Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A28: Focus Session: Statistical Physics of Active Systems Away from Detailed Balance |
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Chair: Aparna Baskaran, Brandeis University Room: 336 |
Monday, March 18, 2013 8:00AM - 8:36AM |
A28.00001: Motility-Induced Phase Separation in Active Matter: a generic formalism for active brownian particles and run-and-tumble particles Invited Speaker: Julien Tailleur In this talk I will show that several classes of active particles admit an identical coarse-grained description in terms of fluctuating hydrodynamic fields. This equivalence holds as long as the microscopic parameters (e.g. swim speed $v$, diffusivity or tumbling rate), that may be spatially varying, depend on the local density $\rho$ of particles but not on their orientation. This equivalence can thus extend to interacting particles and shows that motility-induced phase separation is generic in these systems: a steeply enough decreasing $v(\rho)$ generates phase separation in dimensions d=1,2,3. I will discuss the consequences of this phenomenon for pattern formation in bacterial colony and effective temperatures in Active Matter. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A28.00002: Defect Interactions in Active Nematics Xu Ma, Mark J. Bowick, Luca Giomi, M. Cristina Marchetti Topological defects play prominent roles in passive nematic systems, but defect-antidefect pairs ultimately attract and annihilate in a finite time as the system coarsens and approaches its uniform ground state. The situation changes in active systems, which generate energy at the level of the microscopic constituents. We discuss analytic and numerical studies of two-dimensional active nematics focusing on the ability of activity to generate both defect production and annihilation and to stabilize defect-antidefect pairs at arbitrarily long times. In particular we analyze the dynamics of defect pair annihilation as a function of activity and friction and compare to experimental systems consisting of active bundled microtubule suspensions. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A28.00003: Structure and Dynamics of a Phase-Separating Active Colloidal Fluid Gabriel Redner, Michael Hagan, Aparna Baskaran We examine a minimal model for an active colloidal fluid in the form of self-propelled Brownian spheres that interact purely through excluded volume. Despite the absence of an aligning interaction, this system shows the signature behaviors of an active fluid, including anomalous number fluctuations and phase separation. Using simulations and analytic modeling, we quantify the phase diagram and separation kinetics. We show that this nonequilibrium active system undergoes an analog of an equilibrium continuous phase transition, with a critical point and a binodal beneath which the system separates into dense and dilute phases whose concentrations depend only on activity. The dense phase is a unique material that we call an active solid, which exhibits the structural signatures of a crystalline solid near the crystal-hexatic transition point, and anomalous dynamics including superdiffusive motion on intermediate timescales. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A28.00004: Thermally Active Colloids Jack Cohen, Ramin Golestanian We present a model of thermally active colloids that can propel and act as heat sources through absorption of light. We study the resulting dynamics of a system of many of these interacting colloids. The interplay between light absorption, long range fields and diffusion leads to novel collective dynamics. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A28.00005: Active Colloids, a new building block for smarter materials Invited Speaker: Jeremie Palacci Self-propelled micro-particles are intrinsically out-of-equilibrium. This renders their physics far richer than that of passive colloids while relaxing some thermodynamical constraints and give rise to the emergence of complex phenomena e.g. collective behavior, swarming\textellipsis I will present a new form of self-assembly originating from non-equilibrium driving forces. When activated by light, a set of new self-propelled particles spontaneously assemble into \textit{living crystals} which form, break, explode and reform somewhere else. We will show that this complex dynamics originates in the competition between self-propulsion of the particles attractive interactions induced respectively by osmotic and phoretic effects. The particles can moreover be steered by an external magnetic field. Light activated and steerable self propelled particles new perspectives in the design and the properties of smarter materials. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A28.00006: Asymmetric gears in a bacterial bath: Crossover between equilibrium and active motion Ayhan Duzgun, Jonathan Selinger A fundamental distinction between active matter and equilibrium systems is that active matter is not governed by the conventional laws of thermodynamics. As a specific example, recent experiments have put asymmetric gears into a ``bacterial bath,'' in which bacteria consume food, propel themselves forward, collide into the gears, and induce asymmetric rotation, thus converting chemical energy into mechanical work (Sokolov et al, 2010). By comparison, the same gears would not rotate in a thermal bath, because the second law of thermodynamics prohibits converting equilibrium thermal energy into mechanical work. This experiment leads to the basic question of what makes the difference between self-propelled motion and equilibrium thermal motion. To address this question, we perform simulations of a gear in a bacterial bath, following the approach of Angelani et al (2009); these simulations confirm that bacterial motion leads asymmetric rotation. We then modify the equations of motion, interpolating between bacteria and equilibrium Brownian particles, and determine the motion of the gear. These results help to identify what features of active bacterial motion are necessary to violate the laws of thermodynamics and generate rotation, and how these features can be controlled. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A28.00007: Microfluidic ratchets: from bacterial separation to sperm guidance Carlos Condat, Ivan Berdakin, Veronica Marconi, Alejandro Guidobaldi, Laura Giojalas, Alejandro Silhanek, Yogesh Jeyaram, Victor Moshchalkov, Lyn Venken, Jozef Vanderleyden It has been shown that a suitably built asymmetric microdevice can be used to separate and select self-propelled microorganisms. The efficiency of this rectification effect depends on the detailed dynamics of the individual microorganism. In the case of run-and-tumble bacteria we show that the distribution of run lengths and the partial preservation of run orientation memory through a tumble are important factors when computing the rectification efficiency. In addition, we show that this ratchet effect can be used to separate or concentrate sperm cells. Using a simple phenomenological model we optimize the geometry of the confining habitat in order to accumulate the cells. Both swimming strategy and swimmer size should be taken into account to optimize the design of a micro-patterned architecture for a device that can be used for effective physical bacterial separation or sperm guidance. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A28.00008: Establishing the Turing mechanism using synthetic cells Camille Girabawe, Nathan Tompkins, Ning Li, G. Bard Ermentrout, Irving R. Epstein, Seth Fraden In 1952 Alan Turing published his seminal pape~\textit{The Chemical Basis of Morphogenesi~}in which he described a basis for physical morphogenesis due solely to a reaction-diffusion system. His mechanism has been tested extensively but remains controversial and not fully demonstrated for cellular systems. Now 60 years after its debu, we describe an experimental system that demonstrates all six of his phenomenological predictions with additional support that these observations are due specifically to the Turing mechanism itsel. Further we demonstrate a nonlinear phenomena in the same system that was not predicted by Turing and which is not explained by a linear solution analysis of the governing system equations. Finally we also demonstrate that this system undergoes chemical and physical morphogenesis as Turing suggeste. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A28.00009: Do Ion Channels Spin? Robert Shaw Ionic current flowing through a membrane pore with a helical architecture may impart considerable torque to the pore structure itself. If the channel protein is free to rotate, it will spin at significant speeds. Order of magnitude estimates of possible rotation rates are presented, as well as a few arguments why such motion could improve ion transport. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A28.00010: Interactions Between Flocks and Obstacles Pearson Miller, Nicholas Ouellette The collective behavior of interacting active particles has generated considerable interest in recent years. Many models for such behavior have been proposed, ranging from simple systems of discrete particles with ad hoc interaction rules to continuum models with assumed interaction potentials to complex, bio-inspired models of collective animal motion. But in almost all cases, the resulting emergent behavior is studied in isolated systems far from boundaries. In contrast, we present results from a computational study of a simple discrete flocking model in the presence of obstacles. We consider both the behavior of the system in restricted domains bounded by solid walls and the scattering of developed flocks off of stationary targets, and discuss discuss the relationship of our results to liquid and granular systems. [Preview Abstract] |
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