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 B09: Active Materials II |
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Sponsoring Units: DSOFT Chair: Chris Amey, Brandeis University Room: Room 132 |
Monday, March 6, 2023 11:30AM - 11:42AM |
B09.00001: Locomotion of Active Polymerlike Worms in Porous Media Rosa C Sinaasappel, Antoine Deblais There have been many recent advancements in understanding active matter as a framework to study microbial motility. However, most of these studies focus on the motility of rigid and point-like shapes in free environments. In nature, microorganisms tend to be elongated and flexible and live in complex and crowded environments. |
Monday, March 6, 2023 11:42AM - 11:54AM |
B09.00002: Folding patterns in heterogeneous active polymers — implications for genome organization Andriy Goychuk, Deepti Kannan, Arup K Chakraborty, Mehran Kardar The folding of polymers into specific conformations is a recurring theme among living cells. Equilibrium theories have long tackled this problem by invoking chemical affinities, which lead to relatively stable conformations with little thermal fluctuations. However, what are the implications of patterns of active processes, which are known to emerge in living cells or on the genome, for polymer folding? To address this question, we idealize active processes as "athermal excitations": sequence-specific active random forces that can drive coherent polymer motion. Our analysis predicts that a local increase in activity (hence larger active forces) will induce bending and expansion of the polymer backbone, whereas decreased activity leads to straightening and contraction. Moreover, pairs of loci that exhibit correlated active (sub)diffusion will attract through effective long-ranged harmonic interactions, whereas anticorrelations lead to pairwise repulsion. Taken together, these nonequilibrium mechanisms can address a large shape space of steady-state polymer conformations, which show significant population heterogeneity and are only realized on average in the form of contact probabilities. These folding patterns could have important implications for the organization of chromatin—a large polymer consisting of DNA in complex with many proteins that can dissipate energy. |
Monday, March 6, 2023 11:54AM - 12:06PM |
B09.00003: An Investigation of Segmental Dynamics of Poly(pentyl malonate) Electrolytes Neel J Shah, Naresh C Osti, Xiaopeng Yu, Nitash P Balsara In this study we investigated the effect of added salt on the segmental dynamics of a novel polymer electrolyte. Our system of interest is poly(pentyl malonate) (PPM) with added LiTFSI salt. We created a series of PPM/LiTFSI mixtures, with salt concentrations from 0 ≤ r ≤ 0.10 (where r is the ratio of the moles of lithium to moles of oxygen in the polymer backbone) and measured the segmental dynamics using quasielastic neutron scattering (QENS). QENS data can be used to determine segmental dynamics on length scales ranging from 3 to 60 Å and on the nanosecond timescale, well-suited to probing polymer electrolyte dynamics. Assuming the Rouse model, we calculated the monomeric friction coefficient from QENS experiments and found that the friction coefficient increases exponentially with increasing salt concentration. We examine the relationship between the friction coefficient and ion transport in this polymer electrolyte system and compare these results to a previously studied polymer electrolyte system: poly(ethylene oxide) (PEO) with added LITFSI salt. |
Monday, March 6, 2023 12:06PM - 12:18PM |
B09.00004: Interactions of an elastic ring with active nematic fluid Saaransh Singhal, Aparna Baskaran, Thomas G Fai We investigate how active stresses organize in the presence of soft interfaces by simulating a hydrodynamic model of active nematics coupled with a closed extensible elastic ring using immersed boundary methods. In the overdamped limit, the balance of fluid stresses with the elastic stresses allows us to probe the active fluctuations by investigating the dynamics of shape of the elastic ring. We quantify measures of shape fluctuations including local deformation, curvature, and asphericity in the phase space of elastic stiffness and the strength of the active stress. These shape properties provide insights into the physics of the active nematic fluid under deformable confinements. |
Monday, March 6, 2023 12:18PM - 12:30PM |
B09.00005: Behavior of acoustically powered micro-swimmers in a liquid crystale Andrey Sokolov, Jaideep Katuri, Alexey Snezhko Suspensions of microswimmers in liquid crystals demonstrate remarkably complex dynamics and serve as a model system for studying active nematics. So far, experimental realization of microswimmers suspended in liquid crystalline media has relied on biological microorganisms that impose strict limitations on the compatible media and makes it difficult to regulate activity. Here, we demonstrate that acoustically powered bubble microswimmers can efficiently self-propel in a lyotropic liquid crystal. The velocity of the swimmers is controlled by the amplitude of the acoustic field. Histograms of swimming directions with respect to the local nematic field reveal a bimodal distribution: the swimmers tend to either fully align with or swim perpendicular to the director field of the liquid crystal, occasionally switching between these two states. The bubble-induced streaming from a swimmer locally melts the liquid crystal and produces topological defects at the tail of the swimmer. We show that the defect proliferation rate increases with the angle between the swimmer's velocity and the local orientation of the director field. |
Monday, March 6, 2023 12:30PM - 12:42PM |
B09.00006: Symmetry Breaking of Self-Propelling Topological Defects in Thin-Film Active Chiral Nematics Weiqiang WANG, Rui Zhang, Haijie Ren Current interests in active matter are inspired by its relevance to living systems, which often-times consist of mirror symmetry breaking molecules and structures. As a paradigmatic active matter system, active nematic liquid crystals have shown interesting dynamical phenomena. For example, it is found that +1/2 defects can self-propel along their symmetry axis in a two-dimensional active nematic. In this work, we combine continuum theory and hydrodynamic simulation to consider the self-propulsion dynamics of active +1/2 disclinations in a thin-film chiral nematic. In a flat film, these active defects are found move slower if the chirality is higher. Importantly, we further predict that +1/2 disclinations can exhibit chiral locomotion in a curved film. When these +1/2 disclinations interact, the symmetry breaking dynamics can give rise to rich and novel collective self-propulsion patterns. We explain our findings by considering the coupling between the active stress and the symmetry-breaking structure of the system. Finally, we discuss the experimental setting that could verify our predictions. As such, our work reveals an emergent symmetry breaking mechanism in active matter. |
Monday, March 6, 2023 12:42PM - 12:54PM |
B09.00007: Probing 3D active system with dynamic phase separation Liang Zhao, Itamar Kolvin, Raymond Adkins, Zvonimir Dogic We study 3D active fluids which merge passive liquid-liquid phase separation with a microtubule-based active fluid. Phase separation breaks the spatial symmetry of a homogeneous active material by confining the activity to one of the two coexisting phases. The resulting mixture exhibits an interplay between the activities of the microtubules in the bulk and the fluctuations of the soft and easily-deformable phase interfaces. To determine the influence of dimensionalities we study the structure and dynamics of bulk-separated interfaces in three-dimensional samples, as well as the shape of 3D droplets on a 2D substrate. Our work demonstrates how active stresses generated in the bulk internally driven fluids can generate interfacial structures that are not accessible in equilibrium systems. In turn, the soft deformable interfaces can be used as elastic probes to measure active stress which is a defining property of active fluids. |
Monday, March 6, 2023 12:54PM - 1:06PM |
B09.00008: Tuning colloidal gel mechanics by active doping. (Edmond) Tingtao Zhou, John F Brady The mechanical behaviors of disordered colloidal gels are intimately connected with their complex energy landscape and inherent structures. By embedding a small fraction of active particles into the gel, we demonstrate the ability to quickly and precisely control the gel pore structure and mechanics. Using Brownian dynamics simulations, we show that varying the dynamical properties of the active particles can fine-tune the pore-size distribution and gel connectivity, as well as the mechanical response under external loading. More dramatic changes of the gel ductility and local crystalline order can be induced by a high-activity (swim speed) treatment for a finite time period. The non-equilibrium energy source from the active particles assists the gel in overcoming kinetic barriers without thermal annealing. Our results pave the way to design principles for adaptive colloidal gels. |
Monday, March 6, 2023 1:06PM - 1:18PM |
B09.00009: X-ray Photon Correlation Spectroscopy of Cylindrical and Lamellar Block Copolymers of Controlled Grain Size. Omar Taleb, Patrick Blatt, Kyoungmin Kim, Onyekachi Oparaji, Suresh Narayanan, Qingteng Zhang, Daniel T Hallinan This work studied structural dynamics of block copolymers as a function of grain size and morphology. The block copolymers studied were cylindrical and lamellar polystyrene-b-poly(ethylene oxide) (PS-b-PEO). This examination is the first of structural dynamics for a cylindrical block polymer using X-ray photon correlation spectroscopy (XPCS). The PS-b-PEO grain size was controlled by solvent vapor annealing. Morphologies and grain sizes were evaluated using small-angle x-ray scattering (SAXS) and atomic force microscopy (AFM). The results of SAXS and AFM showed that the grain size variation depends on which homopolymer formed the matrix majority. Grain size of a PS matrix majority PS-b-PEO increased with high solvent evaporation rate. By contrast, the PEO majority matrix grain size decreased with increasing solvent evaporation rate. Autocorrelation function g2, structural relaxation time τst, and stretching exponent β from XPCS measurements were evaluated as a function of morphologies and grain sizes. |
Monday, March 6, 2023 1:18PM - 1:30PM |
B09.00010: Be the flow or go with it: Dynamics of individual bacteria in bacterial turbulence Dipanjan Ghosh, Paxson J Picken, Xiang Cheng Swimming bacteria suspended in fluids at high densities display mesmerizing self-organized flows known as “bacterial turbulence.” While driving the turbulent flow, individual bacteria within bacterial turbulence display complex dynamics, which remain largely unexplored. Here, we image the motion of fluorescently-tagged E. coli cells in bacterial turbulence at different bacterial densities and compare the dynamics of actively swimming bacteria and immobile cells. At high densities, strong advective flows transport active and immobile bacteria at the same speed. However, for weaker turbulent flows at lower densities, the self-propulsion of an active bacterium competes with the background advection. This competition leads to a diversity of bacterial dynamics, ranging from being purely advected by the flow to actively swimming faster than the flow. By simultaneously imaging bacterial turbulence using bright-field and fluorescence microscopy, we quantify the coupling between the motion of an active bacterium and the background turbulent flow and identify the unusual interplay between active swimming and local advection. Taken together, our study reveals the transport of individual bacteria in dense suspensions and sheds light on the microscopic origin of bacterial turbulence. |
Monday, March 6, 2023 1:30PM - 1:42PM |
B09.00011: Repeatable Power Amplified Actuation of Confined Polymer Gels Nolan Miller Many organisms have evolved the ability to rapidly release potential energy as a tool for survival using Latch Mediated Spring Actuation (LaMSA). In these systems, latches mediate the transition from stable to unstable energy release and play a critical role in integrating the actuator and spring capabilities. One type of latch that is used in natural systems is the snap-through elastic instabilities. Recent work has demonstrated that synthetic gels, swollen with volatile solvents, can be engineered to undergo multiple snap-through events, providing a pathway for energy-efficient, autonomous, high-power movements. However, an energy relationship between snapping, diffusion, and buckling has yet to be defined; this interaction is driven by the interplay between the solvent-infused gel and the surrounding environment. Our approach utilizes the snapping physics of ribbon-shaped gels to predict the number of snaps and to quantitatively describe how geometry, material properties, and mass transport influence snapping behavior. Initial results from this current work have identified relationships between lateral confinement, ribbon modulus, and snapping kinetics that describe trends and limitations of an energy model. Expanding the understanding of this autonomous system will elucidate power amplification using an environmental driving force. |
Monday, March 6, 2023 1:42PM - 1:54PM |
B09.00012: Transition to Freedericksz-type Spontaneous Flow and Spatiotemporal Chaos in Three-Dimensional Active Fluids Abhinav Singh, Philipp Suhrcke, Quentin Vagne, Frank Jülicher, Ivo F Sbalzarini Active fluids display rich behavior from spontaneous flow to turbulent mixing at low Reynolds number. Recent experiments suggest fundamental differences in spontaneous symmetry breaking of a 3D extensile active liquid crystal in comparison to the two dimensional counterpart. We investigate the role of confining boundary conditions on the active length scale of a three-dimensional active polar fluid. Using perturbation theory and linear stability analysis, we find that the nature of the spontaneous flow transition depends on the polarity boundary conditions and on the sign (contractile vs. extensile) of the active stress. We show that perpendicular anchoring of polarity at the confining walls only allows for extensile spontaneous flow, whereas parallel anchoring leads to in-plane spontaneous flow for contractile stress and out-of-plane wrinkling for extensile stress. We confirm these theoretical predictions in direct numerical solutions of the nonlinear 3D active Ericksen-Leslie hydrodynamic model. We further show transitions to traveling waves and spatiotemporal chaos when active stress increases beyond the spontaneous flow regime, confirming the existence of active turbulence in 3D. |
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