Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B28: Focus Session: Statistical Physics of Active Systems Away from Detailed Balance: Motors, Swimmers and All That |
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Sponsoring Units: GSNP Chair: Alexander Grosberg, New York University Room: 336 |
Monday, March 18, 2013 11:15AM - 11:51AM |
B28.00001: Mechanics and Stability of Healthy and Cancerous Tissues Invited Speaker: Thomas Risler We study the stability of the interface between a multilayered epithelium and its adjacent stroma. Treating the epithelium as a viscous fluid with cell division, we find a novel hydrodynamic instability that leads to the formation of fingering protrusions of the epithelium into the stroma [1]. Coupling cell division in the epithelium to the local concentration of nutrients diffusing from the stroma enhances the instability by a mechanism similar to that of the Mullins-Sekerka instability in single-diffusion processes of crystal growth [2]. This instability provides physical insight into a potential mechanism by which interfaces between epithelia and stroma undulate, and potentially by which tissue dysplasia leads to cancerous invasion. Later in the process of cancerous invasion, mechanics may also play an important part. We have recently proposed that one aspect of homeostasis is the regulation of tissues to preferred pressures, which can lead to a competition for space of purely mechanical origin and be an underlying mechanism for tumor growth. Surface and bulk contributions to growth lead to the existence of a critical size that must be overcome by metastases to nucleate macroscopic secondary tumors [3]. This property qualitatively explains the observed size distributions of metastases. Following these ideas, the influence of an externally applied osmotic stress on the long-term growth of cellular spheroids has been experimentally demonstrated [4].\\[4pt] In collaboration with M. Basan, F. Montel, M. Delarue, J. Elgeti, G. Cappello, J.-F. Joanny, Institut Curie, Centre de Recherche, UPMC Univ Paris 06, and CNRS, UMR 168, F-75005, Paris, France; and J. Prost, Institut Curie, Centre de Recherche, UPMC Univ Paris 06, CNRS, UMR 168, and 4ESPCI ParisTech, F-75005, Paris, France. \\[4pt] [1] M. Basan, J.-F. Joanny, J. Prost, and T. Risler, Phys. Rev. Lett., 106 (15), 158101 (2011).\\[0pt] [2] T. Risler and M. Basan, under review\\[0pt] [3] M. Basan, T. Risler, J.-F. Joanny, X. Sastre-Garau, and J. Prost, HFSP J., 3 (4), 265-272 (2009)\\[0pt] [4] F. Montel, M. Delarue, J. Elgeti, L. Malaquin, M. Basan, T. Risler, B. Cabane, D. Vignjevic, J. Prost, G. Cappello, and J.-F. Joanny, Phys. Rev. Lett., 107 (18), 188102 (2011). [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B28.00002: In-silico studies of the collective motility of cells crawling on a thick elastic substrate Aparna Baskaran, Arvind Gopinath, Michael Hagan Self-propelling cells crawling on elastic substrates are an example of a collective system that is driven away from equilibrium. Experiments show that such cells communicate with their neighbors by sensing the deformation of the underlying elastic substrate. We propose a minimal, over-damped Brownian dynamics simulation to mimic and study this emergent collective motility. The simulations incorporate intrinsic activity at the single cell level due to self-propulsion, noise and inter-cell interactions via the underlying elastic substrate. Elastic interactions are modeled on a pair-wise additive basis by treating each cell as a force dipole deforming the substrate. We find that self-propulsion, combined with elastic interaction is sufficient to generate the coordinated large scale streaming, migration, jamming and swirling motions observed in experiments. We extract the length and time scales characterizing these correlated motions and thresh out their dependence on activity and elastic interactions. The results are rationalized by deriving a mean-field hydrodynamic theory and studying the linear stability of the equations. Our results provide a unified picture of the patterns of collective migration resulting from mechanical interactions without overlying chemical cues. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B28.00003: Growth of Bacterial Colonies Mya Warren, Terence Hwa On hard agar gel, there is insufficient surface hydration for bacteria to swim or swarm. Instead, growth occurs in colonies of close-packed cells, which expand purely due to repulsive interactions: individual bacteria push each other out of the way through the force of their growth. In this way, bacterial colonies represent a new type of ``active'' granular matter. In this study, we investigate the physical, biochemical, and genetic elements that determine the static and dynamic aspects of this mode of bacterial growth for E. coli. We characterize the process of colony expansion empirically, and use discrete and continuum models to examine the extent to which our observations can be explained by the growth characteristics of non-communicating cells, coupled together by physical forces, nutrients, and waste products. Our results challenge the commonly accepted modes of bacterial colony growth and provide insight into sources of growth limitation in crowded bacterial communities. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B28.00004: Collective motion of squirmers in a quasi-2D geometry Andreas Z\"{o}ttl, Holger Stark Microorganisms like bacteria, algae or spermatozoa typically move in an aqueous environment where they interact via hydrodynamic flow fields. Recent experiments studied the collective motion of dense suspensions of bacteria where swarming and large-scale turbulence emerged. Moreover, spherical artificial microswimmers, so-called squirmers, have been constructed and studied in a quasi-2D geometry. Here we present a numerical study of the collective dynamics of squirmers confined in quasi-2D between two parallel walls. Because of their spherical shape the reorientation of squirmers is solely due to noise and hydrodynamic interactions via induced flow fields. This is in contrast to elongated swimmers like bacteria which locally align due to steric interactions. We study the collective motion of pushers, pullers and potential swimmers at different densities. At small densities the squirmers are oriented parallel to the walls and pairwise collisions determine the reorientation rate. In dense suspensions rotational diffusion is greatly enhanced and pushers, in particular, tend to orient perpendicular to the walls. This effects the dynamics of the emerging clusters. In very dense suspensions we observe active jamming and long-lived crystalline structures. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B28.00005: Identifying and quantifying interactions in a laboratory swarm James G. Puckett, Douglas H. Kelley, Nicholas T. Ouellette Emergent collective behavior, such as in flocks of birds or swarms of bees, is exhibited throughout the animal kingdom. Many models have been developed to describe swarming and flocking behavior using systems of self-propelled particles obeying simple rules or interacting via various potentials. However, due to experimental difficulties and constraints, little empirical data exists for characterizing the exact form of the biological interactions. We study laboratory swarms of flying {\it Chironomus riparius} midges, using stereoimaging and particle tracking techniques to record three-dimensional trajectories for all the individuals in the swarm. We describe methods to identify and quantify interactions by examining these trajectories, and report results on interaction magnitude, frequency, and mutuality. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B28.00006: Stochastic pattern transitions in large scale swarms Ira Schwartz, Brandon Lindley, Luis Mier-y-Teran We study the effects of time dependent noise and discrete, randomly distributed time delays on the dynamics of a large coupled system of self-propelling particles. Bifurcation analysis on a mean field approximation of the system reveals that the system possesses patterns with certain universal characteristics that depend on distinguished moments of the time delay distribution. We show both theoretically and numerically that although bifurcations of simple patterns, such as translations, change stability only as a function of the first moment of the time delay distribution, more complex bifurcating patterns depend on all of the moments of the delay distribution. In addition, we show that for sufficiently large values of the coupling strength and/or the mean time delay, there is a noise intensity threshold, dependent on the delay distribution width, that forces a transition of the swarm from a misaligned state into an aligned state. We show that this alignment transition exhibits hysteresis when the noise intensity is taken to be time dependent. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B28.00007: Concentrating Swimming Bacteria using Funnels: Connecting Simulation Results to Simple Random-Walk Models Yu-Guo Tao, Gary W. Slater Rectification of swimming bacteria has been observed when confined in a closed environment partitioned using porous walls with funnel shaped channels. Using Monte Carlo simulations that take into account the mechanical and thermodynamic properties of round-shape cells as well as the effect of noise on the run/tumble process, we show that the long-time behaviour of the system can be mapped onto a simple one-dimensional biased random-walk process. This implies that the many variables that are needed to describe the geometry of the system and the properties of the cells can be reduced to only two generalized variables plus the size of the system itself. We examine how these two variables depend on the initial variables and draw conclusions on the performance of the system when used as a tool to separate cells. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B28.00008: Hysteresis in transition between individual and collective behavior in suspension of swimming bacteria Andrey Sokolov, Igor Aranson We present a new method for control of motility and tumbling rate of swimming bacteria Bacillus Subtilis via precise and rapid control of temperature of the bacterial suspension. Transitions between individual and collective behaviors in a response to cyclical temperature change in a range of temperatures between 5C and 35C with the rates from 0.1C/s to 1C/s were investigated. Temperature decrease typically results in a decrease of bacterial motility while preserving low tumbling rates. The temperature increase above 20C triggers a ``heat shock'': a significant jump in tumbling rate resulting in temporal decrease of the average swimming speed and termination of collective motion. At temperatures below 20C due to relative low tumbling rates we discovered a hysteresis in the transition between individual and collective swimming: velocity correlation length vs. average swimming speed of bacteria exhibits hysteric behavior. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B28.00009: Flocking in Flow Nicholas Ouellette, Nidhi Khurana Models of active, self-propelled particles with simple interaction rules have long been shown to produce large-scale emergent behavior reminiscent of collective animal motion seen in nature. Such model flocks can be shown to be robust against random noise terms added to the equations. But real animals, such as birds, fish, or insects, live in fluid environments, where the background flow field is nonzero and is often turbulent. In this case, the fluctuations experienced by the individuals in the aggregation are not random, but rather are correlated in space and time. We explore the impact of such spatiotemporally correlated perturbations on flocking by numerically simulating the behavior of a simple flocking model in a turbulent-like flow field produced by a kinematic simulation. The introduction of flow strongly changes the flock formation dynamics. Additionally, we find that under some conditions the background flow tends to break stable flocks into smaller units. We study these clusters, and discuss their relation to the underlying flow field. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B28.00010: Pitfalls in mining active transport trajectories Kejia Chen, Bo Wang, Sung Chul Bae, Steve Granick Single particle tracking is useful in characterizing active motion. However, there are many pitfalls in mining such data, from separating the intermittently alternating active and passive motion to fitting a model to the motion. Using statistical tools, we carefully identified such pitfalls and developed new methods to avoid them. Applying this algorithm to endosomal active transport within living cells, imaged by fluorescence microscopy with nm resolution, we observed L\'{e}vy walk behavior in multiple cells lines and for different cargo types. This L\'{e}vy walk behavior could be easily missed without those statistical tools, which can be very useful in characterizing active motion and identifying regulators in other active systems. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B28.00011: Casimir Effect and Fluctuation-Induced Attractive Forces in Active Matter Cynthia Reichhardt, Lena Lopatina, Charles Reichhardt We consider the fluctuation-induced forces between plates in walls immersed in a bath of active matter, similar to the forces in the classical Casimir effect. The active matter could represent swimming bacteria. We find that the active matter causes a strong attractive force between two plates, whereas for strictly Brownian particles, there is little effect or no attraction between the plates. We discuss how the motion of the active particles, the breaking of detailed balance by the walls, and the geometry of the sample leads to a reduced particle density between the plates and produces a density-induced pressure on the plates. This result also indicates that for movable objects immersed in an active matter bath, larger objects will aggregate over time, suggesting that active matter could be used as the catalyst for a novel self-assembly method. Finally, we discuss other geometries that can produce a repulsive force between the walls, as well as the effect of flocking particles. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B28.00012: Pattern Formation in Growing Polar Bacteria Xingbo Yang, M. Cristina Marchetti, Davide Marenduzzo We analyze a continuum model of a bacterial suspension that includes motility suppression from steric repulsion, polar alignment, and bacteria reproduction and death. Using a combination of linear stability analysis and numerical solution of the nonlinear equations, we demonstrate that the model exhibits a rich variety of emergent structures, corresponding to generic patterns seen in experiments. Motility suppression in a crowded environment gives rise to a density phase separation, regulated by the growth/death of the bacteria, as demonstrated earlier by Cates et al. [PNAS 107, 11715--11720(2010)], with spherically symmetric patterns similar to those observed in \textit{S. typhimurium}. The addition of polar alignment yields new ring/band and swirl/spiral structures resembling those observed in \textit{E.coli } colonies. The stationary/traveling nature of the patterns and their symmetry is classified and summarized in a phase diagram. [Preview Abstract] |
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