Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H61: Active Matter IVFocus
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Sponsoring Units: GSOFT DBIO GSNP Chair: Daniel Pearce, University of Geneva Room: BCEC 258B |
Tuesday, March 5, 2019 2:30PM - 3:06PM |
H61.00001: Dynamic instabilities of contractile acto-myosin rings Invited Speaker: Karsten Kruse The actin cytoskeleton is a prototypic example of active matter and shows a wealth of dynamic phenomena alien to conventionally studied materials. In living cells, actin filaments and myosin motors often organize into dynamic rings. This is notably the case during the late stages of cell division of animal cells, where such a ring contracts and thereby cleaves the mother cell into two daughters. Similar structures are observed in dividing fission yeast, where their functional role is less clear, though. Furthermore, rings spanning several cells appear upon closure of tissues. It has been found experimentally that myosin forms stationary or moving clusters in acto-myosin rings. The mechanism underlying their formation and their role in ring dynamics remain unclear. Based on the analysis of a phenomenological theory, we show that traveling and stationary clusters are generic features of contractile actin rings. Our numerical results indicate that there is a direct transition from homogenous actin and myosin densities to chaotic dynamics along rings. We then use a mesocopic approach to study a possible molecular mechanism for the observed generic phemomena. It suggests that the transient formation of bipolar filaments provides a common mechanism underlying the experiementally observed patterns. The theory also shows that stationary myosin clusters are associated with elevated contractile stress, whereas traveling clusters are not. We compare these theoretical results to experimental observations in mammalian and fission yeast cells. |
Tuesday, March 5, 2019 3:06PM - 3:18PM |
H61.00002: Tuning surface aggregation of active particles. Raghunath Chelakkot, Suchismita Das Collection of disk-like self-propelled particles, interacting via a short-range repulsive force is an effective model to understand the emergent properties in active matter. It is well known that for sufficiently high propulsion speed and density they aggregate to form clusters and at low density, they are in a homogeneous fluid state. |
Tuesday, March 5, 2019 3:18PM - 3:30PM |
H61.00003: Response of a system of passive or active particles to periodic forcing Michael Wang, Alexander Grosberg The presence of active, persistent forces in various biological and artificial systems changes how those systems behave when forced. We present a minimal model of a system of passive or active swimmers driven on the boundaries by a time-dependent forcing. We extract the linear response functions of the system and interpret them in terms of the storage and dissipation of energy through the particles within the system. We find that while a slowly driven active system responds similar to a passive system with a redefined diffusion constant, a rapidly driven active system exhibits a new behavior that may be related to a change in the motoring activity of the active particles due to the forcing. |
Tuesday, March 5, 2019 3:30PM - 3:42PM |
H61.00004: Diffusion of active matter with inertia Mario Sandoval-Espinoza, John F Brady In this work we study the motion of active Brownian particles (ABPs) while keeping both translational and rotational inertia. Following a Langevin formalism, we theoretically find that whereas translational inertia does not play a role in the ABPs effective diffusion, rotational inertia is able to enhance the ABPs’ diffusion. To elucidate such a new effect, one has to properly take into account the rotational inertia contribution in the orientation correlations. The mean-square speed for this system is also studied and its dependence on translational and rotational inertias is determined theoretically. To validate our analytical results, Brownian dynamics simulations are performed. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H61.00005: Critical motility-induced phase separation belongs to the Ising universality class Benjamin Partridge, Chiu Fan Lee Active matter is an extreme kind of non-equilibrium system in that detailed balance is broken at the microscopic scale. A typical active system can be a collection of particles that continuously exert mechanical forces on their surrounding environment, and systems of interacting active particles can display novel phenomena, ranging from the emergence of collective motion in two dimensions when the active particles are aligning, to motility-induced phase separation (MIPS) when the particles interact solely via volume exclusion interactions. However, even though active matter breaks detailed balance in a fundamental way, it remains unclear whether the hydrodynamic, universal behavior of active matter necessarily differs from that of equilibrium systems. The investigation of universal behavior, besides being of central interest to physics, allows us to transfer knowledge of a well-known system to a different system of novel interest. Here, we do exactly that by demonstrating that the critical behaviour of MIPS belongs to the Ising universality class with conservative dynamics. We do so using three approaches: hydrodynamic argument, field-theoretic description of a microscopic model, and simulation of a lattice model. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H61.00006: Topotaxis of Active Particles Koen Schakenraad, Linda Ravazzano, Joeri Wondergem, Roeland Merks, Luca Giomi Recent biophysical experiments have shown that the amoeba Dictyostelium Discoideum, while moving persistently in an environment filled with obstacles, can navigate away from more crowded areas towards regions with more space: an effect called topotaxis. We theoretically study active particles in a crowded environment and discuss whether or not persistent motion on itself, in the absence of any more complicated biological interactions, can drive this behavior. |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H61.00007: The Effect of Confinement on Active Brownian Particles Camilla M. Kjeldbjerg, John F Brady Active Brownian Particles (ABPs) are subject to confinement effects when their run or persistence length, l= U0τR, is comparable to the characteristic size of the confining geometry. Here, U0 is the intrinsic swim speed and τR is the reorientation time of the ABPs. Furthermore, ABPs accumulate at no-flux surfaces owing to their persistent swimming. These two effects can produce some unusual and startling effects. For example, it has been seen in simulations (Ray, et al. PRE 2014) that two parallel walls attract each other when placed in a dilute bath of ABPs. In this work, we provide a simple model based on the Smoluchowski equation for the ABPs and an overall macroscopic momentum balance to predict analytically this attractive force. We extend this simple modeling to predict the partitioning of ABPs between a confined channel of width H and an infinite reservoir, showing that the concentration within the channel over that in the bulk increases as l/H. The theoretical results are compared to Brownian dynamics simulations. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H61.00008: WITHDRAWN ABSTRACT
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Tuesday, March 5, 2019 4:30PM - 4:42PM |
H61.00009: Spatial Variation of Transport Properties of Active Brownian Particles Hyeongjoo Row, John F Brady Self-propelling microswimmers such as motile bacteria and Janus particles can be modeled as Active Brownian Partices (ABPs) and characterized by their transport properties: swim speed, translational diffusivity, and rotational diffusivity. We have developed a general theory to decribe suspensions of ABPs with spatialy varying transport properties. We find that the number density distribution of ABPs is primarily governed by the swim speed and is always lower in the region with the faster swim speed as first shown by Tailleur & Cates (PRL 2008). We also show that the translational diffusivities of ABPs smooth out the effect of the variation of swim speed. The theory implies that spontaneous reverse-osmosis is achievable owing to the spatial variation of the swim speed of active matter. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H61.00010: Curvature-dependent tension and tangential flows at the interface of motility-induced phases Adam Patch, David Yllanes, Daniel Sussman, M. Cristina Marchetti Purely repulsive active particles spontaneously undergo motility-induced phase separation (MIPS) into condensed and dilute phases. Remarkably, the mechanical tension measured along the interface between these phases is negative. In equilibrium, this would imply an unstable, expanding interface. However, these out-of-equilibrium systems display long-time stability and intrinsically stiff phase boundaries. In this work, we use active Brownian particle simulations and a novel frame of reference at the phase boundary to carefully study the emergent tangential currents at the interface, finding correlations between local interface curvature and measured values of interfacial tension. The combined observation of tangential currents in the gas and local “self-shearing” of the surface of the dense phase suggest a stiffening interface that redirects particles along itself to heal local fluctuations. In this way, the wildly fluctuating MIPS interface restores itself via an out-of-equilibrium Marangoni effect. |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H61.00011: Binary mixtures of hard sphere active Brownian particles Thomas Kolb, Daphne Klotsa We computationally study the phase behavior and dynamics of a binary mixture of active Brownian particles, where each ‘species’ is distinguished by its persistence of motion (effectively two species: fast, slow active particles). We find that our binary active system demonstrates motility-induced phase separation (analogous to monodisperse active systems) depending on the activity ratio, and concentration of each species. We observe a variety of steady states, which emerge as we vary the ratio of constituent particle activity, ranging from volatile partially segregated steady-state clusters, prone to fission, to a homogenously distributed, relatively stable, single cluster. We extend current theory for passive-active mixtures to account for active-active mixtures as well. |
Tuesday, March 5, 2019 5:06PM - 5:18PM |
H61.00012: Non-equilibrium work and reversibility in active particles in disordered media Joshua Steimel, Alfredo Alexander-Katz Active particles can in principle extract/do work on the environment. Here we explore experimentally a single active spinning particle in an inclined plane sliding in a disordered array of obstacles. Our results show that there are several interesting features in this system depending on the reversibility of the system during a cycle of the time reversible protocols. In particular we observe the rectification of the activity which allows the spinner to drift faster than the reference drift in the absence of obstacles. Our results should be of interest to understand under what conditions non-equilibrium systems are able to extract work from their environment and connect it to the thermodynamic counterparts. |
Tuesday, March 5, 2019 5:18PM - 5:30PM |
H61.00013: Extreme Active Matter at High Densities Chandan Dasgupta, Rituparno Mandal, Pranab Jyoti Bhuyan, Pinaki Chaudhuri, Madan Rao Extreme active matter, consisting of self-propelled particles characterized by large persistence time τp and high Péclet number, exhibits remarkable behavior at high densities. In the limit τp → ∞, the fluid jams as the self-propulsion force f is decreased below a critical value f*(∞). The system is stuck at a force-balanced configuration for f < f*(∞), with stresses concentrated along force chains. For large but finite τp, the approach to dynamical arrest at low f goes through a phase characterized by intermittency in kinetic and potential energy. This intermittency is a consequence of long periods of jamming separated by bursts of plastic yielding associated with Eshelby deformations akin to those found in the response of dense granular materials to an externally imposed shear. In the vicinity of the boundary between the intermittent and "normal" fluid phases, correlated plastic events result in large-scale vorticity and turbulent motion. Thus, dense extreme active matter brings together the physics of glass, jamming, plasticity and turbulence, in a new state of driven classical matter. |
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