Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B61: Nonequilibrium Statistical Mechanics of Driven Systems and Pattern Formation IIFocus
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Sponsoring Units: GSOFT GSNP Chair: Suriyanarayanan Vaikuntanathan, University of Chicago Room: BCEC 258B |
Monday, March 4, 2019 11:15AM - 11:27AM |
B61.00001: Excitations in an active smarticle gas Akash Vardhan, Zachary Jackson, William C Savoie, Kurt A Wiesenfeld, Daniel Goldman <p style="margin: 0px 0px 10.66px;"><font color="#000000"><font face="Calibri"><font size="3">Spontaneous stable excitations can arise unexpectedly from homogenous many body systems, e.g. oscillons observed in vibrating granular media, and rotons in superfluid helium. We have observed several such long lived excitations in a gas of active matter particles called “smarticles”. Smarticles are small 3 link planar robots. Each smarticle measures 14X2.5X3 cm</font>3</font><font face="Calibri" size="3">, where only the center link is on the ground. In the experiments they are placed on a frictional surface, and change their shape periodically, while interacting with each other via collisions. Although fairly limited in their locomotive capabilities on their own in the considered experimental configuration, interesting behavior is revealed when we allow these smart, active granular materials to mingle. We study the simplest case where two smarticles interact with each other. These two particle excitons demonstrate bouts of coordinated motion synchronized both in their gaits, and spatial orientations. We also witness a unique case of locomotion emerging from their mechanics, wherein the collective executes general planar motion.</font></font></p> |
Monday, March 4, 2019 11:27AM - 11:39AM |
B61.00002: Attractor symmetry and stability in symmetric self-driven oscillator networks Ian Hunter, Michael Norton, James Sheehy, Bolun Chen, Youssef Fahmy, Lanijah Flagg, Chris Simonetti, Seth Fraden The dynamics governing networks of identical oscillators are unchanged by node interchange symmetries, or automorphisms, of the network. Equivariant dynamical system theory predicts such networks consequently must possess steady states, and flow invariant manifolds where particular nodes, exchanged by subgroups of network symmetries, are synchronized. Homogeneous microreactors containing the oscillatory Belousov Zhabotinsky (BZ) reaction, coupled by diffusion, allow the experimental study of symmetric self-driven oscillator networks. A ring of 4 inhibitory-coupled BZ reactors was studied as a model system. This system exhibits symmetric gaits found in quadrupedal animals as its attractors. Experimental invariant manifolds, steady states, and stabilities are compared to those theoretically predicted using methods generalizable to other networks. |
Monday, March 4, 2019 11:39AM - 11:51AM |
B61.00003: Domain wall creep and depinning: a scalar field model approach Nirvana Caballero, Ezequiel Ferrero, Alejandro B. Kolton, Javier Curiale, Vincent Jeudy, Sebastian Bustingorry, Thierry Giamarchi Domain wall motion is at the heart of new magneto-electronic technologies and hence the need for a deeper understanding of domain wall (DW) dynamics in magnetic systems. In this context, numerical simulations using simple models can capture the main ingredients responsible for the complex observed DW behavior. We present a scalar-field model for the magnetization dynamics of quasi-two-dimensional systems with a perpendicular easy axis of magnetization which allows a direct comparison with typical experimental protocols, used in polar magneto-optical Kerr effect microscopy experiments. We show that the thermally activated creep and depinning regimes of DW motion can be reached, and the effect of different quenched disorder implementations can be assessed with the model. Moreover, our model has material-dependent tunable parameters and allows to reproduce experimental velocity-field curves. In particular, we show how the structural disorder should be modeled to reproduce Pt/Co/Pt velocity-field curves in a broad range of temperatures (ranging from 4K to room temperature). We also use this model to make a connection with DW with defects such as bubbles and overhangs. We examine in particular observables such as the two-dimensional structure factor both numerically and analytically. |
Monday, March 4, 2019 11:51AM - 12:03PM |
B61.00004: Modeling the relative dynamics of DNA-coated colloids Miranda Holmes-Cerfon, James Lee-Thorp A versatile way to makes colloids with programmable interactions is to coat them with strands of sticky DNA. However, the DNA changes the dynamics of the colloids, in ways that can push the system out of equilibrium on experimental timescales. We construct a coarse-grained theoretical model for the relative dynamics of DNA-coated colloids (which is similar to models of molecular motors), and use it to argue that (a) DNA induces friction between the points of contact that can be about 100 times larger than hydrodynamic friction, for relevant experimental parameters, and (b) the friction for particles rolling could be several orders of magnitude smaller than the friction for sliding, when the DNA is very short and stiff. Therefore, we speculate that in certain experimental systems such colloids could act like gears, and assemble into metastable states that would not be observed in their true equilibrium. |
Monday, March 4, 2019 12:03PM - 12:15PM |
B61.00005: Driving Frustrated Lattices of Active Droplets Anton Molina, Stefan Karpitschka, Manu Prakash Self-organization is the process by which interacting building blocks arrange themselves into an ordered structure. While there are many examples of self organization, there are no examples which explore the use of external fields to drive self-organizing particles in a geometrically frustrated environment. Here, we explore frustrated self-organization in a novel system comprised of self-propelled droplets. Ensembles of these droplets interact with one another, via a vapor-mediated and long-ranged potential, displaying dynamic behavior that resembles chemotaxis. We present an experiment that confines droplets to hexagonal lattice sites, using gravity to drive the system away from equilibrium. Hence, we study the relationship between single particle dynamics and the statistics of ensemble organization. In this geometrically frustrated environment we observe a phase transition of droplet self-assembly as a function of the driving from equilibrium. Comparison to simulation allows us to connect our experimental results to statistically relevant distributions, thereby connecting observations of single particle dynamics and local accommodation of frustration to statistical physics. |
Monday, March 4, 2019 12:15PM - 12:27PM |
B61.00006: Self-assembly of nanoscale braided structures Edvin Memet, Mohammad Shahjamali, Cheng Zeng, Isaac Bruss, Vinothan N Manoharan, Lakshminarayanan Mahadevan Capillarity, along with bending and cohesive forces, can induce assembly of nanopillar/micropillar arrays into a wide variety of structures. Experiments we performed involving seven hexagonally arranged pillars reveal a surprising transition from straight to twisted conformations. Separately, we also found that large arrays (e.g. 30x30, 10x40) of high-aspect ratio pillars can assemble into various structures, including chiral, globally twisted morphologies. Here we use numerical and scaling methods to understand how the rate of drying, capillarity, bending, and cohesion control the physics of elastocapillary-driven self-assembly. |
Monday, March 4, 2019 12:27PM - 12:39PM |
B61.00007: Simulations on encapsulation of multiple cargoes in bacterial microcompartments Lev Tsidilkovski, Farzaneh Mohajerani, Michael F Hagan Bacterial microcompartments are self-assembling protein shells that encapsulate enzymes or other proteins, similar to eukaryotic organelles. Bacteria use them to sequester toxic intermediates from the cytoplasm, or to accelerate the rates of chemical reactions through co-localization of reagents. |
Monday, March 4, 2019 12:39PM - 12:51PM |
B61.00008: Driven Widom-Rowlinson lattice gas Ronald Dickman, Royce Zia In the Widom-Rowlinson lattice gas, two particle species (A, B) diffuse freely via particle-hole exchange, subject to both on-site exclusion and prohibition of A-B nearest-neighbor pairs. As an athermal system, the overall densities are the only control parameters. As the density increases, a phase transition occurs, leading to ordered states with A- and B-rich domains separated by hole-rich interfaces. Using Monte Carlo simulations, we analyze the effect of imposing a drive on this system, biasing particle moves along one direction. Novel features emerge, including structure factors with kink singularities (best fitted to |q|), maxima at non-vanishing wavevector values, oscillating correlation functions, and ordering into multiple striped domains perpendicular to the drive, with a preferred wavelength depending on density and drive intensity. Interfaces between the domains are statistically rough, in sharp contrast with those in the Katz-Lebowitz-Spohn model, in which the drive suppresses interfacial roughness. Defining an order parameter (to account for the emergence of multistripe states), we map out the phase diagram in the density-drive plane. |
Monday, March 4, 2019 12:51PM - 1:27PM |
B61.00009: Self-assembly, aggregation, and phase behavior of driven and active colloids Invited Speaker: Erik Luijten Colloidal suspensions are a prototypical example of systems that can be either passive of active. I will demonstrate how various forms of dynamics and different types of interactions result in unexpected and until now largely unexplored aggregation and phase behavior. These observations, obtained through a combination of experiments and computer simulations, reveal striking connections between colloidal self-assembly and collective dynamics, and between dynamic behavior and equilibrium thermodynamics. Moreover, a remarkable variety of collective dynamics can be realized through simple variation of external electric fields. These observations provoke new thoughts on the nature of “soft” materials and our ability to manipulate them. |
Monday, March 4, 2019 1:27PM - 1:39PM |
B61.00010: Feasibility of prediction in driven-dissipative system displaying effective ergodicity breaking Chonkit Pun, W. Klein, Harvey Gould Using machine learning techniques we forecast the size of avalanches in the Olami-Feder-Christensen model using spatial information at different values of noise. We use the convolutional neural network and find that prediction is possible at low noise effective non-ergodic phase and not possible at the high noise effective ergodic phase. By looking at the higher level structures learned from the convolutional neural network, we can identify precursors of large events. Our goal is to understand the theoretical limitations of forecasting extreme events in complex systems. |
Monday, March 4, 2019 1:39PM - 1:51PM |
B61.00011: A 3-species Exclusion Process: a model motivated by Leaky Initiation and Interference in Protein Synthesis Debashish Chowdhury, Bhavya Mishra The totally asymmetric simple exclusion process (TASEP) was originally introduced fifty years ago as a model for the synthesis of biopolymers, called protein, by macromolecular machines called ribosome. In that formulation, the mRNA template for protein synthesis, that also serves as the track for the motor-like movement of the ribosomes, is represented by a lattice. We introduce a 3-species exclusion model that captures some of the hitherto neglected, but crucially important, details of the initiation of protein synthesis. We study the interference of the synthesis of two different proteins, the templates for which are encoded on two different, but overlapping, segments of the same lattice. We formulate the process of search for the start sites for protein synthesis as first-passage problems and calculate the mean initiation times for the synthesis of the two species of proteins. We show how the mean initiation times get affected by ``leaky scanning’’ and the interference of the three exclusion processes. |
Monday, March 4, 2019 1:51PM - 2:03PM |
B61.00012: Pressure exerted by confined active particles Micha Kornreich, Paul M Chaikin Mechanical pressure, defined as the force applied perpendicular to the surface of an object per unit area, takes dramatically different forms in and out of equilibrium [1]. To experimentally measure the pressure exerted by an out of equilibrium system, we confine self-propelled particles in a negative dielectrophoretic trap. Knowing the potential profile, the density profile reveals the pressure. In equilibrium, pressure is exclusively governed by bulk properties such as density and temperature. In contrast, when the particles are activated, using either an in-plane rotating magnetic field or light-activated self-phoretic particles, we find that the force exerted on the confining trap boundaries depends on the shape of the boundary dielectrophoretic force fields. |
Monday, March 4, 2019 2:03PM - 2:15PM |
B61.00013: Surfing on protein waves: proteophoresis as a mechanism for bacterial genome partitioning Jean-Charles Walter, Jérôme Dorignac, Vladimir Lorman, Jérôme Rech, Jean-Yves Bouet, Marcelo Nollmann, John Palmeri, Andrea Parmeggiani, Frédéric Geniet Efficient bacterial chromosome segregation typically requires the coordinated action of a three-component, fueled by adenosine triphosphate machinery called the partition complex. We present a phenomenological model accounting for the dynamic activity of this system that is also relevant for the physics of catalytic particles in active environments. The model is obtained by coupling simple linear reaction-diffusion equations with a proteophoresis, or “volumetric” chemophoresis, force field that arises from protein-protein interactions and provides a physically viable mechanism for complex translocation. This minimal description captures most known experimental observations: dynamic oscillations of complex components, complex separation and subsequent symmetrical positioning. The predictions of our model are in phenomenological agreement with and provide substantial insight into recent experiments. From a non-linear physics view point, this system explores the active separation of matter at micrometric scales with a dynamical instability between static positioning and travelling wave regimes triggered by the dynamical spontaneous breaking of rotational symmetry. |
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