Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P53: Nonequilibrium Statistical Mechanics and Hydrodynamics of Active Matter II |
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Sponsoring Units: GSOFT GSNP DFD Chair: Katherine Klymko, Univ of California - Berkeley Room: LACC 513 |
Wednesday, March 7, 2018 2:30PM - 2:42PM |
P53.00001: Active Torque Dipoles Create a Nonequilibrium Cholesteric Phase in Wet and Dry Active Matter Ana Fialho, Elsen Tjhung, Davide Marenduzzo Despite the fact that most biological active components are intrinsically chiral (e.g. actomyosin, swimming bacteria), many active models still disregard chiral processes and describe active units simply as force dipoles. |
Wednesday, March 7, 2018 2:42PM - 2:54PM |
P53.00002: A new class of dry, dilute, active matter Benoit Mahault, Xinchen Jiang, Aurelio Patelli, Eric Bertin, Xiaqing Shi, Hugues Chate Simple models such as the Vicsek model have been decisive for our current understanding of dry dilute active matter. In spite of, but also thanks to their simplicity, Vicsek-style models have helped uncover a wealth of new physics, such as the generic long-range correlations and anomalous fluctuations at play in their ordered phases. Until now, three main universality classes have emerged, characterized by the symmetries of particles’ motion (polar/apolar) and alignment (ferromagnetic/nematic). |
Wednesday, March 7, 2018 2:54PM - 3:06PM |
P53.00003: Congestion model of active particle phase separation Isaac Bruss, Sharon Glotzer
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Wednesday, March 7, 2018 3:06PM - 3:18PM |
P53.00004: Hydrodynamics of Confined Incipient Active Nematics Minu Varghese, Arvind Baskaran, Michael Hagan, Aparna Baskaran The hydrodynamic instabilities inherent in bulk active nematics have been known to organize into large scale coherent flows when guided by confinement. Such long range correlations in the flow is possibly enabled by nematic ordering conveying the effect of confinement to points away from the boundaries. |
Wednesday, March 7, 2018 3:18PM - 3:30PM |
P53.00005: Collective Behavior under Environmental Perturbations Michael Sinhuber, Kasper van der Vaart, Nicholas Ouellette A wide range of animal species including birds, fish, and insects show complex, self-organized collective behavior in the wild. The nature of this collective behavior and its distinction from the aggregate behavior of individuals has been the subject of experimental and theoretical research for decades. However, it remains largely unknown how these collective systems respond to ubiquitous environmental perturbations such as currents, wind gusts, or sound or light signals. We have used laboratory experiments on Chironomus riparius, a midge species that predictably forms collective mating swarms, to investigate the response of swarms to perturbations that mimic expected environmental conditions in a well-controlled way. To do this, we used a multi-camera setup to record three-dimensional information about trajectories, velocities and accelerations of all individual midges in the swarm. We then characterize the individual and swarm responses to environmental perturbations using statistical and topological analysis. |
Wednesday, March 7, 2018 3:30PM - 3:42PM |
P53.00006: Self-organized band state of driven hard needles Michael Stefferson, Hui-Shun Kuan, Matthew Glaser, Meredith Betterton Systems of driven particles display new nonequilibrium phases with self-organized structures. We study an active-matter model of self-propelled hard needles in two dimensions, containing interacting anisotropic Brownian particles with a non-conservative driving force along each rod's polar director. We present a dynamic density functional theory (DDFT) for this system derived from microscopic Langevin equations of driven rods with steric interactions. This model facilitates a full numerical solution of the equations, without requiring the approximations of hydrodynamic expansion. Comparison to Brownian dynamics particle simulations allows us to test the predictions of the DDFT model. We find that driving leads to a spatially inhomogeneous nematic band state that, in contrast to previous work, is stabilized by spatially varying polar order perpendicular to the band. This result challenges the assumption made in previous work that a self-propelled system with apolar interactions directly maps onto an active nematic. We discuss the implications for the microscopic mechanism of band stabilization. |
Wednesday, March 7, 2018 3:42PM - 3:54PM |
P53.00007: Quantifying hidden order out of equilibrium Stefano Martiniani, Paul Chaikin, Dov Levine While the equilibrium properties, states, and phase transitions of interacting systems are well described by statistical mechanics, the lack of suitable state parameters has hindered the understanding of non-equilibrium phenomena in diverse settings, from glasses to driven systems to biology. Here, we describe a simple idea enabling the quantification of order in non-equilibrium and equilibrium systems, both discrete and continuous, even when the underlying form of order is unknown. The length of a losslessly compressed data file is a direct measure of its information content, a quantity directly related to a system's entropy. Using data compression to study several out-of-equilibrium systems, we show that our approach is capable of reliably identifying dynamical phase transitions and their character, and to quantitatively predict certain critical exponents, without any knowledge of the relevant order parameters. This approach thus provides a new and essential way of quantifying order in systems ranging from condensed matter systems in and out of equilibrium, to cosmology and biology. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P53.00008: A case for active self-assembly: The Kagome lattice Stewart Mallory, Angelo Cacciuto The challenges associated with developing robust self-assembly processes in the laboratory mostly stem from an incomplete understanding of the underlying self-assembly pathway. Using numerical simulations, we study the self-assembly pathway of active triblock Janus colloids, which are designed to target the formation of a colloidal Kagome crystal. We demonstrate that the self-assembly of this elusive structure can be significantly improved by self-propelling the particles along the axis connecting their hydrophobic hemispheres. This study represents a meaningful extension of our previous work on the active self-assembly of compact, finite-sized target structures. In this talk, we outline a basic strategy of how self-propulsion can be used to improve the self-assembly of macroscopic colloidal crystals. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P53.00009: The dynamic structure factor of active Brownian particles Austin Dulaney, John Brady The classical problem of diffusion from a point source for a passive system is known to behave in a purely diffusive fashion. Experiments suggest that for active systems a wavelike character is observed. We study the dynamic structure factor of active Brownian particles (ABPs) with no initial net polar order for the case of ABPs released from a point source with no net nematic order in the system. The transient behavior is captured by the Smoluchowski equation for the evolution of the particle probability density function in orientation and position space. Using a moments expansion method we obtain a wave equation for the number density of ABPs. The number density is characterized by wavelike behavior at short times and becomes diffusive at times long compared to the reorientation time. The diffusive behavior is characterized by an effective diffusivity, which is the sum of the translational diffusivity and swim diffusivity Dswim = U02 τR / 2, where U0 is the speed of the particle and τR is the reorientation time. Our results are corroborated by Brownian dynamic simulations. |
Wednesday, March 7, 2018 4:18PM - 4:30PM |
P53.00010: Phase Separation in Binary Mixtures of Active Brownian Particles Thomas Kolb, Daphne Klotsa, Ehssan Nazockdast Active systems are composed of self-propelled (i.e. active) particles that locally convert energy into motion and exhibit emergent collective behaviors, such as fish schooling and bird flocking. Most works, so far, have focused on monodisperse, one-component active systems or binary mixtures where only one species is active (the other is Brownian). However, real systems are heterogeneous and may consist of several active components (e.g. active processes in the cell). We perform overdamped Brownian dynamics simulations of binary active systems, where both species are active. The difference between the species is only their activity (Péclet number). We find that our binary active systems also demonstrate motility-induced phase separation but we also see a separation between more and less active species that is driven by the ratio of their respective activities. We discuss the dynamics of mixed-activity clusters and how they vary as a function of activity and density. |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P53.00011: Active polymer hydrodynamics: simulations and theory Achal Mahajan, Alexandra Zidovska, Michael Shelley, David Saintillan Recent spectroscopy experiments on interphase chromatin have uncovered the existence of long- |
Wednesday, March 7, 2018 4:42PM - 4:54PM |
P53.00012: Negative apparent viscosities, non-monotonic flow curves and multiple mechanical equilibria in the rheology of active suspensions Aurore Loisy, Jens Eggers, Tanniemola Liverpool Active suspensions, such as swarms of bacteria and the cytoskeleton of living cells, consist of anisotropic motile units interacting in a passive medium. Because these active units self-organize and collectively induce mechanical stresses in the bulk, they may, under shear, reduce the apparent viscosity of the suspension. Recently, using a highly sensitive rheometer, Lopez et al. (PRL 115, 2015) were able to measure zero and possibly negative values of the apparent viscosity in a suspension of E. Coli. In this talk, we will use a minimal model of an active liquid crystal to demonstrate that a negative apparent viscosity is realisable in this system and is associated with a negative slope in the steady-state stress vs. strain rate flow curve. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P53.00013: Using relative entropy to quantify non-equilibrium activity in biological active matter Garrett Watson, Tzer Han Tan, Jordan Horowitz, Nikta Fakhri Stochastic force generation within cells drive the system out-of-equilibrium. These forces play a crucial role in vital processes such as intracellular transport. It is important to characterize the nature of such nonequilibrium steady states to understand cellular dynamics and function. Here we apply a framework based on the notion of relative entropy or Kullbeck-Leibler Divergence (KLD) to quantify the extent of non-equilibrium activity in biological active matter. In this framework, from the data of a single stationary trajectory obtained sampling any physical variable of the system, we can estimate the average dissipation rate of the mechanism that generated the data. We validate our framework numerically by using hidden Markov models to simulate different levels of out-of-equilibrium cellular activity. Using experimental time series data of probe particles embedded in the actomyosin cortices, we establish a lower bound for the entropy production of cortical activity. Our results demonstrate a reliable way to quantify the non-equilibrium activity in mesoscopic systems. |
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