Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session S25: Emerging Trends in Soft Microscale MechanicsFocus Recordings Available
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Sponsoring Units: DSOFT DPOLY Chair: Rae Anderson, University San Diego Room: McCormick Place W-187A |
Thursday, March 17, 2022 8:00AM - 8:36AM |
S25.00001: Filamentous Viruses as Probes of Polymer Solution Structure and Dynamics Invited Speaker: Jacinta C Conrad We examine the diffusive transport of highly anisotropic, filamentous viruses in semidilute solutions of near-neutral and charged polymers, in which length scales and dynamics can be systematically tuned through polymer concentration and solution conditions. Like spherical nanoparticles, the viruses diffuse faster than expected based on the bulk solution viscosity, and the deviation increases with polymer concentration. We show that the dynamics of viruses are affected both by anisotropy, which alters the hydrodynamic coupling to the polymer dynamics along and normal to the virus's long axis, and by flexibility. Further, we show that virus dynamics are sensitive to solution properties arising from electrostatic interactions between charged polymers. Our results provide insight into the transport properties of viruses in complex media ranging from membranes and to biopolymer solutions. |
Thursday, March 17, 2022 8:36AM - 8:48AM |
S25.00002: Microrheology of an active suspension of swimming bacteria Mauricio M Gomez, Hunter A Seyforth, William B Rogers, Jennifer L Ross, Wylie W Ahmed Optical tweezers allow the quantitative study of active biological suspensions, e.g., bacterial colonies, enzyme baths, and microswimmers. These active suspensions have the potential to do productive work on embedded objects and their environment. Here, we study the nonlinear response and force fluctuations of a probe particle suspended in a moderately dense suspension of swimming E.coli (0.2 vol frac). We study three physical processes: (1) the force fluctuations transferred to the probe particle, (2) the friction on the probe particle at varying Pe (a measure of persistent versus random motion), (3) the force relaxation as the particle returns to its equilibrium position. We find that at Pe<<1, the active bath experiences shear-thinning approaching the solvent viscosity but not lower. Between Pe 0.85 and 5.1, the active bath shear thickens, and at Pe > 8.5, the effective viscosity plateaus. These results are supported by recent theoretical predictions of the nonlinear rheology of an isotropic bath, extending experimental evidence to moderate densities. Our results set the stage for understanding the basic properties of force and energy transfer from active suspensions to embedded probe particles. |
Thursday, March 17, 2022 8:48AM - 9:00AM |
S25.00003: Kinetic viscoelasticity during early polymer-polymer spinodal dewetting Jyotsana Lal, Laurence B Lurio, Dennis Liang, Suresh Narayanan, Seth B Darling, Mark D Sutton The dewetting kinetics of a supported polymer bilayer were measured in-situ using coherent |
Thursday, March 17, 2022 9:00AM - 9:12AM |
S25.00004: Mapping nonlinear stress propagation in non-equilibrium complex fluids Karthik Peddireddy, Ryan Clairmont, Philip D Neill, Jonathan Garamella, Ryan McGorty, Rae M Anderson Topologically-novel entangled polymers and enzyme-driven active matter systems are examples of complex fluids that exhibit intriguing spatiotemporally varying responses to strain. Yet, how local nonlinear stresses propagate through these soft systems remains an open question. Here, we combine optical tweezers microrheology (OTM) with differential dynamic microscopy (DDM) to map the time-dependent deformation fields arising from local nonlinear straining in ‘topologically-active’ DNA fluids. Specifically, we measure the stresses imposed in the fluids by local nonlinear strains and simultaneously image labeled DNA molecules surrounding the strain site. Using DDM, we characterize the macromolecular dynamics and deformation as a function of strain rate and distance from the applied strain. We perform these measurements on blends of circular DNA undergoing enzymatically-driven topological conversion and fragmentation. Our coupled OTM-DDM platform directly links nonlinear stress propagation to macromolecular dynamics and network structure and is applicable to a wide range of active and heterogeneous complex fluids. |
Thursday, March 17, 2022 9:12AM - 9:24AM |
S25.00005: DNA dynamics in crowded cell-sized droplets Mehdi Shafiei Aporvari, Steven Dang, Juexin Marfai, Kara Coursey, Ryan McGorty, Rae M Robertson-Anderson Understanding the transport and conformational dynamics of biological macromolecules such as DNA in crowded and confined environments is critical to understanding biological processes such as viral infection, liquid-liquid phase separation, and transcription. Here we investigate the dynamics, conformations, and organization of DNA molecules confined in cell-sized droplets and crowded by dextran polymers. Specifically, we use differential dynamic microscopy to measure ensemble DNA dynamics and single-molecule conformational tracking to measure DNA trajectories, shapes, and localization. We directly connect these measurements to the microrheological properties of the crowded, confined environment by performing similar measurements with microspheres. We determine the impact of droplet size and crowding conditions, as well as DNA length and topology, on the measured transport and structural properties. |
Thursday, March 17, 2022 9:24AM - 9:36AM |
S25.00006: The Active Regulation of Pressure and Volume in Cell Aggregates Michael P Murrell, Vikrant Yadav, Sulaiman Yousafzai, Sorosh Amiri We explore the relationship between the non-equilibrium generation of myosin-induced 'active' stress within the cell cytoskeleton and the pressure-volume relationship of cellular aggregates as models of simple tissues. We find that due to active stress, aggregate surface tension depends upon aggregate size. As a result, pressure and number density depend on size, although as they have the same dependence, the relationship between them resembles an equilibrium equation of state. This suggests that bulk and surface properties of tissues balance during growth to yield a constant average mechanical energy per cell. These results describe basic physical principles that govern the growth and form of simple tissues. |
Thursday, March 17, 2022 9:36AM - 9:48AM |
S25.00007: Active pressure and local entropy production of bacterial suspensions Satyam Anand, Xiaolei Ma, Shuo Guo, Stefano Martiniani, Xiang Cheng The mechanical pressure of active matter cannot be trivially related to other thermodynamic quantities, unlike systems in equilibrium. For systems in a non-equilibrium steady state, entropy production rate (EPR) has recently been studied based on the statistical irreversibility between time-forward and time-reversed trajectories of observed degrees of freedom, both globally and locally. Here, we explore the relation between the local mechanical pressure and local EPR of active bacterial suspensions. By means of experiments and Brownian dynamics simulations, we characterize the correlation between local active pressure and local EPR for swimming bacteria rectified by funnel-shaped geometries. While the active pressure is measured in experiments by tracking the displacement of a tracer particle held by optical tweezers at the tip of the funnel, the local EPR is calculated from the Kullback-Leibler divergence of the discretized sequence of collisions with the tracer particle. We further develop a simple theoretical model, which quantitatively captures the relation between active mechanical pressure and local EPR. Our study reveals the intrinsic relation between active pressure and the time irreversibility of nonequilibrium systems. |
Thursday, March 17, 2022 9:48AM - 10:00AM |
S25.00008: Active microrheology of multi-phase field models of biological tissue Austin Hopkins, Michael Chiang, Benjamin Loewe, Davide Marenduzzo, Cristina Marchetti The rheology of biological tissue plays an important role in many developmental processes, from organ formation to cancer invasion. We use a multi-phase field model of motile cells to simulate microrheology experiments of a tissue monolayer. When unperturbed, the tissue exhibits a transition between a rigid glassy state and a fluid state tuned by cell motility and deformability as measured by the ratio of the costs of steric cell-cell repulsion and cell-edge deformation. We find that solid-like tissues exhibit a finite threshold force for the onset of motion of a probe, corresponding to a finite yield stress for the tissue. We study the dependence of the yield stress on cell motility and deformability and show that it vanishes in the liquid state. The onset of motion is qualitatively different in low and high deformability regimes. At high deformability, the tissue is compliant and adapts to deformations, resulting in a smooth onset of motion. At low deformability, the probe induces both deformations and translations of cells in its immediate neighborhood, and the onset of motion appears discontinuous. Near the onset of motion, cell shapes are radially compressed in front of the probe with a wake of elongated shapes behind it, suggesting that cell shapes play a role similar to that density in systems of rigid particles. |
Thursday, March 17, 2022 10:00AM - 10:12AM |
S25.00009: Sticky Crumpled Matter Andrew B Croll, Wenjie Xia, Wathsala Jayawardana, Theresa M Elder, Yangchao Liao, Zhaofan Li |
Thursday, March 17, 2022 10:12AM - 10:24AM |
S25.00010: Auxetic Bending Response in Crumpled Membranes of Magnetic Nanoparticles Edward P Esposito, Heinrich M Jaeger Soft quasi-2D materials are frequently studied as mechanical systems due to their bending response which can allow substantial elastic deformation. Thin, soft materials wrinkle and crumple easily in practice. And although macroscale crumpled sheets can display an auxetic-like elastic response, such effects have not been well-studied in crumpled nano-films. We will show that crumpled nano-films can have an auxetic bending response. Using freely suspended ~10nm thick membranes made of self-assembled superparamagnetic nanoparticles, we can manipulate thin, crumpled sheets with applied magnetic fields and track the micron-scale deflections in 3D with high-resolution confocal microscopy. By analyzing the curvatures induced by the bending, we demonstrate an auxetic bending response due to the geometric structure of the crumples, in contrast with the standard linear-elastic bending response of uncrumpled films. The ability to design mechanical elements with an auxetic bending response due to crumpling, even with non-auxetic materials, should open new design pathways for mechanical micro-structures. |
Thursday, March 17, 2022 10:24AM - 10:36AM |
S25.00011: Autonomous Oscillations in a Colloidal Fluid Driven by Cyanobacterial Circadian Rhythms gregor leech, Alyxandra Vyn, Michael J Rust, Moumita Das, Jennifer L Ross, Rae M Robertson-Anderson Cyanobacteria rely on a circadian oscillator system of proteins, KaiA, KaiB and KaiC, to regulate a variety of cellular processes. Powered by ATP phosphorylation, KaiA and KaiB proteins alternately bind to KaiC to produce periodic structural changes in KaiC. We repurpose this circadian system to design colloidal fluids with autonomous structural oscillations enabled by rhythmic crosslinking of colloids by KaiC-KaiB complexes. We show that the colloids can oscillate between liquid-like states, with individual colloids diffusing freely, to states in which colloids form large clusters with minimal motion, in sync with the binding cycles of KaiC and KaiB. We use differential dynamic microscopy (DDM) and spatial image autocorrelation analysis (SIA) to characterize the rhythmic dynamics and structure of the colloidal fluids and their dependence on the state of the Kai oscillator. Our findings demonstrate that a cyanobacterial circadian oscillator can drive autonomous alterations in abiotic colloidal materials, with applications in next-generation drug delivery, filtration, and infrastructure repair. Our results further equip researchers with new biochemical machinery for developing self-directed, programmable, and reconfigurable biomaterials. |
Thursday, March 17, 2022 10:36AM - 10:48AM |
S25.00012: Sustained Order-Disorder Transitions in a Model Colloidal Network Crosslinked via Oscillator Proteins Lauren Melcher, Elisabeth Rennert, Jennifer L Ross, Michael J Rust, Rae M Robertson-Anderson, Moumita Das Biological systems have the unique ability to self-organize and generate autonomous motion and work. Motivated by this, we investigate a 2D model colloidal network that can repeatedly transition between disordered states of low connectivity and ordered states of high connectivity when crosslinked by rhythmic oscillators motivated by bacterial proteins that maintain circadian rhythms. We use Langevin dynamics to investigate the time-dependent changes in structure and collective properties of this system as a function of colloidal packing fractions and crosslinker oscillation periods and characterize the degree of order in the system by using network connectivity, bond length distributions, and collective motion. Our simulations suggest the conditions for producing distinct states of this colloidal system with pronounced differences in the degree of microstructural order and desired residence times in the states. We are now extending our model to include filaments in place of disc-shaped colloids to mimic actin filaments and microtubules. Our results will aid in the experimental design of smart active materials that can cycle between ordered and disordered states. |
Thursday, March 17, 2022 10:48AM - 11:00AM |
S25.00013: The origin of multi-periodic cycles in cyclically sheared amorphous solids Asaf Szulc, Ido Regev, Muhittin Mungan Plasticity in amorphous materials, such as glasses, colloids, or granular materials, is mediated by immobile local rearrangements called "soft spots" or "shear transformation zones." Experiments and simulations have shown that soft spots are two-state entities interacting via quadrupole displacement fields generated when they switch states. When a system is subjected to cyclic strain driving, soft spots can return to their original state when the strain direction reverses. In this case, the system may cycle periodically between the same microscopic states, with a periodicity that is a multiple of the period of the external drive. In this work, we focus on cycles that have periodicity larger than the periodicity of the drive. We use particle simulations to create graphs representing the system's different states. We use the graph topology and comparison between the displacement fields generated by the soft spots to identify all the soft spots active during such a cycle. We combine these results with a model of interacting hysterons (an abstraction of two-level systems) and show that multi-periodicity results from periodic stability oscillations of some soft spots around the drive amplitude. We then discuss the conditions and mechanisms which allow these oscillations to occur. |
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