendrites can elicit much higher activity than what would be expected from a linear superposition of input currents. We used a simple rate model [1] that accounts for this effect to investigate the influence of these nonlinear interactions on a Hebbian plasticity mechanism in the basal synapses. We found an increased correlation between proximal and distal input sequences. Moreover, this effect was robust against strong distracting basal presynaptic inputs, in contrast to canonical point-neurons, where this alignment could be easily corrupted by distracting input patterns. This indicated that the nonlinear interaction between basal and apical inputs allowed apical inputs to act as a form of “teacher signal” for basal plasticity without adjusting learning rules in the basal synapses.

[1] Shai AS, Anastassiou CA, Larkum ME, Koch C (2015) Physiology of Layer 5 Pyramidal Neurons in Mouse Primary Visual Cortex: Coincidence Detection through Bursting. PLOS Computational Biology 11(3): e1004090.   

Statistical Mechanics of Sadowsky Ribbons under applied Force and Torque

In our work, we study the statistical mechanics of the microscopic Sadowsky ribbon, a long developable ribbons of finite width and very small thickness. The Sadowsky ribbon is isometric to a flat strip at any temperature with a nonlinear coupling between the bending and torsional degrees of freedom. Our simulation was done using Monte Carlo algorithm and different equilibrium distribution of ribbon conformation was found for different values of applied force and torque. The shape of the ribbon characterized by its linking number, twist, and writhe, which are related via the Călugăreanu-White-Fuller theorem. At zero force and torque, the Sadowsky ribbon displays an underlying helical structure that disappears at large values of force and torque, indicating a phase transition from the ordered helical phase to the disordered entangled phase. The transition occurs at a finite Lifshitz point. The binormal-binormal correlation function exhibits a transition from exponential to oscillatory behavior at this critical point, and oppositely for the tangent-tangent correlation function. The plots of topological numbers as a function of force and torque are reminiscent to the role of magnetization in Ising models and can be viewed as order parameters of this ribbon system.   

Biophysical effects of melatonin and azithromycin on model pulmonary membranes

Pulmonary cell membranes are the first line of defense against respiratory viral infections, including the recent coronavirus (SARS-COV-2). Common drugs, e.g. azithromycin and melatonin, have shown promise as agents in slowing down and ameliorating the symptoms of respiratory viral infections. The generic effects of these drugs, across a range of viral infections, suggest an indirect mechanism by which they inhibit viral activity – plausibly by altering membrane biophysical properties. Here, we report experimental results on the structural and mechanical effects of azithromycin and melatonin on model pulmonary membranes. Surface pressure-area isotherms of Langmuir monolayers show that both drugs increase lipid packing and decrease the monolayer compressibility at physiologically relevant surface pressures. Complementary differential scanning calorimetry (DSC) measurements support these results and indicate an increase in the transition temperature upon the inclusion of the drug molecules. Our observations are consistent with previous studies reporting on melatonin induced stabilization of lateral lipid domains. Further measurements using Brewster-angle microscopy and neutron reflectometry illustrate how the inclusion of the drug molecules influences lateral membrane organization.   

Quantitative spectroscopy of single molecule interaction times

Single molecule fluorescence tracking provides information at nm-scale and ms-temporal resolution about the dynamics and interaction of individual molecules in a biological environment. While the dynamic behavior of isolated molecules can be characterized well, the quantitative insight is more limited when interactions between two indistinguishable molecules occur. We address here this aspect by developing a solid theoretical foundation for a spectroscopy of interaction times, i.e. the inference of interaction constants from imaging data. The non trivial crossover between a power law to an exponential behavior of the distribution of the interaction times is highlighted here, together with the dependence of the exponential term upon the product of the microscopic reaction rates (affinity). Our approach is validated on simulated as well as experimental datasets.   

A dynamic Monte Carlo approach for studying competing nucleoprotein bindings on single-stranded DNA

The central step of homologous recombination (HR) involves the search for homology between two DNA molecules and the subsequent exchange of the DNA strands. To have a better understanding of the recombinase-mediated DNA strand exchange, we introduce a dynamical Monte Carlo model. In this computational model, we study the dynamics of the recombinase filament nucleation, extension, and disassembly and its competition with other binding nucleoproteins. These biochemical reactions are experimentally observed at the single-molecule level using Total Internal Reflection Fluorescence Microscopy. This model allows building a mechanistic model of the filament formation that matches the single-molecule data.   

Fluid dynamic simulations of a microcavity bioreactor to promote organoid growth

Organoids are complex 3D multicellular tissues used to emulate cellular interactions within an organ. Fluid forces are a key element to grow organoids in vitro, they determine nutrient and oxygen supply, and generate shear stress. During cellular development shear stresses are necessary for organogenesis, proliferation, nutrient delivery and cell signaling. Organoids cultured without fluid in motion lack physiological shear forces and proper nutrient supply. However, organoid bioreactors with large fluid forces generate high shear stress which negatively affects cell viability and growth. Our objective was to design a fluidic device based on microcavity driven flow to provide an environment with physiological shear and improved nutrient supply. We used computer fluid dynamics to simulate cavity velocity as a function of microcavity length, height, and inlet velocity. We were able to simulate the conditions in which we obtained a flow of 0.01-0.1 Pa which allows us to obtain the correct parameters before micro-fabricating the device. After fabricating the device, we used particle image velocimetry of glass micro-beads to confirm the simulation values. In the future, we will culture stem cell organoids inside microcavities and measure their cell proliferations at 0.01-0.1 Pa.   

Development of a Mechanical Culture System for Stem Cell Stimulation in Bioprinted Scaffolds

Mechanical forces promote stem cell differentiation, tissue and organ development. Research has shown that stem cells stimulated by tensile strain can lead to higher regulation of self-renewal genes and higher proliferation rates. Our goal is to develop a culture system to mechanically stimulate stem cells in three-dimensional scaffolds using bioprinting. Bioinks are used as a scaffold and extracellular matrix where the cells will grow. In this work, we used a custom-made tensile stress bioreactor to grow bioprinted structures containing adipose-derived stem cells using a bioink made of alginate, cellulose and clay. We tested different concentrations of each component under tensile stress and analyzed the different strains for each bioink. We varied the concentrations of each material from 1% to 5%, and found that the optimal mixture for extrusion to keep shape consisted of 3% alginate, 3% cellulose, and 3% laponite. Adding clay to the mixture ensured that structure can be maintained after printing. Cellulose added tensile strength to the alginate mixture. The alginate fibers were strengthened when added with both cellulose and laponite, making the mixture of the three preferred over a bioink made of only alginate materials for tensile loading experiments.   

Democratic leadership controls intermittent collective motion

Most animal behavior studies consider collective motion as an uninterrupted, continuous process. Yet, in many animal systems, collective motion is not a continuous process, but occurs in bursts, which are preceded and followed by resting phases. The abrupt behavioral changes involved in this intermittent process might jeopardize group cohesion. Here, we study intermittent collective motion using sheep as model system. We discover that in this system, each collective motion phase possesses a temporal despotic leader that guides the group. We also show that group coordination and cohesion result from a strongly hierarchical interaction network, where each individual is attracted to, rather than aligned with, the individual directly in front. Consequently, there exists an information flow from the temporal leader to all group members. Importantly, individuals alternate in playing the roles of leaders and followers. We demonstrate that the unveiled democratic leadership mechanism – likely to be at work in other gregarious animal systems – confers the group with the capacity to pool information – known as collective intelligence– to perform a series of complex tasks, such as efficient target localization, consensus decision-making, and coherent group navigation.   

Structure and Self-Assembly of Biomolecules through Molecular Simulations

In the field of bio-inspired materials, the non-covalent self-assembly of relatively simple peptide based molecules has gained increasing attention for the formation of biologically functional materials, all with nanoscale order. Self-assembly is often associated with human medical disorders. Our work concerns the modeling of small biological molecules, such as peptides and lipids as well as of proteins, where the self-assembly propensity and the conformational properties, are studied through all-atom Molecular Dynamics simulations in explicit solvent. Of particular interest is diphenylalanine (FF) peptide, the study of which reveals a strong self-assembling propensity in water in contrast to its behavior in methanol. Furthermore, a systematic comparison in the self-assembly features among dipeptides in aqueous solutions which belong to two different classes “Val-Ala” (Alanine-Isoleucine and Isoleucine-Isoleucine) and “Phe-Phe” (FF), has been performed. A clearly stronger attraction is observed between Phe-Phe dipeptides. Our results are in qualitative agreement with experimental observations. Conformational properties of proteins in aqueous solution, are also studied, in terms of their structural stability and their thermosensitivity and possible mutations are explored.   

Self-organizing pattern formation in Myxococcus xanthus

The soil-dwelling bacterium Myxococcus xanthus is a model system for emergent self-organization in response to environmental cues. When starved, M. xanthus self-organize into fruiting bodies, large multicellular aggregates in which sporation occurs. To investigate this behavior, we have developed high-throughput imaging and custom image analysis code that identifies fruiting bodies. Inputting time lapse videos of fruiting body formation allows us to track the evolution of hundreds of individual fruiting bodies. Our methods present a new avenue for high-throughput characterization of fruiting body formation, an important first step in being able to determine how multicellular organization is encoded by the organism’s genome.   

A transferable, explicit-solvent polarizable coarse-grained model for proteins

The application of classical molecular dynamics (MD) simulations at atomic resolution, to the majority of biomolecular processes, remains limited because of associated computational complexity. Lowering the level of biomolecule’s spatial representation to a coarse-grained (CG) form by locally averaging atomic positions can provide the necessary computational speed-up to explore biomolecule’s conformational landscape. Current transferrable CG forcefields in literature are either limited to only proteins with the environment encoded in an implicit form (cannot study environmental heterogeneity) or cannot capture transitions into secondary/tertiary protein structures from a primary sequence of amino acids. In this work, we present a transferrable CG forcefield with an explicit representation of the environment for simulations with proteins. Electronic polarization has been incorporated into the forcefield by augmenting the protein model with auxiliary charges, which can react to environmental stimuli. The non-bonded pair potentials were parametrized with solvation, vaporization, and partitioning free energies of equivalent chemical species. We also present validations of our CG forcefield with simulations of multiple well-studied protein systems.   

Antibacterial Behaviors of Escherichia Coli Promoted by the Addition of Zinc Oxide Nanoparticles: A ReaxFF Molecular Dynamics Study

The use of zinc oxide nanoparticles (ZnO NPs), biologically safe and compatible, has attracted a great amount of attention in the community of biotechnology due to their ability to act as antibacterial biomaterials. For example, ZnO NPs could interact with a single cluster of bacteria like Escherichia coli (E. coli) which prevents an aggregation of E. coli clusters. While many studies showed the antibacterial behavior of the ZnO NPs experimentally, an atomic level understanding of bioactivities of the ZnO NPs is missing. Here, we perform reactive molecular dynamics (RMD) simulations to clarify reaction processes of ZnO NPs and E. coli, at a molecular level. Our RMD results reveal that the addition of ZnO NPs could successfully delay the growth of E. coli clusters. More importantly, our work identified key reaction pathways for the antibacterial behaviors of ZnO NPs. As such, Our work will help guide the development of ZnO NPs-based medical solutions for a wide range of biological applications.   

Surface exploration and formation of aggregates in bacteria

The near-surface swimming patterns of bacteria are strongly determined by the hydrodynamic interactions between bacteria and the surface, which trap bacteria in smooth circular trajectories that lead to inefficient surface exploration. Here, we show that surface exploration results from a complex interplay between motility and transient surface adhesion events. These events allow bacteria to break the smooth circular trajectories and regulate their transport properties [*]. In particular, we find that there is an optimal adhesion frequency that maximizes surface exploration. In a second step, we will discuss the formation of bacterial aggregate on living tissues.
[*] Perez-Ipina et al., Nature Physics 15 (6), 610-615 (2019).   

Surface exploration in bacteria

The near-surface swimming patterns of bacteria are strongly determined by the hydrodynamic interactions between bacteria and the surface, which trap bacteria in smooth circular trajectories that lead to inefficient surface exploration. Here, we show that surface exploration results from a complex interplay between motility and transient surface adhesion events. These events allow bacteria to break the smooth circular trajectories and regulate their transport properties [*]. In particular, we find that there is an optimal adhesion frequency that maximizes surface exploration.
[*] Perez-Ipina et al., Nature Physics 15 (6), 610-615 (2019).   

Quantifying Activity and Response of an Active Bath

Active systems such as suspensions of active colloids, bacteria, and enzymes have been observed to enhance diffusion at the microscopic scale. These suspensions also exhibit anomalous mechanical properties not observed in typical equilibrium fluids. To investigate the interplay between enhanced diffusion and fluid response, we study a suspended colloidal particle in an active bath using optical tweezers to conduct force measurements. We quantify the space-time correlations and energy dissipated by non-equilibrium processes to extract the activity of the system. A simple theoretical model of a particle in an active bath is used to compare experimental results to predictions. Microrheological approaches are used to quantify the mechanics of the active bath. Lastly, the relaxation time of the system due to both activity and material response is quantified. This approach quantitatively characterizes the activity and mechanical response of an active bath. Preliminary results suggest this approach can extract the timescale of the active process driving non-equilibrium fluctuations.   

Feed-forward correction of neural timing errors through fluctuating scalar input

Complex learned behaviors like playing piano often rely on precise timing control. However, it remains unknown how the nervous system generates precise timing, in particular among multiple motor components that must synchronize for long periods (e.g. two hands). Motivated by the songbird brain, where the putative sequence-generating nuclei governing left and right vocal muscle timing do not connect (birds have no corpus callosum) but could receive shared low-dimensional input, we present a model in which neural sequence generators are synchronized simply by a scalar modulatory input. We show that feed-forward scalar modulation alone can correct timing errors if (1) the modulation fluctuates in time and (2) it is spread non-uniformly over positions in the sequence so as to reflect its timecourse spatially. This yields attractor sequences within a spatiotemporal landscape of propagation speeds toward which delayed sequences advance and advanced sequences slow down. We give a mathematical recipe for constructing an appropriate spatial profile given the temporal structure of the modulation and show how it can correct sizable timing errors in a songbird-inspired neural network. This work reveals a simple mechanism for ongoing correction of timing errors in neural motor signals.   

Mechanical stimulus with real-time feedback on beating cardiac cells

Direct excitation of beating of cardiac cells by mechanical stimulus is
of interest to compare with electrical stimulus as a pacemaker.
Deformation resemble to a cycle of contraction and relaxation on a
cardiac cell induces a beat. We have investigated such mechanical
stimulus response of primary cultured cardiac cells forming aggregations
on petri dish or PDMS substrates by using a tungsten probe tip driven by
PZT bimorph. The stimulus was periodic, and the period was set to that
of intrinsic inter beat intervals of cells Continuous observations
showed that the beat intervals were fluctuated initially and then mostly
the beat stopped in 24h. Evidently, the beat rhythms of cell aggregates
under stimuli were sensitive to the phase difference between their
intrinsic beats and the periodic stimuli. To control such phase
differences, we integrated a function to apply mechanical stimulus with
a real-time feedback control using fast image processing using LabVIEW.
This real-time feedback system allows us to apply probe stimuli
synchronized with the intrinsic cardiac beats with variable phase
differences. In this presentation, we will discuss the beat intervals
and the variability of cardiac cell aggregates under phase controlled
mechanical stimulus.   

The effects of pore geometry for DNA entering and clogging nanopore

Solid state nanopores have a capability to detect single DNA molecules with sub-molecule resolution. However, long chain DNA molecules frequently clog at a pore although DNA enters individually. The clogging probabilities exceed 0.3 for lambda DNA (48.5kbp) and 0.7 for T4 DNA (166kbp). We have investigated a cause of such clogging enhanced by electroosmosis by DNA itself since negatively charged DNA attracts positive ions as well as pore wall, which generates counterflow for DNA translocation. Focused ion beams were used on SiN (200 nm) thin films to create nanopores. Fluorescently stained DNA molecules were directly observed by using an optical microscope. To reduce such counterflow, we have tested nanopores with various shapes with the diameter from 100 to 500 nm. We found a pore with sharp corners rather than circular reduced clogging probabilities for both lambda DNA (48.5kbp) and T4 DNA (166kbp). Further investigations showed the traces of DNA entering pores are strongly affected by the nanopore shape. To evaluate the flow by electroosmosis and electrophoresis quantitatively, a finite element analysis was also performed. Finally, we discuss the interactions between nanopore wall and highly crowded DNA inside pores.   

Agent-based Modeling for Biofilm Growth under Mechanical Confinement

Bacterial biofilm is a basic form of multicellular colonies grown on various surfaces. Mechanically confinement alters the morphological evolution and the collective cell ordering of a growing biofilm. Here we develop an agent-based model to elucidate the underlying biomechanical mechanisms of mechanically confined biofilm growth. The agent-based model quantitively reproduces density and morphology of biofilms from experiments under confinements of hydrogels of different stiffnesses. Large-scale agent-based simulations also provide 3D cell trajectories and the growth stress profiles in the growing biofilm as well as the hydrogels. A morphological transition occurs as hydrogel stiffness increasing, where radial alignment on the bottom surface and onion-like ordering on the top surface of biofilms emerges. As biofilm continues growing, the growth stress exceeds the cohesive strengths of the hydrogels or of the hydrogel-glass substrate, leading to fracture of the hydrogels or the hydrogel-glass interface and subsequently biofilm invasion into the fractured space. Our model highlights how mechanics shapes growing biofilms.   

Optogenetic data reveal changes in gamma-band oscillations due to cocaine addiction

Cocaine addiction is a significant healthcare issue. We investigated the effect of acute cocaine injection on the medial prefrontal cortex (mPFC) activity of mice. The study's main goal was to determine the changes in the gamma oscillations and their relationship to short-term neuroadaptation that may mediate addiction. The in vivo locale field potentials (LFPs) in response to a brief 10 ms laser pulse delivered to the mPFC were recorded for 2 s. For each of the 17 mice in the experiment, we repeated the optogenetic stimulation 100 times. Given the nonlinear nature of the mPFC network response and the nonstationarity of the recorded data, we used the empirical mode decomposition method to obtain orthogonal intrinsic mode functions (IMFs). The number of IMFs was selected such that the frequency bands of the decomposed signal do not overlap and they match the known brain wave frequency bands delta (0.5-4 Hz), theta (4-8 Hz), alpha (8-12 Hz), beta (12-35 Hz) and gamma (> 35 Hz). Our results showed significant changes in the gamma band between control and cocaine cases.   

Symmetries Uncover the Minimal Regulatory Network for Logical Computations in Bacteria

The use of symmetries in physics to reduce a complex system to its underlying components and interactions is a widely known useful tool. Recently we have shown¹ that relevant symmetries of biological network systems allow for a systematic reduction of the networks that preserves information flow. Symmetry fibrations, which consists of grouping nodes that share an isomorphic input tree into equivalence relations called fibers, allows for the collapse of the network while preserving information flow. Further reducing the network by the k-core decomposition of the collapsed network gives the minimal network driving the dynamics of the entire network. In gene transcriptional regulatory networks (TRN), where fibers consist of genes that are synchronized for being co-expressed, this gives the minimal transcriptional regulatory network. This minimal structure is understood as a combination of genetic circuits which perform core logical computations from outside inputs and the current state. Hence, symmetry principles unveil the minimal TRN that corresponds to the core computational machinery.
1)Flaviano Morone, Ian Leifer, and Hernán A Makse. Fibration symmetries uncover the building blocks of biological networks. PNSciences, 117(15):8306–8314, 2020.   

Active Restructuring of Myosin-Driven Actin-Microtubule Networks

The cytoskeleton is a dynamic network of protein filaments and motors, including actin, microtubules and myosin, that enable key processes in the cell such as growth, movement, and cell division. Active rearrangement and contraction of actin networks by myosin motors has been extensively studied in recent years. However, how the interactions between actin and microtubules affects actomyosin activity remains poorly understood. We investigate the active dynamics and restructuring of composite networks of actin and microtubules driven by myosin II motors. We combine spatial image autocorrelation analysis and particle image velocimetry to characterize the time-varying network structure and velocity profiles of actin and microtubules. We show that actin and microtubules exhibit similar contractile flow fields that increase in magnitude and heterogeneity as the concentration of actin or myosin increases. We also find that the correlation length scales of the composite networks generally decrease over time but at a slower rate with higher concentrations of actin or myosin. These findings provide insight into how the cytoskeleton can tune its dynamics and structure to impact cellular processes such as movement and growth.   

B. subtilis uses c-di-GMP accumulation as a long-term decision making factor during biofilm formation

Cells such as bacteria that make decisions about their fate often do so over timescales longer than a single cell cycle, which poses a design challenge. Here, we suggest that B. subtilis uses c-di-GMP accumulation as a long-term decision making factor for biofilm formation. C-di-GMP is implicated in motility switching and biofilm matrix production. Previous literature has shown that during biofilm formation, B. subtilis exhibit a bimodal distribution of c-di-GMP. This is thought to be driven by transcriptional repression of the enzyme that degrades c-di-GMP. We constructed a model that stochastically simulates formation and degradation of c-di-GMP and examined distributions of intracellular concentrations across a population. The model predicts that accumulation in the high subpopulation occurs over five or six cell generations. To experimentally confirm these predictions, we use a fluorescent c-di-GMP sensor. Bulk population measurements of c-di-GMP accumulation rates appear to take 150 minutes to saturate, roughly corresponding to the five cell generation prediction. Together, these results suggest that through c-di-GMP accumulation, B. subtilis uses intracellular signaling factors to integrate information beyond an individual cell cycle to coordinate biofilm formation.   

Multifunctional control of vocal output in the songbird syrinx

Songbirds produce complex vocalizations by coordinating neuromuscular control of syrinx, respiratory system and upper vocal tract. The specific involvement of individual syringeal muscles in achieving fine vocal control is still largely unknown. Here we investigate the contributions of the two main airflow controlling muscles, the dorsal and ventral tracheobronchial muscles in the zebra finch, through a new experimental approach. Ablation of the muscle insertion on the cartilage framework reveals detailed insights into their respective roles in the fine control of song features. Unilateral ablation of a tracheobronchial muscle resulted in mostly subtle changes of the air sac pressure pattern and song features. Effects of ablation varied with the acoustic elements, thus indicating a context-dependent specific synergistic activation of muscles. More pronounced effects on song features and air sac pressure were observed after bilateral ablation of the dorsal tracheobronchial muscles. The results illustrate that the gating muscles serve multiple functions in control of acoustic features, and that each feature arises through context-dependent, synergistic activation patterns of syringeal muscles.   

Generating nonlinear chemical gradients in microfluidic devices by biased mixer input design

Cell migration can be driven by chemical gradients. Our ability to precisely emulate and control the aspects of the biophysical environment (manipulate chemical gradients) is crucial to studying cell function. We present five microfluidic designs for generating non-linear gradients by passing the chemical through a combination of bifurcated, trifurcated, and mixing channels that split, mix, and recombine the flows to produce incremental dilutions into a common chamber. The non-linearity was introduced by adding bias to the mixers inputs. The study began with a 1:1 mixing ratio that generated a reference linear gradient in the chamber, followed by 2:1, 3:1, 4:1, and 5:1 mixing ratios that generated non-linear gradients. The bias was introduced by altering the input channel widths. Both COMSOL simulations and wet-lab experiments were used to study the chemical profiles across and along the gradient chamber.

ELW and OMA contributed as co-first authors.   

Cellular reproduction and death induce diffusion of immotile bacteria in growing biofilms

Biofilms, surface-adhered communities of bacteria, are often studied in the framework of colloidal soft matter physics. Yet, in contrast to non-living colloids, cells are living matter which can reproduce and die. We study the statistical-mechanical consequences of these life events in Vibrio cholerae biofilms. While the bacterial cells are inherently immotile in biofilms, reproduction and death induce cellular rearrangement, providing an effective diffusive behavior of individual cells. We confirm such diffusive dynamics experimentally by developing a protocol to track individual bacteria during biofilm growth. We vary the rate of reproduction and death to explore the relation between these life and death events and the resulting cellular dynamics. We find that cellular rearrangement is strongly time-dependent, exhibiting stages of diffusive, super-diffusive, and sub-diffusive behavior. Further, cellular dynamics in biofilms emerges from an interplay of growth and death and reorganization dynamics in biofilms. The results demonstrate that life and death events are important aspects in dense cellular populations, and provide a rich source of non-equilibrium dynamics in living soft matter.   

Modeling evolution of firefly-like signal vocabularies during species radiation

Fireflies in vast swarms communicate with each other by producing bioluminescence to signal their presence and court mates. In particular, some species emit patterns of short flashes that have the potential to encode information. Male fireflies flash according to a species-specific pattern in order to attract and locate female partners. As multiple firefly species can share the same habitat, potential visual clutter could greatly hinder species discrimination and successful communication among conspecifics. We investigate how firefly flash sequences can co-evolve to be distinguishable by developing a method for simulating flash patterns that minimizes a cost function which incorporates similarity and predation risk. We observe an emergent periodicity in the resulting optimal sequences despite the lack of any constraints on the sequences to contain regular patterns. We also demonstrate a method of reconstructing potential cost functions from the phylogenetic relationships of extant species alongside their characteristic flash patterns.   

Comparison of mechanisms of kinetochore capture with varying number of spindle microtubules

The mechanism by which microtubules find kinetochores during spindle formation is a key question in cell biology. Experimental studies have shown that although search-and-capture of kinetochores by dynamic microtubules is a dominant mechanism in many organisms, several other capture mechanisms are also possible. One such mechanism reported in Schizosaccharomyces pombe shows that microtubules can exhibit a prolonged pause between growth and shrinkage. During the pause, the microtubules pivoted at the spindle pole body search for the kinetochores by performing an angular diffusion. To understand the physical basis for the selection of one mechanism over another, we numerically compare the temporal efficiency of these two mechanisms by distinct models. We find that the capture timescales have non-trivial dependences on microtubule number N, and one mechanism may be preferred over the other depending on this number. While for small N (as in fission yeast), the characteristic capture times due to rotational diffusion are lesser than those for search-and-capture, the situation is reversed beyond a certain N.   

Molecular Dynamics Simulations of Biological Macromolecules

The Guanylate Kinase (GK) enzyme catalyzes the conversion of ATP to ADP which provides energy for cellular processes. Lab research on this important enzyme has shown that the behavior of the molecule follows a viscoelastic regime, and this research project has attempted to model the GK enzyme via computational software. The research required becoming familiar with the Python programming language and doing research into molecular dynamics simulations. The project looked into visualization software such as Anaconda and LAMMPS to create the model. This research project was able to obtain a simple visualization of the GK enzyme using VMD, and is in the process of obtaining further simulations using GROMACS software. Given the complexity of the field of computational biology and the associated programming, full scale representations of the GK enzyme are very difficult, and the work is ongoing. The GK enzyme model will be useful to better understand the molecular dynamics of the enzyme and assist with more targeted research in the fields of biophysics and biochemistry.   

FKPP dynamics mediated by a parent field with a delay

In this poster we describe the result of modification of the Fisher-Kolmogorov-Petrovsky-Piskunov (FKPP) process in which the diffusing substance requires a parent density field for reproduction. This would be relevant in biological context, for example, with diffusing spores (propagules) and stationary fungus (parent). The parent produces propagules at a certain rate, and the propagules turn into the parent substance at another rate. A finite time is typically required for a new parent to mature before it begins to produce propagules. We model this evolution by a modified FKPP process with delay. Other types of delays in the FKPP model have been considered in the past as a mathematical construct. However, in our work, the delay arises in a natural science setting. The speed of the resulting density is shown to decrease with increasing delay time. Moreover, the front speed has a non-trivial dependence on the rate of conversion of propagules into new parent. The fronts in this model are always slower than Fisher waves of the classical FKPP model. The largest speed is half of the classical value - it is achieved at zero delay and when the two rates are matched.   

Coarse-grained Molecular Dynamics Simulations of Ca2+-Calmodulin

Calmodulin (CaM) is a primary Ca²⁺ signal transducer protein that undergoes multiple conformations upon Ca²⁺ binding, which allow it to bind and activate a myriad of target proteins. An appropriate force field to model the effect of Ca²⁺ on the dynamics of CaM is still lacking. Here, we present a coarse-grained approach using the Associative memory, Water mediated, Structure and Energy Model (AWSEM) force field to study the Ca²⁺-induced conformational changes on CaM. We have parameterized the force field for the Ca²⁺-CaM to match the experimental observables. This model provides mechanistic insights on CaM’s ability of target binding and selection.   

EMBED: Essential Microbiome Dynamics, a dimensionality reduction method for longitudinal microbiome data

Dimensionality reduction techniques are crucial in deciphering high dimensional biological data. Gut microbiome is an example of complex and dynamical systems comprising thousands of bacteria whose abundances change stochastically and substantially. To understand the collective dynamics of the gut microbiome as a combination of simple dynamical patterns, we propose Essential MicroBiomE Dynamics (EMBED); a matrix factorization method that embeds longitudinal microbiome abundance data onto a series of lower dimensional Boltzmann Distributions. EMBED learns uniquely from data features (Hamiltonians) and orthogonal latents (Intensive variables) akin to “normal modes” in soft matter physics. We use EMBED to investigate three gut microbiome datasets that represent pathogenesis, diet, and drug related perturbations respectively. For all datasets considered, < 5 modes are sufficient to capture the dynamics with significant accuracy. Notably, EMBED identifies the collective modes representing typical dynamics of gut bacteria that have clear physiological interpretations. Moreover, features can be used to identify bacteria with similar dynamical profiles. We believe that EMBED will be a significant tool in the analysis of other longitudinal sequencing data as well.   

Photonics probing of structural alterations in DNA specific mass density fluctuations in nuclei due to total body irradiation (TBI) via confocal imaging

The quantification of structural alternation in cells at the nanoscale can provide a range of cellular information. Structural alteration in cells may result due to irradiation. Accidental or deliberate exposure to total body irradiation (TBI) has adverse effects on the nuclear DNAs of cells. Here, we report a quantitative analysis of the molecular specific DNA mass density spatial structural changes in gut cell nuclei due to 4 Gy gamma TBI. The structural changes in nuclei due to TBI are studied by quantifying the DNA molecular specific light localization properties. The DNA mass density fluctuations are probed by using a recently developed inverse participation ratio (IPR) technique, on DAPI stained DNA/chromatin molecular mass density via confocal imaging. The gastrointestinal tract is the major target of TBI. So, using the IPR technique nuclei of mice gut cells are studied. The result shows that irradiation effects vary with the post-irradiation time and have adverse effects on the DNA molecular spatial structural arrangement. Our results indicate radiation suppresses DNA spatial mass density fluctuations. And hence, reduction and saturation in DNA density fluctuations are observed on different durations of post-irradiation.   

Phase separation in membranes due to matter exchange

We consider a two-dimensional membrane exchanging matter with a reservoir. Matter absorption and desorption in the membrane occurs with rates k_A and k_D respectively.
By only assuming matter conservation in the system membrane-reservoir, we show that a complex patterned composition distribution emerges in the membrane, depending on the values k_A and k_D. The pattern emergence is due to Spatio-temporal "memory" effects.   

Optical detection of structural abnormalities in chronic alcoholic mice brain tissues using Partial Wave Spectroscopy

Light is a good probe to study the nanoscale structural alteration in brain cells/tissues. The individuals who consumed alcohol with no specific neurological or histopathological disorders have shown signs of regional brain damage and cognitive dysfunction. However, the nano to submicron scale structural alterations in chronic alcoholic’s brain cells/tissues are hardly explored. Here, the recently developed finer focusing mesoscopic physic-based imaging technique, partial wave spectroscopy (PWS), is used to probe the nanoscale structural alteration in alcoholic mice brain tissues. This highly sensitive and versatile PWS technique can quantify the nanoscale mass density/refractive index fluctuations due to alcohol as the degree of structural disorder strength (L_d) parameter. The results show that the average and std of L_d is significantly higher in the brain tissues of chronic alcoholic mice relative to control. This increase in the disorder strength of alcoholic mice brains may be due to the increase in the mass density fluctuations or the rearrangements of the different macromolecular spatial structures. The potential application of the PWS technique to reveal the underlying correlation of nanoscale alterations in brain tissues and chronic alcoholism will be discussed.   

Spectroscopic study of the structural alterations in pups’ brain cells at the nanoscale level due to fetal alcoholism

Mesoscopic physic-based molecular specific light localization and microscopic imaging are highly sensitive spectroscopic techniques to study the structural abnormalities in brain cells/tissues. Fetal alcohol syndrome and other neurological disorders are the severe, and irreversible outcomes of fetal alcoholism. However, the nanoscale damages in cells/tissues are hardly diagnosed. In this work, we probe chronic alcoholic mothers' pup brain cells/tissue using dual spectroscopic approaches in a mouse model. First, the inverse participation ratio via confocal imaging, confocal-IPR, is used to probe DNA and histone spatial structural alterations. Then, a recently developed highly sensitive partial wave spectroscopy (PWS) is used to probe the nanoscales structural alterations in the pup’s brain tissues. The confocal-IPR technique shows an increase in the degree of spatial structural disorder in DNA, while a decrease in histone. An increase in spatial disorder in DNA might be due to DNA unwinding while a decrease in histone could be due to the release of histone from the DNA resulting in the unwinding of the DNA and gene expression. Besides, this result is supported by the PWS which shows an increase in the degree of structural disorder in the brain of pups exposed to fetal alcoholism.   

Models of Galvanotaxis: Coupling Cell Migration and Shape

Eukaryotic cells can undergo galvanotaxis, which involves cells crawling to follow an electrical potential gradient. This occurs naturally during wound healing, when electrical fields guide the cells towards the wound. Keratocytes are prevalent during wound healing and are used as a model system for studying galvanotaxis. However, galvanotaxis is not as well studied. For instance, the specific proteins that allow electric field sensing and much of the pathway for responding to the electric field remain unknown. It is not well understood what cellular features control keratocyte behavior during galvanotaxis and how quickly cells can respond to changes in gradients. In addition to persistent crawling, keratocytes may exhibit oscillatory and circular motion both with and without a field present. We have developed a coarse-grained phenomenological model that can qualitatively recapitulate crawling, oscillatory, and circular motion with and without a field present. We can fit this model to experimental data on the response of keratocytes to a field turning on and off. Key parameters in controlling cell motion include the strength of coupling between shape and velocity and the rate at which the cell repolarizes in response to a signal.   

Possible Impact of Mutation of Sars-COV-2

The novel Sars-Cov-2 virus that causes COVID-19 has undergone some structural changes—in particular, a mutation in the spike protein established what appears to be a dominant strain that is more transmissible by a single amino acid replacement in residue 614 of the spike (S) protein---Aspartic Acid replaced with Glycine. However, the new strain that is potentially more infectious has not been shown to be associated with mortality, indicating that it is likely not more deadly. We study how this mutation is contributing to the epidemiological data showing lower mortality rates in parallel to the observed waves of infection.   

Bacterial Route-finding and Collective Escape in Mazes and Fractals

Bacteria which grow not on the featureless agar plates of the microbiology lab but in the real world must navigate topologies which are non-trivially complex, such as mazes or fractals. We show that chemo-sensitive motile {\em E. coli} can efficiently explore non-trivial mazes in times much shorter than a no-memory (Markovian) walk would predict, and can collectively escape from a fractal topology. The strategies used by the bacteria include individual power-law probability distribution function exploration, the launching of chemotactic collective waves with preferential branching at maze nodes and defeating of fractal pumping, and bet hedging in case the more risky attempts to find food fail.   

The Broken Symmetry of Low Temperature and High Temperature Unfolding of Proteins in Water

We consider a very simple mathematical model of the thermodynamic stability of globular proteins in water at high and low temperatures, stressing the concepts of broken symmetry in the water-protein system. There are two crucial concepts: 1) The continuum of conformations that a protein can have when dissolved in water; 2) The broken symmetry of the order parameter of the packing density of the amino acids at high and low temperatures. Using simple thermodynamic arguments we make predictions about two basic instability regions of a protein at both high and low temperatures. For widely satisfied parametric conditions, we demonstrate the existence of a unique cold denaturation temperature related to the hot denaturation temperature. We point out that there exists some confusion in the literature concerning this thermodynamic analysis, which we clarify in our exact solution. We finally discuss new experimental technologies which could explore thermal stability range of proteins.   

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.   

Emergent in vitro Cancer Tumor Hypoxia and K-Core Collapse

We have developed a oxygen-permeable thin film containing a platinum-porphyrin phosphorescent dye whose excited state is quenched by O2. The dye phosphorescence intensity is a function of oxygen concentration over at least 3 orders of magnitude, when used in a microfabricated 3D ecology we get time and space resolved hypoxia of a PC3 cancer colony. The cancer colony when enclosed generates its own emergent hypoxia due to metabolism to the remarkably low level of 0.01 or lower normoxia and can maintain this for several days. However, the extremely low self-induced hypoxia and acidosis seems to generate a collective response of the cells consisting of polyploidal giant cells with strong interconnects. This ``k-core” interactive colony ultimately reaches a tipping point and collapses.   

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.   

A Stochastic Model of University Virus Transmission

The spread of airborne viruses is a concern on campuses that has been heightened by the current COVID-19 global pandemic, leading to many universities taking various safety precautions to minimize outbreaks on campus. Smaller universities may benefit from using a stochastic compartmental model to help determine the impact of specific policies and scenarios on the spread of COVID-19 on their campuses. The model we developed creates a network of students with randomly generated class schedules and, starting with a randomly chosen infected student, calculates the number of infections on campus as the semester progresses. From this model, we determined the relationship between probability of infection, the peak number of infections, and the impact of various social distancing policies on the spread of the virus on campus. As the infectivity of the virus increases, our results show that the peak of the campus outbreak occurred earlier in the semester, while for lower infectivity rates, it was more likely for the virus to entirely disappear from the community. We also observed naturally occurring “superspreader” events that worked to prolong outbreaks on campus. Finally, we expanded our model to capture more complex and realistic student interactions on and off campus.   

Modeling the Actin Organization in Dendritic Spines using a Minimal Stochastic Model

Synaptic plasticity is a complex process, involving the intricate coupling between biochemical and biophysical events. Actin remodeling plays an important role in this coupling of biochemical and biophysical events at the synapse. For example, changes in size and shape of dendritic spines, small membrane protrusions which form the postsynaptic component of most excitatory synapses, affect the geometry of the synapse and thereby neurotransmitter dynamics. While it is known that actin plays a key role in synaptic plasticity, the relationship between spine size and shape and actin organization remains poorly understood. In this work, we pose an inverse problem -- given a spine shape, what configurations of the actin network can be accommodated? To answer this question, we develop a minimal model of actin dynamics and simulate it using the agent-based modeling framework, Cytosim. Our simulations show that different spine geometries fundamentally accommodate different actin networks, characterized by the distribution of Arp2/3 branch points, numbers of barbed ends, network topology and other metrics. Furthermore, we find the synergy between Arp2/3 mediated branching and cofilin mediated severing for barbed end generation is dependent on the spine shape.   

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   

Comparison of the Effect of Magnetic field on the different type of cells

Abstract

Magnetic fields have a direct effect on the morphology of different types of bacteria and their growth. The human body incubates bacteria all over the place. Therefore, the magnetic field generating electronic devices can have a non-ignorable effect on human bacteria. This study needs to consider the effect of the magnetic field on human cells as well. Different magnetic fields have different effects on variable bacterial species. A weak magnetic field slows the growth rate of bacteria cells. However, the effect of the magnetic field on the eukaryotic cells is needed to be studied in detail. In addition, the bacteria in the human body may be affected differently in the human body environment. We look at all such effects and compare the results of the magnetic field effect on eukaryotic cells and on the prokaryotic cells.   

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 , and preexponential factor ν - during thermal desorption of binary mixtured 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, 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.   

Data-Driven Committor Approximation for Anisotropic Diffusions in Collective Variables

Molecular simulation commonly deals with systems that reside in stable states over very large timescales and transition quickly between on extremely small scales. These transitions are crucial to molecular simulations but difficult to characterize due to the timescale gap. Transition Path Theory (TPT) is a powerful mathematical framework describing the transitions, but involves solving the committor equation, an elliptic boundary value problem on the high dimensional state space. To circumvent high dimensionality, practical use of TPT involves: (a) collective variables (CVs) and (b) assuming that the transition mechanism is restricted to thin channels: the "transition tube" assumption . A common issue with the use of CVs is anisotropy from local metric distortion. Hence, we propose a data-driven method to correct for anisotropy and solve the committor equation without the transition tube assumption in the space of CVs. We provide illustrative examples that our approach approximates the committor and more generally is invariant to the choice of CVs up to diffeomorphism.   

Transition time in a two-state system involving similar and non-similar potential energy curves coupled by δ-potential

The study of the transition time has been remained unexplored in the multistate problems in quantum mechanics. There is hardly a method available to calculate that quantity. In this work, we have proposed a simple method of calculating transition time and study the characteristics of it by observing its dependency on coupling strength and the energy of the system, etc. We have adopted the method of tunneling time estimation to implement the method for transition time in a two-stage model, where states are coupled by a δ-function. An explicit dependence on both the potential energy curves as well as on the coupling strength has been observed. For a simple two constant potentials energy states, we also noticed that the coupling potential appears as almost being transparent and nearly being opaque to the incident wave, depending upon its energy. Our calculated transition time non-monotonically depends on the incident energy and coupling strength.   

Photodissociation of PdCl42- in aqueous/EtOH solution using TDDFT calculations

Nuclear spent fuel discharged from atomic power plant contain platinum group elements (PtGs) such as palladium. Recently, it is proposed to be reused as resource by separation and recovery of PtGs from nuclear spent fuel. We focused on that laser-induced can reduce metal and separate element. We proposed and succeed a method for the separation of PtGs using laser-induced particle formation (LPF) and recovery after growing to nanoparticle. The irradiation of UV laser at 266 nm induced the reduction of Pd²⁺ in aqueous/EtOH solution, which has charge-transfer absorption band at UV range, neutralizing Pd²⁺ into Pd⁰ [1]. In this study, we report dissociation energy and absorbed spectrum for n-Cl (n = 1-4) loss from PdCl₄^2- in the gas phases and aqueous/EtOH solution by TD-DFT and Complete Active Space Multiconfiguration SCF calculations performed with B3LYP, SDD for Pd and 6-311+G(d,p) for Cl using Gaussian 16.
As a results, we assume 4Cl^- loss from the PdCl₄^2- and calculate low-lying potential energy curves where the binding energy of ground state and excited states of PdCl₄^2- for Pd-Cl bond length. We clear that PdCl₄^2- is dissociate into Pd0, 2Cl^- and 2Cl.
[1] M.Saeki, et al., J. Phys. Chem. C 123 (2019) 817   

Improved training set sampling techniques for machine learned interatomic potentials

Machine learned interatomic potentials (MLIPs) offer the prospect of ab initio level accuracy and reliability as linear-scaling, surrogate models. Currently, much work is focused on improving spatial descriptors and model formulations, which are typically trained on a random structures and modulated structures derived from a representative set of known phases. Ab initio molecular dynamics is typically employed to sample the local potential energy surface of the training structures. Here, we explore the biases that molecular dynamics can impart in the performance MLIPs, using the solid-state Zr-O system as test system presenting structural and chemical complexity. To address these biases, we explore improvements in configuration space sampling, including a novel technique that may have applications beyond machine learning. To this end, our methods are implemented in our open source framework for rapidly generating training data on HPC clusters.   

Protonic Bipolar Semiconductor with Proton and Prohol Self Traps in Our Melted Ice Lattice Model of Pure Liquid Water

We began a study of liquid water in July 2013 and found a physics model was still elusive after Volta invented the Voltaic Cell in 1799, 220 years ago. We reported our discoveries at the September 2013 Annual Fall Meeting of Chinese Physical Society that the 1933 Bernal-Fowler Hexagonal Close Packed crystalline Ice model, proven by Pauling's 1935 residual entropy theory observed by Giauque's 1930−1936 low-temperature specific heat experiments, can model thru water's liquid into vapor phase to quantitively account for the vibration frequencies of isolated water molecule in vapor and the 3 thermal equilibrium liquid properties used worldwide: the pH ionization energy of pure water 540 meV and the thermally activated electrical mobility of positive protons 190 meV and negative prohols 210 meV. The still puzzling kinetic energy band and valence electron bond models in spacetime (x,y,z,t) were instantly recognized by our third author with her Chess Mastery, who joined us in August 2018, trying to provide a A1G & A0T (At 1 Glance & At 0 Think) picture to explain to her peers at the Student Session of the 2019 APS March meeting in Boston. It was the occupation by the protons at only one of the two trisector positions of the 1933 Bernal-Fowler Ice Model. Refer to our 2018 JOS article.   

Ultra-sensitive Detection of Hg2+ in Water Using Flower-like Metallic 1T Tungsten Disulfide

The development of an electrochemical sensors for mercury ion (Hg2+) with ultratrace sensitivity, wide linear detection range (LDR), and high selectivity is still challenging. An electrochemical sensor based on flower-like metallic 1T phase tungsten disulfide (WS2) for sensing Hg2+ has been presented. The resulting sensor showed an excellent sensitivity toward Hg2+ with a wide LDR of 1 nM - 1 mM. Besides, the sensor exhibited an ultra-low detection limit down to 79.8 pM (pico molar) toward Hg2+, which is well below the safety limit of Hg2+ in drinking water recommended by the World Health Organization. Further, the sensor exhibited high selectivity for Hg2+ in presensce of different interferring metal ions such as Pb2+, Cu2+, Fe3+, Cr3+, Ag+, Sn2+, and Cd2+. The obtained superior sensing performance
can be ascribed to the large surface area, flower-like morphology, high conductivity of WS2, and the complexation of Hg2+ ions with S2-. Thus it is suggested that the sensor is a potential device for real applications.   

Ice adhesion Strength on Nanostructured Graphite Substrates by Molecular Dynamics Simulations

The issue of ice accumulation at low-temperature circumstances causes multiple problems and serious damages in many civil infrastructures that substantially influence human’s daily life. However, despite the significant consideration in manufacturing icephobic surfaces, it is still demanding to design surfaces with well ice-repellent properties. Here in this study, we used force-probe molecular dynamics simulations to investigate the adhesion mechanism and tensile strength of ice from atomistically smooth and nanotextured graphite substrates. It is found that the ice cube temperature, ice-substrate interfacial energy, size of the surface roughness, the orientation of graphene sheets of graphite surface, and the local order of interdigitated water molecules significantly influence the tensile strength of ice from substrates. Atomistically smooth surfaces are seen to exhibit higher normal failure stress compared to nanostructured graphite substrates. Morover, our results indicate that the MD data for the ice tensile strength on smooth graphite substrate can be collapsed onto two master curves as a function of the ratio between the water-surface interaction energy and ice cube temperature.   

Exploration of High Dimensional Free Energy Landscape by a Combination of Temperature Accelerated Sliced Sampling and Parallel Biasing Abhinav Gupta, Shivani Verma, Nisanth N. Nair Department of Chemistry, Indian Institute of Technology, Kanpur, India

Biased sampling methods such as the Temperature Accelerated Sliced Sampling (TASS)[1], which can explore high dimensional collective variable (CV) space, are of great interest in free energy calculations. Such methods can efficiently sample configurational space even when a large number of CVs for biasing are used while many conventional methods are limited to two or three CVs. Here [2] a modification to the TASS method is proposed, named as Parallel Bias TASS or PBTASS, wherein a multidimensional parallel metadynamics biases [3] are incorporated on a selected set of CVs. In my poster, I will be discussing the implementation and the performance of PBTASS method by comparing the accuracy and efficiency with various other enhanced sampling methods. The applicability of the PBTASS method is presented by studying the conformational sampling of small peptides, protein folding, and biochemical reactions. Quick convergence of free energies, and controlled sampling are the main features of this technique.

REFERENCES:
[1] S. Awasthi and N. N. Nair, J. Chem. Phys. 146 094108 (2017)
[2] A. Gupta et al. ArXiv:2010.01646v1
[3] J. Pfaendtner and M. Bonomi, J. Chem. Theory Comput. 11 5062 (2015)   

Thermal effect in plasmon assisted photocatalyst: a parametric study

Recently, it has been suggested that chemical reactions can be facilitated by using mm-scale composites of metal nanoparticles on porous metal oxides when illuminated at their plasmonic resonance wavelength. This effect was shown recently to be predominantly associated with the heating induced by illumination [Dubi et al., Chem. Sci., 2020]. In this work, we study the parametric dependence of the temperature distribution in these composites numerically and provide analytic expressions for simple cases. It turns out that the physical picture emerging from the collective thermal response is quite different from the one that emerged from single particles. We show [Un & Sivan, Nanoscale, 2020] the temperature rise distribution in them is typically weakly-dependent on the illumination wavelength, pulse duration, particle shape, size, and density; but is strongly sensitive to the beam size and the host thermal conductivity. These results have a direct implication in the route for optimization of the reaction rates. On a more general level, this work would also be instrumental in uprooting some common misconceptions associated with the role of thermal effects in applications that rely on heat generation from a large number of particles.   

“Hot” electron generation in plasmonic nanostructures – thermal vs. non-thermal effects

We develop [Dubi & Sivan, Light Sci. Appl., 2019] a coupled Boltzmann-heat equations formulation for calculating the electron distribution in plasmonic nanostructures under continuous-wave illumination, taking into account non-equilibrium and thermal effects on the same footing. This approach allows us to determine self-consistently and uniquely the increase in electron and lattice temperatures above ambient conditions although the system is inherently away from thermal equilibrium. Our results provide the first-ever quantitative prediction of the high energy non-thermal electron densities, and show that close to the Fermi level, the non-equilibrium is dominated by holes (rather than by electrons)! Most importantly, we find that most absorbed power causes heating, and only an extremely small fraction actually leads to high energy electron generation. Finally, we develop a simple model for the catalytic enhancement for chemical reactions based on illuminated metal nanoparticles. It shows [Sivan et al., Science, 2019; Dubi et al., Chem. Sci., 2020] that the faster chemical reactions reported in many previous papers are extremely unlikely to originate from the high energy non-thermal electrons. Instead, the faster reactions very likely originate from a purely thermal effect.   

A Temperature Accelerated Sliced Sampling Study on Drug Binding/Unbinding Shubhandra Tripathi and Nisanth N. Nair Department of Chemistry Indian Institute of Technology Kanpur 208016, Kanpur, Uttar Pradesh, India

Molecular dynamics simulations using enhanced sampling techniques are widely used to explore the rugged free energy landscapes of drug binding/unbinding in proteins. Collective variable based techniques for enhanced sampling have severe drawback that their free energy convergence is slow when the dimensionality of the landscape goes beyond two or three [1]. Sampling of large number of transverse coordinates are vital to obtain reliable free energy estimates and quick convergence. A controlled biased sampling is essential to overcome the “leak out” of biasing energies. Recently, we proposed a novel sampling technique called “Temperature Accelerated Sliced Sampling” (TASS) to overcome these limitations [1-2]. In this work, we applied the TASS method in modelling binding/unbinding reaction pathways of avibactam, a β-lactamase inhibitor, with Class C β-lactamase [3]. We obtain full atomistic details of the binding/unbinding reactions, the intermediate steps thereof, critical interactions and free energetics.
References
Awasthi, A., & Nair, N. N. WIREs Comp. Mol. Sci., 9(3) e1398 (2018)
Awasthi, S., et al. J. Chem. Phys. 146(9), 094-108 (2017)
Das, C. K., & Nair, N. N. Phys. Chem. Chem. Phys., 20(21), 14482-14490 (2018)   

Reaction Coordinate and Enhanced Sampling for Polymorph Nucleation

Nucleation as an essential pre-crystallization process is important in organic synthesis, drug industry and many other problems in science and engineering. However, given different conditions, different structures of specific molecules, also known as polymorphs, may form which eventually affect the performance of the compound synthesized. Many enhanced sampling based all-atom simulation methods have been developed to study the process of nucleation in systems with competing polymorphs, including entropic order parameters used with metadynamics [1,2]. Here we study such entropic and other order parameters to build an optimized low-dimensional reaction coordinate for biasing in metadynamics. To do so we use the method Spectral Gap Optimization of Order Parameters (SGOOP) [3]. With this we tackle different aspects of the nucleation process in classic problems [4] including the test-piece of urea polymorph nucleation.   

Strong-field-driven dynamics and high-harmonic generation in interacting 1D systems

We explore the role of band structure and Coulomb interactions in solid-state high-order harmonic generation (HHG) by studying the optical response of linear atomic chains and carbon nanotubes to intense ultrashort pulses [1]. We solve the single-particle density matrix equation of motion in the presence of intense optical fields, incorporating tight-binding electronic states and a self-consistent electron-electron interaction. The doping can be tuned to describe metals, regular insulators, and topological insulators. Our study reveals the important role played by electron interactions in HHG, due in part to the presence of collective optical resonances. We find that doped semiconductors generate high harmonics more efficiently than their metallic and undoped counterparts. We also show results for HHG in more realistic quasi-1D structures such as carbon nanotubes, with behavior similar to that of atomic chains. Our findings apply to a broad variety of solid-state and molecular systems and can be extended to optimize HHG platforms or identify new solid-state alternatives in the context of nonlinear plasmonics.
[1] S. de Vega et al. Phys. Rev. Research 2, 013313 (2020).   

Toward Practical Corrections of Artificial Symmetry Breaking in Molecular Self-Consistent Field Calculations

Spin symmetry breaking is a critical part of practical molecular electronic structure calculations. A commonly cited example is the so-called “strongly correlated” dissociation limit of ground-state singlet hydrogen molecule. Spin-restricted singlet Hartree-Fock or Kohn-Sham density functional theory calculations give an artificially high energy, while spin-unrestricted calculations localize spin-up and spin-down electrons to different atoms and return a reasonable dissociation energy. A number of literature reports have documented unexpected spin symmetry breaking in the Hartree-Fock ground states of conjugated π-systems as common as benzene. Such symmetry breaking is typically identified by restricted/unrestricted instability and the resulting broken-symmetry and spin-contaminated unrestricted Hartree-Fock wave functions and Kohn-Sham determinants. We show that symmetry breaking degrades predicted properties of many of these systems. Moreover, we demonstrate that practical spin purification models can improve the quality of calculated minimum energy geometries and vibrational frequencies while retaining reasonable computational cost. These results show that careful consideration of spin symmetry can be important in simulating even "normal" systems.   

Investigation into the effects of Stereochemistry in Heterogeneous Catalysis of Propylene Glycol on a Palladium Surface.

Small organic molecules, as an alternative to fossil fuels, have been utilized as natural sources of biofuels. Affiliated experimental work indicates stereochemistry shows to produce different power densities in organic molecules such as Ascorbic acid and Propylene Glycol. In this paper we investigate the effects of stereochemistry and chirality in heterogeneous catalysis between Pd(111), both enantiomers of 1,2Propylene Glycol (12PG), and 1,3Propylene Glycol (13PG). The isomers are placed in different configurations to analyze the preferred stable orientation and position motifs. Density Functional Theory (DFT) calculations are utilized to optimize the geometries. Both 12PG favored the flat motif described in our experiment. Both leading 12PG and 13PG showed the molecules prefer to have an hydroxy group interacting closer to the slab (center carbon hydroxy group for 12PG) oriented away towards the other group, while the other hydroxy group was father away and oriented towards the slab. Our current results, along with the experimental data, will discuss the most favored binding energies and hydroxy group orientations associated with the motifs found in order to elucidate the stereochemistry effects in fuel cell catalysis.   

Self-formed of 3D Structure on the Edges of 2D Ruddlesden-Popper Perovskites

The observation of low energy edge photoluminescence and its beneficial effect on the solar cell efficiency of RP perovskites have unleashed an intensive research effort to reveal its origin. While, a reliable and consistent explanation is still missing and the underlying material structure on the edges has still not been identified. Using 2D (BA)₂(MA)₂Pb₃Br₁₀ as an example, we show that 3D MAPbBr₃ is formed on the edge due to the loss of BA. This self-formed MAPbBr₃ can explain the reported edge emission, while the reported intriguing optoelectronic properties such as fast exciton trapping from the interior 2D perovskite, rapid exciton dissociation and long carrier lifetime can be understood via the self-formed 2D/3D lateral perovskite heterostructure. The 3D perovskite is identified by the emergence of XRD signature from freezer-milled nanometer-sized 2D perovskite, submicron infrared spectroscopy and its photoluminescence response to external hydrostatic pressure. The revelation of this edge emission mystery and the identification of a self-formed 2D/3D heterostructure provide a new approach for developing the new optoelectronic devices based on the 2D RP hybrid perovskites.   

DFT Studies of the Quinone Electron Acceptor of Photosystem II and Related Model Systems

In photosynthetic and respiratory protein complexes, quinone cofactors participate in both proton and electron transfer. In Photosystem I (PSI), phylloquinones function to transfer electrons through the electron transport chain. In Photosystem II (PSII) and the bacterial reaction center, benzoquinones are used to facilitate both electron and proton coupled electron transfer (PCET). Previous experimental studies determined the redox potentials, solvent interactions and magnetic parameters for a variety of model quinones as well as the native plastoquinone cofactor of PSII (1-3). In this study, we performed density functional theory (DFT) on the reduced semiquinone state of the models in protic and aprotic solvents to determine the electronic structure, energy levels and magnetic parameters. We also created a working model of the plastoquinone pocket of PSII. The DFT calculations were optimized by using a variety of functionals and basis sets and the calculations were validated by comparison with the experimental electro-chemical properties.
1. Weyers et al., 2009, JPC B, 113, 15409.
2. Chatterjee et al., 2012, JPC B, 116, 676.
3. Chatterjee et al., 2011, Biochemistry, 50, 491.   

Computational Characterization of Magneto-Plasmonic Nanowires

Magneto-plasmonic systems constitute an unconventional route for all-optical manipulation of magnetic properties and has strong potential for developing applications in technologies such as high-density data storage and memory devices. Magnetic elements have poor optical properties with very damped localized surface plasmon resonances (LSPR). In contrast, plasmonic elements have very defined and intense LSPRs but their magnetic properties are very much nonexistent. Combining both materials adds a new dimension of functionality and allows for a system to exhibit magneto-optical properties. From this emerges a new phenomena of a ferroplasmon. Here, we present a computational study of the magneto-optical properties of hyrbid nanowires composed of a noble metal paired with soft ferromagnetic material. Our goal is to theoretically design a complex nanowire that will exhibit strong plasmon-induced surface currents and a strong magnetic dipole moment for the optical control of magnetic properties. We use the finite-difference time-domain method (FDTD) to compute the LSPR, magnetic enhancement, and current density of different cases such as capped nanowires, segmented nanowires, and coaxial nanowires. The various cases are compared to one another in order to find the optimal system.   

Characterizing N2O5 Reactivity at Aerosol Interfaces Using Water Cluster Model Systems

The heterogenous reactions of dinitrogen pentoxide (N₂O₅) occurring at the air-water interface of sea spray aerosol (SSA) play a key role in the regulation of gas-phase oxidants and greenhouse gases in the atmosphere. Despite the importance of such reactions, existing atmospheric models consistently fail to reproduce the highly variable rates and product yields observed in ambient SSA, highlighting the disconnect between current predictive parameterizations and the processes they describe. Here, we characterize interfacial reactivity at the molecular level by implementing a custom-designed dual ion trap mass spectrometer to investigate the reactions between size-selected I^-(D₂O)_n=0-10 cluster model systems and N₂O₅ at well-defined temperatures and collision energies. In doing so, we reveal the chemical speciation of N₂O₅ as a function of cluster size and clarify the role of humidity in the chemical ionization detection method currently used for its field observation.   

Seeding studies on the nucleation of NaCl in supersaturated solutions

It is known that the simulation of rare events, such as the appearance of a nuclei in the heart of a liquid, usually requires special methods that artificially accelerate the crystallization without altering the real nucleation mechanism. Nevertheless, these methods are generally computationally expensive. In the seeding method a seed of the crystal phase is inserted into the systems in order to determine the conditions at which the cluster is critical. Hence, this technique allows to estimate the sizes of the critical cluster in a relatively simple and efficient way. Moreover, when used along the classical nucleation theory and an educated choice of the order parameter, employed to distinguish fluid and solid particles, it can provide good estimates of the nucleation rate at relatively low computational cost.

In this work we used the JC/SPC/E force field to study sodium chloride nucleation from supersaturated aqueous solution, at 1 bar and 298.15 K. It will be shown that nucleation rates obtained from the seeding technique along with the mislabeling criterium, agree reasonably well with those obtained from more rigorous techniques as forward flux sampling.   

Monitoring Protonation and migration with UV Spectroscopy – Looking at Solvation effect of the mobile proton on 4-aminobenzoic acid in water clustersMonitoring Protonation and migration with UV Spectroscopy – Looking at Solvation effect of the mobile proton on 4-aminobenzoic acid in water clusters

We utilize cryogenic ion mass spectrometry along with UV action spectroscopy to monitor the effect of protonation on the electronic states of the molecule 4-aminobenzoic acid (4-ABA), both free in the gas phase and within small water clusters. Previous research has concluded that the proton would prefer the so called “O-protomer”, with the condensed phase showing heavy implications of a preferred “N-protomer”, both being determined by ultraviolet spectroscopy.¹ We employ this spectroscopy of the electronic transitions of the N and O protomer to observe the effect of hydration on the charge localization and the electronic structure. Hydration shows a large blue shift in the main identifying feature of the O-Protomer, starting around 340nm for bare 4-ABA and showing stability around 300nm at 4-(ABA)(H₂O)₅. When the 6^th water is added, the cross section of the absorption is dramatically reduced and is further blue shifted than before, possibly indicating a change in the protonation site.

Phys. Chem. Chem. Phys., 2017,19, 17434-17440   

Making third order Møller-Plesset perturbation theory useful: The role of DFT orbitals.

Møller-Plesset (MP) perturbation theory is the simplest route for adding dynamical correlation to mean-field Hartree-Fock (HF). However, the practical utility of MP theory is severely constrained by limitations of HF orbitals. In this work we examine whether density functional theory (DFT) optimized orbitals can be employed to improve the performance of MP theory at both the second (MP2) and third (MP3) order. We find that use of DFT orbitals leads to significantly improved performance for prediction of thermochemistry, barrier heights, non-covalent interactions, and dipole moments relative to standard HF based MP theory. Indeed MP3 (with or without scaling) with DFT orbitals is found to surpass the accuracy of coupled cluster singles and doubles (CCSD) for several datasets. We also find that the results are essentially functional agnostic in most cases, (although range-separated hybrid functionals with low delocalization error are somewhat more reliable on the whole). MP3 based on DFT orbitals thus appears to be an efficient, non-iterative O(N⁶) scaling wave function approach for single-reference electronic structure computations, indicating substantial promise for double hybrid DFT methods with MP3 correlation.   

Coherent Coupling Between Surface Plasmon Polaritons and the Mesoscopic Moments of Single Quantum Dots

In this work, we study the mechanism of the coherent coupling of the surface plasmon polaritons (SPPs) field and the mesoscopic moment of an epitaxially grown semiconductor quantum dot. Specifically, we studied the polarization dependent excitation of semiconductor InGaAs quantum dots (QDs) with a nearby silver plasmonic structure in the weak coupling regime. We have observed a reduction of photoluminescence (PL) signal from QDs near a silver plasmonic structure when the polarization of the excitation light satisfies the launching of an SPP field. This behavior is absent from QDs away from the silver structure. We have also observed signature of energy shift of the QD emissions in the presence of an SPP field indicative of a possible modification of the QD’s confinement potential by the SPP field. Both observations can lead to the development of possible means of dynamic tuning of QDs in applications towards quantum information processing and photonic switching.   

Density Functional Theory Investigations of Change-Transfer Cofactors in Photosynthesis

Energy demands continue to grow as we deplete our fossil fuel resources and damage the global environment. The world is in need of affordable and efficient renewable energy, so we turn to nature to provide a blueprint. Photosynthesis performs highly efficient solar energy conversion and our goal is to better understand and replicate its design principles. Quinones play an important role in charge-transfer during photosynthesis. The two quinones in photosystem II (PSII), the primary and secondary plastoquinone, Q_A and Q_B, have identical chemical structures but perform different functions, namely, Q_A participates in electron transfer reactions while Q_B conducts proton-coupled electron transfer reactions. In this study, we used density functional theory (DFT) to investigate the electronic structure, solvent effects, energy levels and magnetic properties of the primary quinone of PSII and related quinone models. We validated our DFT calculations by comparing the calculated and experimental magnetic parameters (1-3).
1. Weyers et al., 2009, JPC B, 113, 15409.
2. Chatterjee et al., 2012, JPC B, 116, 676.
3. Chatterjee et al., 2011, Biochemistry, 50, 491.   

Spectrally Selective Imaging of Phase Separations in Multicomponent Polymer Films

Understanding the kinetics of phase separations in multicomponent polymer systems has a variety of applications ranging from molecular biology to material engineering. Polymer blend films provide a convenient way to model these processes in a quasi-2-dimensional system, allowing for use of imaging techniques focused on a single plane. The binary case of two polymers or polymers in a solvent has been explored in previous literature, but common experimental methods of observing phase separations in real time pose challenges under the addition of more components. Here we present a method of tracking phase length scales in real time using spectrally selective imaging to single out components of multicomponent films undergoing phase separations.   

Identification of "materials genes" governing heterogeneous catalysis using "clean experiments" and artificial intelligence

Heterogeneous catalysis is an example for a complex materials function governed by the intricate interplay of several processes such as reaction networks and catalyst dynamics. We argue that it is impractical, if not impossible, to explicitly model (e.g. via atomistic simulations) the full catalytic progression under realistic conditions. Instead, we show how artificial intelligence can determine the key microscopic materials parameters, the "materials genes", that govern (actuate, facilitate or hamper) the catalytic performance. We start from a consistent data set obtained from well-documented "clean" experiments [1] containing 10 fully-characterized vanadium-based oxidation catalysts and apply the (multi-task) SISSO symbolic-regression approach [2,3] for the identification of the materials genes responsible for the measured reactivity. The identified parameters not only provide insights on the underlying processes but also guide the choice of new materials to be investigated.
[1] A. Trunschke, et al., Top. Catal., DOI:10.1007/s11244-020-01380-2 (2020). [2] R. Ouyang et al., Phys. Rev. Mater. 2, 083802 (2018). [3] R. Ouyang et al., J. Phys. Mater. 2, 024002 (2019).   

Dipolon Theory of Muon Spin Relaxation Rate in High Temperature Superconductors

We have utilized the dipolon theory [1-4] to explain the muon spin relaxation rate (MSRR) in $YBa_2Cu_3O_{7-\delta}$. Earlier, we predicted first of all two new high energy kinks which were observed later on. Following the dipolon theory we use the four momenta space diagrams in the dressed particle picture for scattering of quasiparticles (QPs) by dipolons involving the QP Green's function, the dipolon propagator and screened electron-dipolon vertex to derive the self-energy $\Sigma^{e-dip}(p)$ and the mean lifetime of scattering of QPs by dipolons required to calculate the penetration depth $\lambda$ \ and MSRR. We also incorporate scatterings by vacancies and phonons. The calculated values [5] of MSRR as a function of $T_c$ agree very well with experiments.
(1) R. R. Sharma, Phy. Rev. {\bf B 63}, 054506 (2001).
(2) R. R. Sharma, Physica {\bf C 439}, 47 (2006).
(3) R. R. Sharma, Physica {\bf C 468}, 190 (2008).
(4) R. R. Sharma, "Dipolon Theory of Kink Structure ...", in "Superconducting ...", Ed. K. N. Courtlandt,
P. 81-100, Nova Sc, Pub., Inc., New York, 2009.
(5) R. R. Sharma, "Theory of Dipolon ...", Physipoetics Press,Lake Barrington, IL, 2021.   

Investigating Heritage Old Copper Culture using X-ray Techniques

Lake Superior is home to one of the largest natural copper deposits in the world. Copper remains an important part of heritage and culture for Native Peoples from this region. Raw copper harvested from present-day Michigan was originally worked by hand, through repeated annealing (heating over fire) and mechanical working (forming using stone tools). By recreating these thermal and mechanical processes on copper deposits, we seek to characterize copper specimens from the Old Copper Culture collection based on origin and refinement techniques, as well as answer questions about early technological advances in metals processing such as smelting, using non-destructive x-ray analysis methods.   

Shape-driven conductivity in graphene

A graphene surface is presented which has a non-vanishing density of states at the Fermi level. This is driven by defects in the sheet structure: these defects are solely odd-membered rings, forced upon the system by topological constraints due to its shape, which is a series of cones and saddles. These defects sufficiently distort the electronic structure of pure graphene such that the density of states at the Fermi level is significant. The system, physically, is globally flat--there is no net gaussian curvature--and the density of states is calculated via first principles. The system further possesses multiple stable surface configurations accessible via mechanical inversion of the cones, each with different densities of states.