Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session F34: Active Matter IVFocus
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Sponsoring Units: GSOFT DBIO GSNP/DFD Chair: Cristina Marchetti, Syracuse University Room: 337 |
Tuesday, March 15, 2016 11:15AM - 11:27AM |
F34.00001: Self-Pumping Active Gel. Kun-Ta Wu, Jean Bernard Hishamunda, Seth Fraden, Zvonimir Dogic Isotropic active gels are the network which is consist of cross-linked building blocks and the structure of which changes randomly and isotropically with time. Dogic et. al. show that pairs of anti-parallel microtubules form extensile bundles, which merge, extend, and buckle. In an unconfined system, the dynamics of these bundles causes spontaneous turbulent-like flow driven by motion of microscopic molecular motors. We found that confining these active gels in a millimeter sized toroids causes a transition into a new dynamical state characterized by circulation currents persisting for hours until ATP is depleted. We show how toroid dimensions impact the properties of self-organized circular currents, how directions of circulation can be designed by engineering ratchet-shaped boundaries, and how circulations of connected toroids can be either synchronized or antisynchronized. Furthermore, we demonstrate that the flow rate in the circulation is independent of curvature and length of flow path. The flow rate persists for centimeters without decay, disregarding conventional pipe flow resistance. Such findings pave the path to self-pumping pipe transport and performing physical work with biological system. [Preview Abstract] |
Tuesday, March 15, 2016 11:27AM - 11:39AM |
F34.00002: Synchronization of oscillations in hybrid gel-piezoelectric active materials. Victor V. Yashin, Steven P. Levitan, Anna C. Balazs We model the hybrid gel-piezoelectric active material that could perform oscillator based unconventional computing tasks (“materials that compute”). The material is assumed to have a cellular structure, where each cell contains a polymer gel, which undergoes cyclic swelling and deswelling due to the oscillatory Belousov-Zhabotinsky (BZ) reaction, and is coupled to a piezoelectric (PZ) film. Upon electrical connection, oscillations in the BZ-PZ units get synchronized, and the mode of synchronization is shown to depend on the number of units in the system, type of circuit connection, etc. Introduction of capacitors into the circuits allows us to further manipulate the synchronization modes, i.e., the distinctive patterns in phase of oscillations. The results indicate the BZ-PZ systems could be used for spatio-temporal pattern recognition. [Preview Abstract] |
Tuesday, March 15, 2016 11:39AM - 11:51AM |
F34.00003: Photo-programming Semicrystalline Shape Actuators Yuan Meng, Jason Yang, Mitchell Anthamatten A semi-crystalline double network is formed that contains two types of molecular junctions: covalent junctions and reversible molecular linkages. The reversible junctions have the ability to rearrange/reshuffle upon irradiation, and, therefore give rise to a competitive double network architecture that actuates upon crystallization without an applied external load. Poly(caprolactone) networks containing reconfigurable allyl-sulfide linkages are melted, strained to various elongations (hundreds of percent), and irradiated. The network connectivity is reconfigured through a series of light-induced AFCT events, causing a unique built-in stress to be introduced. After irradiation and unloading, the resulting double networks assume a mechanical state-of-ease, and polymer strands adopt biased configurations; when cooled, they crystallize along a preferred direction leading to fully reversible shape actuation. Sample networks can be programmed in multi-steps under constant strain or constant stress, leading to different dynamics and equilibrium states-of-ease. Stress free actuation of 18 percent was achieved. [Preview Abstract] |
Tuesday, March 15, 2016 11:51AM - 12:27PM |
F34.00004: Designing Self-powered Nanomotors and Pumps Invited Speaker: Ayusman Sen Self-powered nano and microscale moving systems are currently the subject of intense interest due in part to their potential applications in nanomachinery, nanoscale assembly, robotics, fluidics, and chemical/biochemical sensing. We will demonstrate that one can build autonomous nanomotors over a wide range of length-scales ``from scratch'' that mimic biological motors by using catalytic reactions to create forces based on chemical gradients. These motors are autonomous in that they do not require external electric, magnetic, or optical fields as energy sources. Instead, the input energy is supplied locally and chemically. These "bots" can be directed by information in the form of chemical and light gradients. Furthermore, we have developed systems in which chemical secretions from the translating nano/micromotors initiate long-range, collective interactions among themselves. This behavior is reminiscent of quorum sensing organisms that swarm in response to a minimum threshold concentration of a signaling chemical. In addition, an object that moves by generating a continuous surface force in a fluid can, in principle, be used to pump the fluid by the same catalytic mechanism. Thus, by immobilizing the nano/micromotors, we have developed nano/microfluidic pumps that transduce energy catalytically. These non-mechanical pumps provide precise control over flow rate without the aid of an external power source and are capable of turning on in response to specific analytes in solution. [Preview Abstract] |
Tuesday, March 15, 2016 12:27PM - 12:39PM |
F34.00005: Emergent Ultra-Long-Range Interactions Between Active Particles in Hybrid Active-Inactive Systems Joshua Steimel, Juan Aragones, Helen Hu, Naser Qureshi, Alfredo Alexander-Katz Particle-particle interactions determine the state of a system. Control over the range and magnitude of such interactions has been an active area of research for decades due to the fundamental challenges it poses in science and technology. Effective interactions between active particles have gathered much attention as they can lead to out-of-equilibrium cooperative states such as flocking. Inspired by nature, where active living cells coexist with lifeless, immobile objects and structures, here we study the effective interactions that appear in systems composed of active and passive mixtures of colloids. Our system is a two dimensional colloidal monolayer composed primarily of passive (inactive) colloids and a very small fraction of active (sinning) ferromagnetic colloids. We find an emergent ultra-long-range attractive interaction between active particles induced by the activity of the spinning particles and mediated by the elasticity of the passive medium. Interestingly, the appearance of such interaction depends on the spinning protocol and has a minimum actuation time scale below which no attraction is observed. Overall, these results clearly show that in the presence of elastic components, active particles can interact across very long distances without any chemical modification of the environment. Such a mechanism might potentially be important for some biological systems and can be harnessed for newer developments in synthetic active soft materials. [Preview Abstract] |
Tuesday, March 15, 2016 12:39PM - 12:51PM |
F34.00006: Noise and diffusion in vibrated self-propelled particles Lee Walsh, Sarah Schlossberg, Aparna Baskaran, Narayanan Menon Active-matter systems are often modeled in the lab by studying the two-dimensional dynamics of granular particles driven by vibration in the third dimension. If the vibrational noise is rectified by the shape of the particle, the resulting motion of the particle shows directed motion superimposed on diffusion. We use particles designed for polar motion along a body axis as well as others that break isotropy in various ways. The long-term motion is typically theoretically modeled by a Langevin equation that encodes a self-propulsion velocity along the body axis as well as uncorrelated rotational and translational noise, all of which are treated as independent parameters. For a dilute system of granular tiles confined to a horizontal plane and vertically vibrated, we measure the long-time single-particle dynamics as well as the short-time distributions of translational and rotational motion. From these we characterize the different correlation functions that determine the noise and test the assumptions of the conventional Langevin dynamics used for self-propelled particles. [Preview Abstract] |
Tuesday, March 15, 2016 12:51PM - 1:03PM |
F34.00007: Glassy dynamics of self-propelled particles Elijah Flenner, Ludovic Berthier, Grzegorz Szamel We examine the glassy dynamics of a system of self-propelled, interacting particles. The self-propulsion is described as an internal driving force that evolves according to the Ornstein-Ulenbeck process. It can be characterized by an effective temperature and a persistence time for the self-propelled motion. For a fixed effective temperature, as the persistence time approaches zero the particles dynamics becomes equivalent to overdamped Brownian (thermal) dynamics. Our goal is to investigate how the average structure and dynamics evolves with increasing persistence time, which corresponds to increasing departure from the Brownian limit. To this end we simulate a system whose glassy dynamics has been extensively studied in Brownian dynamics simulations, the Kob-Andersen binary mixture. We examine how the effective mode-coupling transition, the fragility and heterogeneous dynamics change with increasing persistence time. [Preview Abstract] |
Tuesday, March 15, 2016 1:03PM - 1:15PM |
F34.00008: Emergent motion patterns of delay-coupled swarms Klementyna Szwaykowska, Luis Mier-y-Teran-Romero, Ira Schwartz Emergent pattern-forming behaviours of aggregates of interacting autonomous agents are a topic of great interest in complex systems research, with applications including biology, environmental monitoring, and defence. We model, and experimentally verify, pattern formation in a swarm of delay-coupled agents, using a simple but general model of agent interactions. Using mean-field dynamics, we perform a thorough analytical study of the bifurcation structure as a function of network connectivity and delay to describe the emergence of pattern formation. We show that swarm motion patterns observed for a homogeneous swarm with all-to-all communication are robust to decreasing network connectivity and to heterogeneity in the parameters governing individual agent behaviours. We perform systematic numerical studies to show where the mean-field theory deviates from simulation and experiment. [Preview Abstract] |
Tuesday, March 15, 2016 1:15PM - 1:27PM |
F34.00009: Trapping and sorting active granular rods Sriram Ramaswamy, Nitin Kumar, Harsh Soni, Rahul Gupta, Ajay Sood We report experiments and simulations on collective trapping in a horizontal monolayer of tapered granular rods rendered motile by mechanical vibration. A macroscopic fraction of the particles are trapped by a V-shaped obstacle if its opening angle is less than a threshold value of about 120 degrees, consistent with active Brownian simulations [PRL 108, 268307 (2012)]. the transition between trapped and untrapped states becomes sharper with increasing system size in our numerical studies. We offer a theoretical understanding of this nonequilibrium phase transition based on collective noise suppression and an analysis of fluxes. We show also that the trap can serve to separate particles based on their motility and rotational diffusivity. [Preview Abstract] |
Tuesday, March 15, 2016 1:27PM - 1:39PM |
F34.00010: Pattern Formation in Driven Systems Katherine Klymko Model colloidal particles of two types, driven in opposite directions, will in two dimensions segregate into lanes, a phenomenon studied extensively by Lowen and co-workers [Dzubiella et al. Phys. Rev. E 65, 021402 (2002)]. We have simulated mixtures of oppositely-driven particles using three numerical protocols. We find that laning results from enhanced diffusion, in the direction perpendicular to the drive, of particles surrounded by particles of the opposite type, consistent with the observation of Vissers et al. [Soft Matter 7, 6, 2352 (2011)]. By comparing protocols we find that enhanced diffusion follows from a simple geometrical constraint: oppositely-driven particles must, in the time taken to encounter each other in the direction of the drive, diffuse in the perpendicular direction by about one particle diameter. This constraint implies that the effective lateral diffusion constant grows linearly with drive speed and as the square root of the packing fraction, a prediction supported by our numerics. By invoking an analogy between hard particles with environment-dependent mobilities and mutually attractive particles we argue that there exists an equilibrium system whose pattern-forming properties are similar to those of the driven system. [Preview Abstract] |
Tuesday, March 15, 2016 1:39PM - 1:51PM |
F34.00011: How many dissenters does it take to disorder a flock? David Yllanes, M. Cristina Marchetti Minimal models of active particles have had much success in the study of flocking behavior. Typically one considers a system of self-propelled particles with noisy aligning interactions. By varying the density of the system or the intensity of the noise one can switch between a disordered phase where the particles move randomly and independently and a flocking state where the velocities of the particles are aligned. In this work we consider what happens if a fraction $p$ of the particles does not experience the aligning interaction. This is an interesting problem from a statistical mechanics point of view, with applications to collective behavior of living systems, where not all the members of a community (a flock of birds, a herd of sheep, etc.) behave in the same way. By carrying out extensive molecular dynamics simulations we show that even a very small fraction of such "individualistic" particles can have a dramatic effect on the whole system and, indeed, that the flocking can be destroyed for a very low value of $p$. [Preview Abstract] |
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