Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session E38: Active Matter IIIFocus
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Sponsoring Units: GSOFT DBIO GSNP/DFD Chair: Cynthia J. Reichardt, Los Alamos National Laboratory Room: 341 |
Tuesday, March 15, 2016 8:00AM - 8:12AM |
E38.00001: Statistical Mechanics and Hydrodynamics of Self-Propelled Hard Disks Benjamin Hancock, Aparna Baskaran Active particle fluids have constant energy production and dissipation at the level of its constituent particles. Yet, despite its out-of-equilibrium nature, analogies have been drawn between the steady state behavior of active fluids and their passive fluid counterparts. While most of the studies have been phenomenological or numerical, in this talk we present a first systematic derivation of the statistical mechanics and hydrodynamics of self propelled hard disks in particular we focus on two results. First, a dynamical instability signaling the onset of phase separation and cluster formation is derived and compared to existing phenomenological and kinetic estimates. Second, a leading order contribution to the pressure due to particle interactions is derived and compared with simulations of active brownian particles. [Preview Abstract] |
Tuesday, March 15, 2016 8:12AM - 8:24AM |
E38.00002: Reversible Ratchet Effects and Structural Ordering for Self-Propelled Disks on Quasi-One Dimemsional Asymmetric Substrates Danielle McDermott, Cynthia Reichhardt, Charles Reichhardt When a particle is placed in an asymmetric periodic potential and an ac driving force is applied, it is possible to produce a net dc flow through a ratchet effect. When the particles are active, a net dc particle flow can arise even in the absence of external driving, creating an active ratchet effect as has been observed for bacteria in funnel geometries. Here we examine a 2D assembly of self-propelled disks interacting with an asymmetric 1D substrate. We find that at low density, with few particle collisions, this system exhibits a robust ratchet effect in which the particles undergo a net drift in the easy direction of the substrate asymmetry. At higher densities where particle-particle interactions become important, a reversed ratchet effect can arise with the net flow of particles in the hard direction. These reversals occur due to the formation of commensurate chain-like structures of disks. When there are two or more chains of particles in a one substrate well, the effective substrate potential is inverted. This reversible active ratchet effect could be used to separate different species of particles, cause the shepherding of passive particles, or control the migration of micro-organisms, and should be general to a wide class of self driven interacting particle systems. [Preview Abstract] |
Tuesday, March 15, 2016 8:24AM - 8:36AM |
E38.00003: Melting a crystal of self-propelled particles Christopher Olson, Michael Muller, Lee Walsh, Narayanan Menon We experimentally study the kinetics of melting a two-dimensional non-cohesive crystal of hard, square-shaped millimeter-scale particles. Interactions between the square particles have four-fold rotational symmetry, but particles are designed with features such that when vibrated their predominant motion is polar along one body axis. We prepare the initial crystalline state with varying orientations of the particle polarity relative to the symmetry axes of the crystal. We then study the melting of this crystal when vertical vibrations are turned on. Orientational and translational order are initially strongly coupled, and during melting translational order is lost before orientational order. The spatial distribution of order parameters and the time scale for melting kinetics is strongly affected by compatibility between the polarity and the crystal axes in the initial condition. [Preview Abstract] |
Tuesday, March 15, 2016 8:36AM - 8:48AM |
E38.00004: Magnetic properties of self propelled particles Melissa Ferrari, Michelle Driscoll, Jeremie Palacci, Stefano Sacanna, David Pine, Paul Chaikin We study a class of synthetic light-activated colloidal swimmers which self propel osmotically/phoretically close to a surface and self organize into dynamic clusters. Swimming is activated by a photocatalytic hematite cube exposed from the colloidal surface. Hematite is a canted antiferromagnet, with a permanent magnetic moment; the magnetic moment is oriented in a discrete number of directions relative to the exposed hematite face. The permanent moment allows us to orient and direct the swimmers’ motion with an applied magnetic field, and different field configurations allow for a large range of directed motion. Furthermore, the various orientations of the magnetic moment give rise to distinct species of swimmers, which can simultaneously undergo clockwise and counterclockwise orbits in a rotating magnetic field. [Preview Abstract] |
Tuesday, March 15, 2016 8:48AM - 9:00AM |
E38.00005: Guiding catalytically active particles with chemically patterned surfaces William Uspal, Mihail Popescu, Siegfried Dietrich, Mykola Tasinkevych Catalytically active Janus particles in solution create gradients in the chemical composition of the solution along their surfaces, as well as along any nearby container walls. The former leads to self-phoresis, while the latter gives rise to chemi-osmosis, providing an additional contribution to self-motility. Chemi-osmosis strongly depends on the molecular interactions between the diffusing chemical species and the wall. We show analytically, using an approximate ``point-particle'' approach, that by chemically patterning a planar substrate (e.g., by adsorbing two different materials) one can direct the motion of Janus particles: the induced chemi-osmotic flows can cause particles to either ``dock'' at a chemical step between the two materials, or to follow a chemical stripe. These theoretical predictions are confirmed by full numerical calculations. Generically, docking occurs for particles which tend to move away from their catalytic caps, while stripe-following occurs in the opposite case. Our analysis reveals the physical mechanisms governing this behavior. [Preview Abstract] |
Tuesday, March 15, 2016 9:00AM - 9:12AM |
E38.00006: 3-d Brownian dynamics simulations of the smallest units of an active biological material Jutta Luettmer-Strathmann, Nabina Paudyal, Maral Adeli Koudehi Motor proteins generate stress in a cytoskeletal network by walking on one strand of the network while being attached to another one. A protein walker in contact with two elements of the network may be considered the smallest unit of an active biological material. In vitro experiments, mathematical modeling and computer simulations have provided important insights into active matter on large and on very small length and time scales. However, it is still difficult to model the effects of local environment and interactions at intermediate scales. Recently, we developed a coarse-grained, three-dimensional model for a motor protein transporting cargo by walking on a substrate. In this work, we simulate a tethered motor protein pulling a substrate with elastic response. As the walker progresses, the retarding force due to the substrate tension increases until contact fails. We present simulation results for the effect of motor-protein activity on the tension in the substrate and the effect of the retarding force on the processivity of the molecular motor. [Preview Abstract] |
Tuesday, March 15, 2016 9:12AM - 9:24AM |
E38.00007: Smarticles: smart, active granular matter Will Savoie, Arman Pazouki, Dan Negrut, Daniel Goldman We investigate a granular medium composed of smart, active particles, or ``smarticles''. Previously, we discovered that ensembles of ``u''-shaped particles exhibited geometrically-induced cohesion by mechanically entangling via particle interpenetration [Gravish et al, PRL, 2012]; the strength and/or extent of entanglement could be varied by changing particle level entanglement by changes in arm-to-base length of the u-particle. Since changing this parameter on demand is inconvenient, we develop a power-autonomous programmable robot composed of two motors and three links with an on-board microcontroller. This smarticle can be activated to change its configuration (specified by its two joint angles) through audio communication. To complement these experiments, since study large ensembles of smarticles is cost and labor prohibitive, we also develop a simulated smarticle in the Chrono multibody simulation environment. We systematically study ensemble cohesiveness and compaction as a function of shape changes of the smarticles. We find that suitable activation of smarticles allows ensembles to become cohesive to ``grip'' rigid objects and lose cohesion to release on command. [Preview Abstract] |
Tuesday, March 15, 2016 9:24AM - 9:36AM |
E38.00008: Active Brownian particles near straight or curved walls: Pressure and boundary layers Ayhan Duzgun, Jonathan Selinger Unlike equilibrium systems, active matter is not governed by the conventional laws of thermodynamics. Through a series of Langevin dynamics simulations and analytic calculations, we explore how systems cross over from equilibrium to active behavior as the activity is increased. In particular, we calculate the profiles of density and orientational order near straight or circular walls, and show the characteristic width of the boundary layers. We find a simple relationship between the enhancements of density and pressure near a wall. Based on these results, we determine how the pressure depends on wall curvature, and hence make approximate analytic predictions for the motion of curved tracers, as well as the rectification of active particles around small openings in confined geometries. [Preview Abstract] |
Tuesday, March 15, 2016 9:36AM - 9:48AM |
E38.00009: Active particles on curved surfaces Yaouen Fily, Aparna Baskaran, Michael Hagan Active systems have proved to be very sensitive to the geometry of their environment. This is often achieved by spending significant time at the boundary, probing its shape by gliding along it. I will discuss coarse graining the microscopic dynamics of self-propelled particles on a general curved surface to predict the way the density profile on the surface depends on its geometry. Beyond confined active particles, this formalism is a natural starting point to study objects that cannot leave the boundary at all, such as cells crawling on a curved substrate, animals running on uneven ground, or active colloids trapped at an interface. [Preview Abstract] |
Tuesday, March 15, 2016 9:48AM - 10:00AM |
E38.00010: Edge states in confined active fluids Anton Souslov, Vincenzo Vitelli Recently, topologically protected edge modes have been proposed and realized in both mechanical and acoustic metamaterials. In one class of such metamaterials, Time-Reversal Symmetry is broken, and, to achieve this TRS breaking in mechanical and acoustic systems, an external energy input must be used. For example, motors provide a driving force that uses energy and, thus, explicitly break TRS. As a result, motors have been used as an essential component in the design of topological metamaterials. By contrast, we explore the design of topological metamaterials that use a class of far-from-equilibrium liquids, called polar active liquids, that spontaneously break TRS. We thus envision the confinement of a polar active liquid to a prescribed geometry in order to realize topological order with broken time-reversal symmetry. We address the design of the requisite geometries, for example a regular honeycomb lattice composed of annular channels, in which the active liquid may be confined. We also consider the physical character of the active liquid that, when introduced into the prescribed geometry, will spontaneously form the flow pattern of a metamaterial with topologically protected edge states. Finally, we comment on potential experimental realizations of such metamaterials. [Preview Abstract] |
Tuesday, March 15, 2016 10:00AM - 10:12AM |
E38.00011: Effect of micro-stirring on enzymatic reaction kinetics inside a biomimetic container Irep Gozen, Viva Horowitz, Zachary Chambers, Vinothan Manoharan The intracellular environment is dynamic, influenced by the motion of active machinery such as cytoskeleton filaments and molecular motors. To understand whether and how such activity affects the rates of diffusion-limited reactions, we construct a model system consisting of a phospholipid vesicle encapsulating a small number of micro- or nanoparticles, the active motion of which can be induced by chemical or magnetic cues. We aim to determine a relation between active motion of particles and rates of enzymatic reactions in the confined volume. Our findings might illuminate how active motion influences cytoplasmic reaction dynamics, or may have played a role in protocell genetics. [Preview Abstract] |
Tuesday, March 15, 2016 10:12AM - 10:24AM |
E38.00012: Correlated Rotational Noise in Active Brownian Systems Caleb Wagner, Aparna Baskaran We consider a system of self-propelled particles in a viscous medium for which the angle parametrizing the direction of particle propulsion is subject to correlated noise. The physics involved in the correlated noise is explored by deriving a modified Smoluchowski equation that governs the evolution of the probability distribution for particle positions and orientations. More precisely, given noise correlations that decay exponentially in time with decay constant $\nu $, we give the modified Smoluchowski equation as a perturbative expansion in $\nu $. While the physical origins of correlated noise may be diverse, we give one interpretation of the resulting dynamics in terms of inertial effects that are absent from the usual overdamped description of self-propelled particles in a viscous medium. [Preview Abstract] |
Tuesday, March 15, 2016 10:24AM - 10:36AM |
E38.00013: ABSTRACT WITHDRAWN |
Tuesday, March 15, 2016 10:36AM - 10:48AM |
E38.00014: Dancing the night away: Improving the persistence of locomotion on the micron scale Emily W. Gehrels, W. Benjamin Rogers, Zorana Zeravcic, Vinothan N. Manoharan In recent years a range of nano and microscale walkers (motors that are able to move along a preformed track) have been developed. Many of these walkers bind to their tracks using a single binding site at each station along the track. A disadvantage of these systems is that any failure involving a single site becoming unbound leads to the walker falling off of the track and locomotion being prematurely terminated. For this reason, it has been difficult to develop a motor that can reliably take more than a few sequential steps. We present an experimental system of DNA-functionalized colloidal particles which exhibit directed motion along patterned substrates in response to temperature cycling. Many DNA bridges form between each pair of interacting particles, adding redundancy to the binding at each station to realize a system that should be able to consistently take many steps. We take advantage of toehold exchange in the design of the DNA sequences that mediate the colloidal interactions to produce broadened, flat, or even re-entrant binding and unbinding transitions between the particles and substrate. Using this new freedom of design, we devise systems where, by thermal ratcheting, we can externally control the direction of motion and sequence of steps of the colloidal motor. [Preview Abstract] |
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