Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session N09: Emerging Trends in Soft Microscale Mechanics II |
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Sponsoring Units: DSOFT Chair: Megan Valentine, University of California, Santa Barbara Room: Room 132 |
Wednesday, March 8, 2023 11:30AM - 11:42AM |
N09.00001: On the shape of smectic droplets Tim Atherton Smectic liquid crystals are fluids with periodic layered order, which geometrically proves extremely restrictive: Only a few families of surface are compatible with a constant layer spacing. Here we investigate the shape of a smectic droplet---or tactoid---with deformable boundaries, whereby the smectic order competes with the surface tension and anchoring to give a rich variety of morphologies. Using *Morpho*, a programmable environment for shape optimization, we explore the range of possible morphologies as a function of system parameters and identify possible transitions between minimizers. Prospects for simulating tactoids with other exotic liquid crystals are also discussed. |
Wednesday, March 8, 2023 11:42AM - 11:54AM |
N09.00002: Designing three-dimensional disclination line architecture in nematic liquid crystals Alvin Modin, Biswarup Ash, Robert L Leheny, Hillel Aharoni, Francesca Serra Versatile control over liquid crystal alignment is key to the development of novel electro-optical devices. We use photo-alignment to create disclination lines with controlled architecture in nematic liquid crystals. By imposing patterns of topological surface defects at opposing parallel glass substrates, we generate three-dimensional configurations of disclination lines with pre-designed and non-trivial morphology that result from an energetic competition between defect line tension and elastic distortions. The disclination lines' shape can be further tuned by varying the temperature. This capability to precisely synthesize 3D disclination achitecture offers a promising step in constructing and manipulating optical metamaterials. |
Wednesday, March 8, 2023 11:54AM - 12:06PM Author not Attending |
N09.00003: Algorithmic approach to topological classification and visualization of disclination loops in 3D nematic datasets Daniel A Beller While topological defects in two-dimensional nematic liquid crystals carry topological charges that add upon combining, the topological rules of three-dimensional nematics are more subtle. For example, closed-loop disclinations come in four topological varieties, one of which is equivalent to a unit-charge hedgehog point-defect. This fact is important for understanding disordered nematic configurations, such as defect coarsening in isotropic-nematic quenches and the chaotic steady state of 3D active nematics. However, it can be challenging to calculate these classifications when a simulated or experimental dataset contains many disclination loops, possibly linked. Here I present an algorithmic approach to topological classification of disclination loops, together with an implementation in an open-source visualization software package adapted for nematic director or Q-tensor datasets. The approach takes advantage of recent theoretical advances in calculating the "rotation vector," which describes a disclination locally, from nematic orientation data. I will also demonstrate applications to understanding disclination reconnection events. |
Wednesday, March 8, 2023 12:06PM - 12:18PM |
N09.00004: Effect of elastic anisotropy on dynamics of pair-annihilating disclination lines in nematic liquid crystals Jorge Vinals, Lucas J Myers Current experiments in both lyotropic chromonics and mixtures of actin filaments involve highly anisotropic elasticity, with elastic constants differing by orders of magnitude. To understand the motion of disclinations in these elastically anisotropic media we use a Ball and Majumdar singular potential Q-tensor theory to numerically simulate the pair-annihilation of disclination lines. We provide an approximate scaling law for the coalescence time of the annihilation. To verify these numerical results we use a perturbative expansion in the anisotropy parameter to find the director field around a disclination in the presence of another disclination. Using the singularity tracking method proposed by Schimming we find approximate analytic solutions of disclination separation as a function of time during pair-annihilation. |
Wednesday, March 8, 2023 12:18PM - 12:30PM |
N09.00005: Effect of charge patterning and hydrophobicity in microrheological properties of peptide-based complex coacervates Pankaj Kumar Pandey Liquid-liquid phase separation is a developing field for understanding intracellular structure, the genesis of life, and disease. The development of new liquid-based organelles and therapeutics may be amplified by addressing the design principles behind the phase separation, but an understanding of how monomer sequence and hydrophobicity affect the physical properties of the material, and consequently, strategies to influence the phase dynamics are inadequate. To understand charge-driven phase separation between polypeptides, microrheology is used to find that the change in viscosity of coacervates. Charge patterning and hydrophobicity are predicted to have a significant impact on viscosity. Overall, these observations offer a fresh mechanism for comprehending the sequence-level components that influence the coacervate dynamics. |
Wednesday, March 8, 2023 12:30PM - 12:42PM |
N09.00006: Active mixing with Quincke rollers Cécile Clavaud, Isaac Lenton, Scott Waitukaitis The mixing of fluids at the microscopic scale is a notoriously complicated problem due to flow reversibility at low Reynolds numbers. Different approaches have been used in the past, such as micromixers shaped in specific ways to induce chaotic flow curves, or passive particles under shear. |
Wednesday, March 8, 2023 12:42PM - 12:54PM |
N09.00007: Stress-Strain relaxation in regions surrounding an oil droplet in a Drying Colloidal Suspension H Daniel Ou-Yang, Zhiyu Jiang, Megan T Valentine Colloidal dispersions of submicron particles pack and solidify upon drying, however, how stresses are relaxed in regions surrounding embedded oil droplets in an aqueous suspension of colloidal nanoparticles is unknown. This study uses time-lapse confocal fluorescence microscopy to measure drying-induced movements of micron-sized fluorescent tracer particles and deformation of oil droplets in a linear microfluidic channel. Tracer particles were used to map the displacement fields from which the stress-strain fields of the nanoparticles surrounding the oil droplet. The shape of the oil droplets, initially spherical, deform into oblate spheroidal in the concentration gradient of the colloidal particles surrounding them. At long times, when the colloidal particles approach close-packing, the oil oblates lost front-back symmetry – with higher curvature in the more packed front – suggesting an intriguing interpretation that the pressure in the colloidal particle domain is lower at the oil-water interface in the front side facing downstream than the upstream side. The time-dependent stress-strain fields are used to analyze the stress relaxation by considering the force balance between the hydrostatic pressure, colloidal osmotic pressure, interfacial tension of the oil droplets. Comparison of experimental data with diffusion equation-based numerical simulation provides an additional angle to understand the stress relaxation of colloidal nanoparticles surrounding deformable obstacles. |
Wednesday, March 8, 2023 12:54PM - 1:06PM |
N09.00008: Using Mechanophores to Study the Mechanical Behavior of Polymers under High-Strain-Rate Impact Polette Centellas, Edwin P Chan, Katherine M Evans, Christopher L Soles, Sarah E Morgan, Yoan Simon, Kyle Mehringer The recent advances in microballistic impact testing have enabled the study of the high-strain-rate mechanical response of materials on the microscale. However, these approaches provide limited insight into the mechanical behavior of the material as they cannot experimentally measure in-situ stress nor deformation evolution. To address this limitation, we study the high-strain-rate impact behavior of an anthracene-based mechanophore block copolymer material system using microprojectile rebound experiments. The mechanophores exhibit a fluorescence signal that is commensurate with the applied stress and can thereby act as molecular stress sensors in the test specimen. Importantly, these materials have the requisite temporal and spatial kinetics to enable the measurement of stress evolution during a short-duration (~200 ns), small-scale (~20 μm) impact event with impact velocities ranging from 50 to 500 m/s. Using laser scanning confocal microscopy, we show that the fluorescence information from the impacted sites can be used to quantify the local stresses to help provide insight into the energy absorption mechanisms as a function of impact velocities. This study demonstrates mechanophores as a tool for quantifying impact stresses that can help guide materials design for impact mitigation applications such as armor and spacecraft protection. |
Wednesday, March 8, 2023 1:06PM - 1:18PM |
N09.00009: Active hydrodynamic theory of chromatin dynamics Alex S Rautu, Michael J Shelley, David Saintillan, Alexandra Zidovska The organization of chromatin inside the cell nucleus is crucial for the proper functioning of many nuclear processes. In differentiated cells, chromatin is spatially segregated into euchromatin and heterochromatin. The former is loosely packed and transcriptionally active, while the latter is compacted and mostly consists of transcriptionally silent genes. We describe a hydrodynamic model of chromatin and nucleoplasm at micron scales. The chromatin is modeled as a viscous, compressible fluid, as informed by microrheology experiments and their response at long time. Heterochromatin is distinguished by the presence of contractile stresses due to HP1 crosslinking. This stress induces density instabilities and large-scale flows of the chromatin fluid and nucleoplasm. Simulations reveal coarsening of heterochromatic components, which in an open system lead to a finite-size droplet, whereas in a confining domain we observe a redistribution at the boundaries, resembling a wetting phenomenon. Hence these mechanical processes may play an important role in the spatial organization of heterochromatin which is usually enriched near the nuclear periphery. |
Wednesday, March 8, 2023 1:18PM - 1:30PM |
N09.00010: Jamming Transitions in Confined Driven Polymer in Solutions Setarehalsadat Changizrezaei, Colin Denniston In the present work, we study the compression of a confined polymer in a fluid pushed by a large sphere with a diameter comparable to the channel width through the use of lattice-Boltzmann molecular dynamics (LBMD) simulations. We examined the chain's deformation when there is a) purely repulsive and b) attractive Lennard-Jones (LJ) potential applied between the monomers. The sphere's velocity is varied over 3 orders of magnitude and the chain is in a non-dense state at low sphere's velocities for both repulsive and attractive monomer interactions. At v > v*, the back end of the chain is in a jammed state with high density and low mean square displacement(MSD) values and the front end is in unjammed state with low density and high MSD indicating a pseudo two-state coexistence. This transition is also revealed through the study of volume per monomer and MSD as a function of sphere's velocity. |
Wednesday, March 8, 2023 1:30PM - 1:42PM |
N09.00011: Dynamic Covalent Polymer Microspheres under Supersonic Impact Zhen Sang, Svetlana A Sukhishvili, Edwin L Thomas Polymeric microspheres have recently drawn interest for possible applications in solvent-free cold spray coatings. However, issues of material rebounding, self-adhesion and substrate adhesion remain unsolved. Click chemistry is a powerful platform that uses various building blocks to design dynamic covalent polymer networks with multi-functionalities, including self-healing. It is desirable to understand the responses of dynamic networks to supersonic impact. Here, we fabricate Diels-Alder polymer (DAP) dynamic microspheres composed of furan-attached prepolymers and bismaleimide crosslinkers and explore their in-situ high-strain-rate-induced deformations. The glass transition temperature of the microspheres can be tuned via density of DA bonds, while the solid-to-liquid transition occurs at 120 °C. The angled laser-induced projectile impact test (Θ-LIPIT) is employed to launch DAP-microspheres towards hard substrates at velocities between 150 and 800 m/s and angles from 30 to 90 degrees. The in-situ observation of impact events is carried out by a micrometer- and nanosecond-resolution ultra-high frame rate camera, while low-voltage SEM is used to reveal the deformation morphology. The impact-induced adiabatic heating and stress produced gradients of DA bond densities enabling rigid-to-elastomeric transition observed as elastic response, as well as solid-to-liquid transition observed as radially splashed ejecta of the DAP material. |
Wednesday, March 8, 2023 1:42PM - 1:54PM |
N09.00012: Additively Manufactured Elastomeric Lattices for High Throughput Mechanical Quantification of Thin Glassy Polymeric Films Anesia D Auguste, Kenya Hazell, Daniel Long, Richard A Vaia, Andrew Gillman, Lawrence Drummy Understanding the microscale mechanics within nanoscale thin films is crucial to numerous technologies, ranging from flexible wearable devices to soft robotics to smart separation membranes. However, current techniques used to rapidly measure this behavior are primarily limited to linear and/or uniaxial techniques. In this work, we will discuss a high throughput concept to measure the elastic moduli, plasticity mechanisms and failure strain of thin films. We designed an experimental technique utilizing an additively manufactured compliant elastomeric lattice to replace the traditionally rigid copper grid technique developed by Lauterwasser and Kramer. By varying the geometries of the lattices with differing Poisson ratios, we can transduce the macroscopic, uniform, in-plane deformation into a wide range of local deformation fields at each lattice cell. By placing a thin glassy polymeric film on top of the lattice structure, each cell acts as a unique deformation stage, allowing simultaneous mapping of the yield and fracture envelope. Combining this with modeling, optical imaging and spectroscopy enables statistically robust analysis and understanding of various microscale mechanical parameters in parallel of a number of different material systems. |
Wednesday, March 8, 2023 1:54PM - 2:06PM |
N09.00013: The effect of water uptake on the mechanical behavior of hybrid thin films fabricated by sequential infiltration synthesis (SIS) Shachar Keren, Cynthia Bukowski, Myounguk Kim, Alfred J Crosby, Noy Cohen, Tamar Segal-Peretz Hybrid organic-inorganic materials are an exciting subclass of composites due to their unique structures and properties. Control over their mechanical properties is central to their implementation in various advanced applications. In recent years, sequential infiltration synthesis (SIS) has emerged as a promising new technique for fabricating hybrid materials with nanoscale precision. In SIS, inorganic materials are grown within polymers from vapor phase precursors using atomic layer deposition (ALD) chemistry. Several studies have demonstrated the potential of SIS to tune the mechanical properties of polymers. However, a full understanding of the nanostructure mechanical behavior is still an ongoing effort. |
Wednesday, March 8, 2023 2:06PM - 2:18PM |
N09.00014: Effective sliding friction of lubricated soft patterned surfaces Arash Kargar-Estahbanati, Bhargav Rallabandi Recent work on the wet sliding contact of soft robotic fingers showed that the Stribeck curve (coefficient of friction versus dimensionless sliding speed) for patterned surfaces is non-monotonic (Peng et.al, 2021, Nat. Mater.). The elastohydrodynamic flows underlying this non-trivial behavior are only partly understood. In this work, we develop a modeling framework for the lubricated contact of locally patterned but globally flat surfaces. We solve the resulting system of integrodifferential equations numerically for a wide range of geometrical parameters characterizing the surface patterns. Additionally, we investigate the effects of properties of the entrained fluid layer and soft solid on the Stribeck curve. In the limit of small sliding speeds, when the fluid film is thin, the lubrication flow observes each asperity as a separate degenerate contact. At the other extreme, where the fluid film is thicker than the typical depth of the asperity, the entire patterned surface can be approximated by a nearly flat surface with a small roughness. We find analytic results for the coefficient of friction in both limits using the method of multiple scales, and show them to be in agreement with our numerical predictions. By explaining the quantitative behavior of friction in the contact of soft wet objects, this work can pave the way for encoding friction coefficient into haptic signals in robotic and haptic engineering. |
Wednesday, March 8, 2023 2:18PM - 2:30PM |
N09.00015: Evaluating the Influence of Twisted Structure on Mechanical Properties of Biopolymer Films using LIPIT Rebecca (Sujin) Lee Bio-derived nanomaterials such as cellulose nanocrystals (CNCs) and chitin nanocrystals (ChNCs) garnered considerable interest due to their tunable mechanical and biodegradable properties. These nanoparticles possess a high aspect ratio with rod-like morphology which impart them with the ability to self-organize into a “cholesteric” liquid crystal phase in an aqueous suspension. The twisted structure of the cholesteric phase makes CNCs/ChNCs suitable candidates for the bottom-up fabrication of the “Bouligand Structure” found in natural materials that exhibit high mechanical strength. However, the intrinsic brittleness of CNCs/ChNCs films, unlike the natural Bouligand structure, limits their application scope. Moreover, little is understood about the mechanical properties of the pristine CNCs/ChNCs films. In this contribution, we study the relationship between liquid crystal structure and mechanical properties of CNCs/ChNCs films at the micro and nanoscales. Coefficient of restitution experiments based on laser-induced projectile impact testing (LIPIT) is used to evaluate the effects of pitch and pitch angle of the twist on the mechanical properties of CNCs/ChNCs films. These results provide insights into the role of chirality on the mechanical behavior of these composites and enable the development of advanced composite materials with tunable mechanical properties. |
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