Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S53: Nonequilibrium Statistical Mechanics and Hydrodynamics of Active Matter III |
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Sponsoring Units: GSOFT GSNP DFD Chair: Michael Hagan, Brandeis Univ Room: LACC 513 |
Thursday, March 8, 2018 11:15AM - 11:27AM |
S53.00001: Non-Markov Model for Self-Propelling Droplets Katherine Newhall , Pepijn Moerman , Eric Vanden-Eijnden , Jasna Brujic Active droplets and particles make an experimentally-tractable model systems for autophoretic micro-swimmers that interact with their own and each other’s trails through the concentration gradient they leave behind. Here, a non-Markov model is derived to describe their collective motion. The model is shown to capture experimental features, such as the enhancement of diffusion as the fuel concentration is decreased. It also explains the paradoxical observation that the lower the microscopic diffusivity of the swimmers, the higher their effective diffusion coefficient at long times. |
Thursday, March 8, 2018 11:27AM - 11:39AM |
S53.00002: Melting a Crystallite of Self-propelled Grains Lee Walsh , Narayanan Menon We experimentally study the kinetics of melting a two-dimensional non-cohesive crystallite of hard square-shaped millimeter-scale particles. Interactions between the square particles have four-fold rotational symmetry, but particles move when vibrated along a single polar body axis. We prepare the initial state with an ordered crystalline structure in the positions and orientations of the squares. The separation between the symmetries of particle shape versus motility allows us to vary the orientations of the polarity independently from the symmetry axes of the crystal. The kinetics of melting differ significantly across various initial configurations of square crystallites, which can either frustrate or promote the melting process. We find that inward-facing particles maintain an orientationally ordered state while quickly losing translational order. Further, this shows a progressive melting from the outer surface of the cluster. Thus, this initial orientation controls the sequence of phases observed in the interior of the crystallite, as well as the spatial distribution of the melting front. |
Thursday, March 8, 2018 11:39AM - 11:51AM |
S53.00003: Response of an active gas to shearing Caleb Wagner , Michael Hagan , Aparna Baskaran We explore how introducing activity influences the response of a gas to external driving forces. We consider an active gas of self-propelled spheres in the presence of a moving wall, and study the ability of the active gas to transport momentum introduced by shearing at the boundary. We identify several new and surprising behaviors. For large enough activity, increasing density decreases the ability of the gas to transport momentum, while for small activity the opposite effect is observed. In addition, a vorticity is observed on the scale of the particle diameter, which for certain parameter values results in a flow reversal near the boundary. This phenomenon is not captured in a fluid dynamical description, but we can explain it in terms of a kinetic sorting mechanism unique to active systems. Our present results are intended to form the basis for future study of active rheology at a more fundamental level, in terms of a Chapman-Enskog type theory. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S53.00004: Hydrodynamic Instability of a Chiral Active Fluid Sofia Magkiriadou , Vishal Soni , Stefano Sacanna , Denis Bartolo , Michael Shelley , William Irvine We study the collective dynamics of spinning colloidal magnets suspended in water. When they spin, these particles attract and form a cohesive material that behaves like a fluid. We probe, experimentally, the fluid dynamics of this two-dimensional magnetic gyrofluid. In particular, we find that, under certain conditions, it can undergo a hydrodynamic instability that causes it to break up into droplets. We investigate this instability, and we use a hydrodynamic model to understand its origin. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S53.00005: Using probability current fluctuations to quantify dissipation: A case study Junang Li , Jordan Horowitz , Todd Gingrich , Nikta Fakhri Biological systems consume ATP to maintain nonequilibrium structures that perform crucial functions such as molecular sensing and force generation. Like macroscopic machines, not all of the chemical fuel is converted into useful work. Some of the harvested free energy is lost due to dissipation, and this dissipation is accompanied by broken detailed balance in microscopic state transitions. Previous experiments have successfully detected probability currents in a variety of active biological systems that reflect broken detailed balance. Here, we use a bead-spring model to demonstrate how fluctuations in these probability currents can be utilized to indirectly quantify the dissipation rate. By leveraging the thermodynamic uncertainty relation, we can use statistical fluctuations in currents to bound the dissipation rate, even in high-dimensional systems operating outside a linear-response regime. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S53.00006: Study of Bend Instability & Efficiency in Microtubule and Motor Protein based Active Gels Pooja Chandrakar , Guillaume Duclos , Zvonimir Dogic In the last few years, the use of cytoskeletal filaments and motor proteins to create active systems has emerged as a promising method for exploring the field of active matter. We study an active system which consists of microtubules and kinesin motors and explore the hydrodynamic instability which is inherent in many active materials with orientational order. This instability depends on the microscopic parameters of our system such as motor concentration and channel dimension. In addition to exploring these parameters we compare the activity of microtubule bound SNAP-tagged kinesin to the activity of kinesin clusters formed by biotin-streptavidin bonds. An active gel with SNAP-kinesin uses the fuel more efficiently compared to kinesin clusters and lasts for more than hundred hours. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S53.00007: Collective Behavior of Swimmers in Fluids at Intermediate Reynolds Numbers Thomas Dombrowski , Shannon Jones , Amneet Pal Singh Bhalla , Boyce Griffith , Daphne Klotsa Emergent active matter behavior is observed in both biological and artificial systems. So far, most active-matter systems that include many-body hydrodynamic interactions have focused on small length scales in Stokes flows. However, a whole region of parameter space, that is mesoscale active matter, i.e. active matter of inertial particles in fluids at intermediate Reynolds numbers (Re), remains largely unexplored. We use the immersed boundary method to model the collective behavior of model swimmers at intermediate Re. We will talk about our results on the hydrodynamic interactions between swimmers, and we show how the onset of swimming, fluid flows and forces between them varies as a function of Re and how it differs from Stokes flows. We discuss next steps for how to build a statistical framework of inertial swimmers. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S53.00008: Highly-correlated, Spontaneous Gear-like Motion in an Active Granular System Zhejun Shen , Lee Walsh , Narayanan Menon We study a system of vibrated self-propelled granular particles on a horizontal plate within a circular boundary. The particles are square and designed to have polar motion along one body diagonal. When they hit the boundary they align along the boundary but also "walk" along the boundary. Given a large enough initial density, particles spontaneously migrate to the boundary, form a ring and perform a stable 1D rotational gear-like motion with a direction chosen by their net polarization. For a fully polarized single ring we find that the collective velocity surpasses the free single-particle velocity. This collective velocity increases as the density of particles in the ring increases, which is counterintuitive for a normal traffic problem. The spatial correlations of particle velocity in the fully polarized ring shows anti-correlations between nearest neighbors, which indicate they form pairs in the ring. The temporal correlation shows that velocity fluctuations are also anticorrelated in time. We also use two or more rings of particles to explore the effects of pressure and drag between layers. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S53.00009: Vesicle Transformations under Swim Pressure Yao Li , Pieter Rein ten Wolde All types of living cells and their organells are bound by membranes, which are mostly immersed in active environments. In active matters, an important quantity swim pressure was recently shown to depend on boundary shapes. Consequently, understanding how swin pressure interplays with fluidic membranes is highly desired. For the first time, we study vesicle transformations under swim pressure exerted by filled self-propelled particles. Our simplest model gives a novel phase diagram for vesicle shapes with a variation of vesicle volumes and particle driving speeds. Moreover, we surprisingly find a first order phase transition between spherical and prolate shapes without volume constraint. The origin of this phase transition is then well revealed by a theoretical calculation for single particle. Our predictions are directly verifiable in experiments on vesicles filled with Pt-coated Janus particles. Furthermore, they can help to understand and exploit various active behaviors of membranes in both living and designed matters. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S53.00010: Do hydrodynamic interactions affect the swim pressure? Eric Burkholder , John Brady The dynamic behavior of active matter has been a subject of great interest in recent years, though the role of fluid-mediated hydrodynamic interactions (HI) remains largely unknown. We study the motion of a spherical active Brownian particle (ABP) of size a, moving with a fixed speed U_{0}, and reorienting on a time scale τ_{R} in the presence of a confining boundary. The ABP, or swimmer, interacts with the boundary through a combination of hard-core collisions and HI; the strength of HI is characterized by a dimensionless parameter Δ. We compute the average force per unit area exerted on the wall by the swimmer, which is equivalent to the mechanical pressure Π = p_{f }+ n_{0}k_{B}T + Π^{swim}, where p_{f} is the fluid pressure, n_{0} is the far-field number density of swimmers and Π^{swim }is the swim pressure. In the absence of HI, Π^{swim} depends only on the mechanical properties of the swimmer. We show that HI quantitatively modify this pressure because the run length λ = U_{0}τ_{R }and the translational drag on the swimmer ζ may now depend on Δ, however the swim pressure has the same scaling: Π^{swim}=n_{0}ζ(Δ)U_{0}λ(Δ)/6. Similarly, if the swimmers move by a fixed force F^{swim}, Π^{swim} = n_{0}F^{swim}λ(Δ)/6. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S53.00011: Self-organized vortex crystal states in a generalized Navier Stokes model for active fluids Martin James , Wouter Bos , Michael Wilczek Generalized Navier Stokes equations have recently been used to model active fluid dynamics. These models capture the fundamental features of active fluid motion - self-propulsion and inter-particle interations - by means of a modified stress tensor which injects energy at intermediate scales and an advective nonlinearity which transfers energy across scales. In this contribution, we explore the dynamical phases exhibited by these models, focusing in particular on a self-organized quasi-stationary vortex crystal state preceded by a long turbulent transient phase. Our results show that the nonlinear energy transfer plays an important role in the formation of this vortex crystal state. The inverse energy cascade in this system shifts the dominant wavenumber from the naively expected one based on a linear stability analysis. We explore and study the parameter choices for which the system relaxes to this phase. Our results provide insights into turbulent pattern formation in active systems. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S53.00012: New Stochastic Field Theory for Understanding Active Matter Patrick Underhill , Yuzhou Qian , Peter Kramer The out-of-equilibrium nature of active matter can give rise to interesting properties in both small and large concentrations of the active objects. This includes local and global correlations between active particles and enhanced fluctuations and mixing. It is important to understand how hydrodynamic interactions between active agents cause and/or alter the suspension properties. Agent based models can incorporate fluctuations and interactions, but are limited to smaller systems. It can also more difficult to extract physical insight from the results. One of the most successful alternative approaches has been a mean field theory. However, in some situations the mean field theory makes predictions that differ significantly from experiments and direct (agent or particle based) simulations. There are also some quantities that cannot be calculated by the mean field theory. In this talk, we will describe our new approach which uses a stochastic field to overcome the limitations of the mean field assumption. It allows us to calculate how interactions between organisms alter the correlations and mixing even in conditions where there is no large-scale group behavior. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S53.00013: Collective phenomena in laned-active-matter: blockages, percolation, and traffic Skanda Vivek , David Yanni , Peter Yunker , Jesse Silverberg We study the emergence of collective behavior in laned-active matter. Our approach combines empirical measurements and an active matter model. We explore dynamics in the presence of blockages and find connections with clogging, jamming and percolation transitions. Confinement to lanes decreases the number of accessible configurations of constituents, but also suppresses kinetically-trapped, disordered pathways, thus avoiding clogging. The results of these studies may inform how traffic can be engineered to deal with blocked lanes. |
Thursday, March 8, 2018 1:51PM - 2:03PM |
S53.00014: Flocking from a quantum analogy: Spin-orbit coupling in an active fluid Benjamin Loewe , Anton Souslov , Paul Goldbart Quantum analogues have proven to be valuable tools in the study of both equilibrium and non-equilibrium statistical systems. At their core, these analogies allow one to explore some complex classical systems in terms of simpler quantum ones, thus facilitating the use of the powerful toolkit of quantum mechanics. We enlarge on the well-known relationship between the Schrödinger equation and the diffusion equation in order to incorporate self-propulsion, and thus, to build quantum analogues of systems of two-dimensional self-propelled particles. Crucially, we show how, on the quantum side, spin and spin-orbit coupling capture both a particle’s orientation and self-propulsion. Interestingly, the microscopic active system that stems from this analogy is characterized by a coupling of translational and rotational noises, which resembles the Heisenberg uncertainty principle. Finally, by coarse-graining the microscopic model, we obtain explicit expressions for the coefficients in the Toner-Tu equations, which describe the hydrodynamic limit of the system. The connection between self-propelled particles and quantum spins may help realize exotic phases of matter using active fluids via analogies with systems composed of strongly correlated electrons. |
Thursday, March 8, 2018 2:03PM - 2:15PM |
S53.00015: The physics of high-density crowds Arianna Bottinelli Communication and information transfer between human subjects give rise to social conventions, shared norms, collective motion, and other efficient self-organized phenomena. During mass events such as concerts, parades, sporting events, and pilgrimages, crowd density can become extremely high, causing the breakdown of conventional information transfer and the emergence of potentially deadly collective motions. |
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