Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A61: Nonequilibrium Statistical Mechanics of Driven Systems and Pattern Formation IFocus
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Sponsoring Units: GSOFT GSNP Chair: Arnold Mathijssen, Stanford University Room: BCEC 258B |
Monday, March 4, 2019 8:00AM - 8:12AM |
A61.00001: Phase Coexistence in a 2D System of Granular Self-propelled Particles Zhejun Shen, Narayanan Menon We study a system of vibrated self-propelled granular particles on a horizontal plate within a circular boundary covered by an acrylic top. The particles are square and designed to have polar activity along one body diagonal. With fixed particle size, we find that the activity of particles (quantified by the persistence length of motion along the mobility direction) can be controlled by varying the gap between top cover and bottom base. We then study the phase behaviour of the particles as the total number of particles in the system is varied. For a fixed value of activity, particles are uniformly distributed in the system at low density. As the density is increased, particles separate into a high-density ordered region at the boundary of the system, while the remaining particles remain uniformly distributed in the in the central area. Thus the system shows phase coexistence between a mobile, liquid phase and a dense, low-mobility ordered phase. |
Monday, March 4, 2019 8:12AM - 8:24AM |
A61.00002: Calculation of Critical Nucleation Rates by the Persistent Embryo Method: Application to Quasi Hard Sphere Models Shang Ren, Yang Sun, Feng Zhang, Alex Travesset, Cai-Zhuang Wang, Kai-Ming Ho We study crystal nucleation of the Weeks-Chandler-Andersen (WCA) model, using the recently introduced Persistent Embryo Method (PEM). The method provides detailed characterization of pre-critical, critical and post-critical nuclei, as well as nucleation rates that compare favorably with those obtained using other methods (umbrella sampling, forward flux sampling or seeding). We further map our results to a hard sphere model allowing to compare with other existing predictions. Implications for experiments are also discussed. |
Monday, March 4, 2019 8:24AM - 8:36AM |
A61.00003: Magnetotactic bacteria droplets : a controllable motor Benoit Vincenti, Gabriel Ramos, Maria-Luisa Cordero, Carine Douarche, Rodrigo B Soto, Eric Clement We confine magnetotactic bacteria in spherical droplets suspended in oil. The application of a uniform and constant magnetic field induces a focusing of the bacteria at the North and South poles of the droplets due to both alignment with the magnetic field and confinement by the droplet curvature. The focusing of the bacteria at these particular points can eventually create a vortex when the bacteria are densely packed. The spatial characteristics of the vortex flow are extracted with PIV analysis. The dependences of the vortex flow with respect to the magnetic field strength and the droplets radii are quantitatively analyzed and correlated to the flow created outside the droplet by the collective rotation of bacteria. The outstanding applications allowed by such a system are finally discussed in terms of energy harvesting. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A61.00004: Boosted Annealing of Colloidal Monolayers using Active Dopants Sophie Ramananarivo, Etienne Ducrot, Jeremie Palacci Active particles are microscopic particles, which can inject energy locally and made available by recent progress in colloidal science. They are ideal "pump-probes" to explore the emergent properties in non-equilibrium soft systems and control the behavior of soft matter and self-assembly at the microscale. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A61.00005: Rectification in non-equilibrium gyroscopic networks Zhenghan Liao, William T. M. Irvine, Suriyanarayanan Vaikuntanathan One of the goals of molecular engineering is to find design principles for rectification in microscopic conditions, which could lead to the fabrication microscopic motors. Past studies have successfully achieved rectifications in few-body or terminal systems, however, the extension to many-body system is not straightforward. We study energy rectification in a many-body model that combines the active bath and the gyroscopic metamaterials, and numerically and theoretically show that spontaneous heat rectification is achieved at the steady state. We then focus on understanding two issues: the mechanism of this rectification, and the relationship between the network geometry and the flow pattern. Given the knowledge of chiral eigenmodes in isolated gyroscopic networks, the rectification mechanism can be understood as a result of the weighted excitation of these modes. By expanding the heat flux as a summation over paths in the weak interaction regime, we can understand the flow pattern in a complex network from simple paths, which in turn enables us to control the rectification by the design of the network geometry. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A61.00006: Characterizing elusive correlation lengths by computable information density Stefano Martiniani, Yuval Lemberg, Paul M Chaikin, Dov Levine The analysis of computable information density (CID) has recently been introduced as a general approach to quantifying order in equilibrium and non-equilibrium many-body systems, both discrete and continuous, even when the underlying form of order is unknown. The approach has been shown to reliably identify phase transitions, determine their character, and to quantitatively predict certain dynamical critical exponents without prior knowledge of the order parameters. A natural question is whether CID may also inform us on the existence of diverging correlation lengths, and their exponents, in the proximity of phase transitions. We study the CID flow under a renormalization group transformation, and show how this can be exploited to extract correlation lengths and their critical exponents without knowledge of the system's specific correlation functions. To demonstrate the greater generality of the approach, we consider a system for which a simple analysis based on pair-correlation functions cannot detect the diverging correlation length. Hence, with this work, we introduce a new approach for the identification of elusive correlation lengths. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A61.00007: Surfactant effect on collective dynamics of surface-associated bacterial particles Mingfei Zhao, Xin Yong Bacteria exhibit collective behavior on moist agar surface to expand and acquire new territories. Self-produced surfactant could entail a significant surface activity due to the reduction of the interfacial friction between moving bacteria and substrates. The bacteria density determines surfactant concentration, so that surface motility will vary locally. The contribution of the density-dependent motility to the collective behavior of bacteria communities remains unclear. We use a discrete self-propelled particle model to simulate an ensemble of rod-like bacteria without the background fluids. The self-propulsion speed is coupled with the local surfactant concentration, solved by a diffusion equation that describes the surfactant production and transport. Simulations between constant self-propulsion speeds will serve as the comparison to test the influence of surfactant. Our investigation will provide a deep understanding of how surfactant influence the collective behavior of bacteria, and further inspire new designs of active materials. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A61.00008: Transverse Temperature Interfaces in the Katz-Lebowitz-Spohn Driven Lattice Gas Ruslan Mukhamadiarov, Priyanka ., Uwe Claus Tauber We explore the intriguing spatial patterns that emerge in a two-temperature Katz-Lebowitz-Spohn (KLS) model in two dimensions, a driven lattice gas with attractive nearest-neighbor interactions and periodic boundary conditions. The domain is split into two regions with hopping rates governed by different temperatures T > Tc and Tc, respectively, where Tc indicates the critical temperature for phase ordering, and with the temperature boundaries oriented transverse to the drive. In the hotter region, the system behaves like the (totally) asymmetric exclusion processes (T)ASEP, and experiences particle blockage in front of the interface to the critical region. We argue that transport in the subsystem is impeded by the lower current in the cooler region, which tends to set the global stationary particle current value. We observe the density profiles in both high-and low-temperature subsystems to be strikingly similar to the well-characterized coexistence and maximal-current phases in (T)ASEP models with open boundary conditions. If the lower temperature is set equal to Tc, we instead detect the corresponding critical power law density decay. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A61.00009: Dynamical fine-tuning to external forcing in disordered networks of bistable springs Hridesh Kedia, Deng Pan, Jeremy L England A driven many-body system can absorb energy from an external drive quite differently depending on the system's internal configuration. Thus, the nonequilibrium exploration of configuration space can be strongly biased by the matching between external drive and system response properties. We demonstrate such emergent, adaptive fine-tuning in simulations of an externally forced, disordered mechanical network of bistable springs under a variety of driving conditions. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A61.00010: Ordering and Synchronization in Driven Systems Prashant Sharma Active matter systems display a rich phenomenology of ordered phases and dynamic pattern formation that can be described by models of self-propelled particles with interactions. We introduce a class of models relevant for studying magnetic nanoparticles (and magnetotactic bacteria) in a fluid at low Reynolds number, and explore the different ordered structures observed in these models and their relevance to experimental systems. We analyze the stability of the ordered structures using ideas of stochastic thermodynamics and discuss how this approach solves the problem of general stability analysis of non-stationary ordered states of active matter. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A61.00011: Chemical oscillators on star graphs, theory and experiment Michael Norton, Nathan D P Tompkins, Baptiste Blanc, Matthew Cambria, Jesse Held, Seth Fraden Oscillator networks represent a large class of physical systems in both the natural and engineered worlds. Here we present experimental data for chemical oscillators on star graphs with inhibitory coupling. We examine dynamics as a function of star-degree (number of nodes coupled to a central hub) and coupling strength. We control both by loading a water-in-oil Belousov-Zhabotinsky emulsion (drops ∼100μm) into etched-Si wafer wells. We observed three dynamical attractors: (1) a phase locked state in which the arm-nodes form a synchronized cluster, (2) center-silent dynamics in which the hub well is inhibited by the arm nodes and (3) unlocked dynamics. We developed theory at two levels: a chemically realistic discrete reaction-diffusion model and a phase model; we found excellent agreement to experiment. In particular, in the locked state, we find non-trivial dependence of the locking angle between the arm nodes and the hub as a function of star degree. Finally, we demonstrate that the system can be dynamically reconfigured through photo-inhibition of targeted drops. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A61.00012: Control of a microfluidic three-ring chemical oscillator network Maria Eleni Moustaka, Michael Norton, Chris Simonetti, Seth Fraden Inspired by the collective behavior and synchronization patterns, which are a common phenomenon in nature, our work focuses on studying the non-linear dynamics of microfluidic networks of Belousov-Zhabotinsky (BZ) chemical oscillators. In our experiments, the auto-catalytic, light-sensitive, BZ reaction is confined to micro-fabricated wells constructed from the elastomer PDMS. Using soft lithography, PDMS networks are arranged into wells with controlled topology. Each well can be regarded as a single network node that sends and receives inhibitory signals. We are particularly interested in the dynamics of a 3-node ring network. This network exhibits spontaneous chiral behavior. In theory, control over the chirality can be achieved by exploiting the light sensitivity of the BZ catalyst, which can modulate the frequency of a node. In experiment, we perturb the network by changing the light intensity and duration in each of the three BZ wells. This technique provides a model of gait switching in central pattern generators and a dynamic method of information storage. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A61.00013: Minimal Model for an Energy Cascades in Driven Nonequilibrium Systems Guram Gogia, Justin Burton Complex systems that are fed energy at their constituent-level frequently exhibit self-organizational and emergent properties. The external driving can be periodic, but more commonly it comes in the form of noise. Here we show how a spatially extended layer of charged particles exhibits intermittent switching between crystalline and gas-like states. The particles are driven by individual charge fluctuations which feed energy into one vibrational degree of freedom. A small amount of disorder leads to recurrent energy cascades (melting) in the system. Using normal mode analysis, we show that the fraction of vertical vibrational modes with low participation ratio determine the response of the system to the noisy driving. We propose a minimal model of the transfer of the kinetic energy between vertical and horizontal degrees of freedom using modified Lotka-Volterra equations. The model reproduces all of the salient features of the energy cascades observed in the experiment and simulation. Similar to the Reynolds number in fluid flow, we characterize a dimensionless number that is the ratio of the energy input and dissipation. Intermittent energy cascades are observed for a narrow range of this number, in striking resemblance to transition to turbulence in pipe flow. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A61.00014: Thermodynamic Efficiency in Dissipative Chemical/Supramolecular Processes Emanuele Penocchio, Riccardo Rao, Massimiliano Esposito Out-of-equilibrium chemistry is not anymore a prerogative of nature. In recent years, fuel-driven self-assembly became a paradigmatic example of how chemists were able to access the realm of nonequilibrium processes. In the meantime, theoretical physicists achieved a deep understanding of these phenomena, which resulted in rigorous formulations of nonequilibrium thermodynamics for chemical systems. In this work, we crossbreed experimental and theoretical cutting edge research by building a quantitative thermodynamic description for two classes of chemical dissipative processes: energy storage and dissipative synthesis, which boast experimental examples in supramolecular chemistry. The former consists in storing chemical energy in the form of high-energy molecules, whereas the latter in synthesizing molecules by consuming fuel species. As nascent thermodynamics did for heat engines, we treat these systems as chemical engines and develop a quantitative framework for evaluating their efficiency. In doing so, we set the foundation for performance analysis of generic dissipative chemical processes. |
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