Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J14: Focus Session: Physics of Active Materials |
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Sponsoring Units: GSNP Chair: Aparna Baskaran, Brandeis University Room: D227 |
Tuesday, March 22, 2011 11:15AM - 11:51AM |
J14.00001: Active Currents and Stresses on the cell surface: Clustering, Instabilities and Budding Invited Speaker: We study the contractile dynamics of a collection of active polar filaments, such as actin, on a two dimensional substrate, using a continuum hydrodynamic description in the presence of spatiotemporal noise. The steady states, characterized by a variety of phases generically consisting of a transient collection of inward pointing asters. We next study the dynamics of particles advected along these active filaments. This is relevant to the dynamics and organization of a large class of cell surface molecules. We make several predictions regarding the statistics of fluctuations of these passive advective particles which we confirm using fluorescence based experiments. We then show how such active patterning of filaments can give rise to membrane stresses leading to membrane shape deformations. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J14.00002: Pattern formation in Active Polar Fluids Arvind Gopinath, Michael Hagan, Aparna Baskaran Systems such as bacterial suspensions or cytoskeletal filaments and motility assays can be described within the paradigm of active polar fluids. These systems have been shown to exhibit pattern formation raging from asters and vortices to traveling stripes. A coarse-grained description of such a fluid is given by a scalar density field and a vector polarization field. We study such a macroscopic description of the system using weakly nonlinear analysis and numerical simulations to map out the emergent pattern formation as a function of the hydrodynamic parameters in the context of two specific microscopic models - a quasi-2D suspension of cytoskeletal filaments and motor proteins and a system of self propelled hard rods that interact through excluded volume interactions. [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J14.00003: Non-Equilibrium Dynamics of the Metaphase Spindle Daniel Needleman, Jan Brugues A wide variety of cellular structures exist in a nonequilibrium steady-state with a constant flux of molecules and energy continuously modifying and maintaining their architecture. Understanding such self-organizing structures is not only crucial for cell biology, but also poses a fundamental challenge for physics, since these systems are materials that behave drastically differently from those that have been traditionally studied in condensed matter physics. Physical theories of active materials have been used to describe the cytoskeleton, but it is still unclear how applicable these theories are to complex biological systems \textit{in vivo}. We are experimentally testing if such phenomenological theories of cytoskeletal behavior can be profitably used to model the metaphase spindle. Our approach is to use polarized light microscopy, spinning disk fluorescence microscopy, single molecule imaging, and magnetic tweezers to quantitatively measure spatial-temporal correlation functions of spontaneous fluctuations in the director, concentration, and internal stress in spindles. We are comparing these measurements with predictions from various continuum theories to determine how best to describe the non-equilibrium dynamics of these structures. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J14.00004: Modeling active materials based on self-oscillating gels Victor V. Yashin, Anna C. Balazs The Belousov-Zhabotinsky (BZ) reaction in solution is a classical example of an active medium that demonstrates various chemical oscillations and waves, which can be observed visually. Grafting a ruthenium metal-ion complex, the catalyst to the BZ reaction, to a chemo-responsive polymer gel creates an active material (BZ gel), which exhibits periodic volumetric changes in the course of the reaction. The redox oscillations of the catalyst affect the polymer-solvent interactions and cause the periodic swelling and de-swelling of the gel, so that chemo- mechanical energy transduction occurs within the material. We consider a model that couples the polymer gel dynamics and the BZ reaction kinetics; the latter is described by the modified Oregonator model. The model equations are solved numerically in 2D. We demonstrate that the dynamical behavior of the BZ gel can be controlled by a heterogeneous distribution of the catalyst and by such structural features as the solvent-filled voids. The dynamics of an active membrane having the self-oscillating pores is considered as an example. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 1:03PM |
J14.00005: Homeostatic pressure, tumor growth and fingering of epithelial tissues: Some generic physics arguments Invited Speaker: We propose 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 pressure lead to the existence of a critical size that must be overcome by metastases to reach macroscopic sizes. This property qualitatively explains the observed size distributions of metastases, while size-independent growth rates cannot account for clinical and experimental data. It also potentially explains the observed preferential growth of metastases on tissue surfaces and membranes, suggests a mechanism underlying the seed and soil hypothesis introduced by Stephen Paget in 1889, and yields realistic values for metastatic inefficiency [1]. Treating epithelial tissues as viscous fluids with effective cell division, we find a novel hydrodynamic instability that leads to the formation of fingering protrusions of the epithelium into the connective tissue. Arising from a combination of viscous friction effects and proliferation of the epithelial cells, 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.\\[4pt] [1] M. Basan, T. Risler, J.-F. Joanny, X. Sastre-Garau, and J. Prost, \textit{HFSP Journal}, \textbf{3}, 4, p.265 [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:15PM |
J14.00006: Hydrodynamics of an Active Smectic Tapan Chandra Adhyapak, Sriram Ramaswamy, John Toner We show that self-driven particles, in suspension or on a substrate, can support striped phases with long-range order in three dimensions and quasi-long-range order in two dimensions. This is in contrast to the situation for smectic phases at thermal equilibrium, which have the same spatial symmetry. We analyse the fluctuation properties of stable active smectics as well as the nature of characteristic instabilities that these systems can display. Our results apply to any active system that spontaneously develops layers, including apolar orientable cells, monolayers of rods either fluidized or shaken and, most significantly, the Rayleigh-Benard instability. [Preview Abstract] |
Tuesday, March 22, 2011 1:15PM - 1:27PM |
J14.00007: Spontaneous Oscillations in Nonlinear Active Solids Shiladitya Banerjee, Tanniemola B. Liverpool, M. Cristina Marchetti We present a generic continuum model of a nonlinear active gel with both passive and active crosslinks. The model is relevant for actin gels with passive elasticity provided by ABPs such as filamin-A or $\alpha$-actinin and dynamic active crosslinkers such as myosin-II. We consider an one dimensional continuum active solid where compressional deformations are coupled to molecular motor dynamics. Three kinds of nonlinearities are incorporated : (a) nonlinear load dependence of unbinding rate of molecular motors, (b) pressure nonlinearities stemming from excluded volume interactions, and (c) nonlinearity due to convection of bound motors along the gel. Unbinding rate nonlinearity stabilizes the oscillatory instabilities predicted by the linear theory and lead to sustained oscillations at intermediate concentrations of ATP. Pressure nonlinearity due to excluded volume interactions stabilizes the contractile instability and leads to a contracted ground state. Our work provides a generic framework for the description of the large scale properties of nonlinear isotropic active solids. [Preview Abstract] |
Tuesday, March 22, 2011 1:27PM - 1:39PM |
J14.00008: Self-diffusiophoresis in the strongly advecting regime Gareth Alexander, Andrea Liu Certain forms of biological motility, such as actin-based propulsion and chromosomal translocation in certain bacteria, have recently been proposed to have their physical origins in the phenomenon of self-diffusiophoresis. In diffusiophoresis, a particle in a fluid with an inhomogeneous concentration of solute will move along the concentration gradient with a well-defined velocity due to surface interactions with the solute. If the particle has the means of generating the concentration gradient itself--by catalyzing a chemical reaction on one side of its surface, for example--then self-diffusiophoresis serves as a mechanism of self-propulsion. Until now, self-diffusiophoresis has been studied under conditions of rapid diffusion, or small P\'eclet number, where the effects of advection on the solute dispersion can be neglected. However, in the biological examples of interest, the P\'eclet number is high. We present an analysis of the large P\'eclet number limit, where diffusion is slow and advection by the fluid flow is the primary means of solute dispersal. The resulting motion is still described in terms of a slip velocity generated in a thin boundary layer, but with a different origin, arising not from diffusion but from local outward flow to carry away the solute together with fluid continuity. A simple model is developed on this basis, contrasted with the rapid diffusion regime, and applied to provide insight into relevant biological processes. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J14.00009: Phase transitions and solitons in a rule-based model of active particles Thomas Ihle I study the Vicsek model [Phys. Rev. Lett. {\bf 75} (1995) 1226] by means of kinetic theory. In this non-equilibrium model, self-driven particles try to align their travel directions with the average direction of their neighbours. At strong alignment, rotational symmetry is spontaneously broken and a global flocking state forms. The alignment is defined by a stochastic rule, not by a Hamiltonian. The corresponding interactions are non-additive and are typically of genuine multi-body nature. Due to this and the discreteness of the time evolution, the kinetic equations are different from the usual ones found in textbooks. I derive the phase diagram for the flocking transition and show that it agrees very well with simulations at large particle velocities and is qualitatively different from the one of a continuous version of the Vicsek-model. The theory starts with the Liouville equation, the hydrodynamic equations are derived by a Chapman-Enskog expansion. These equations contain more terms than previously postulated; their coefficients are given in terms of microscopic parameters. I show how a large-wavelength instability of the flocking state leads to an inhomogeneous soliton state which is very stable and shows a first-order phase transition to the disordered state. I determine the speed of the solitons, investigate the hysteresis of the transition and estimate the system size beyond which the first order nature of the transition should be visible in computer simulations. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J14.00010: Collective motion of vibrated polar granular disks Olivier Dauchot, Deseigne Julien, Hugues Chate In many interesting situations, the interactions among self-propelled agents lead to the spontaneous emergence of self-organized collective motion. The ubiquity of the phenomenon at all scales raises the question of the existence of some underlying universal mechanisms. Recent numerical and analytical studies have confirmed the existence of a transition from a disordered state at large noise to a state with various collective properties reflecting the local symmetry of the particles and their interactions. Though, there are still very few experimental situations where the onset of collective motion can be attributed to spontaneous symmetry breaking. Here, we report on experiments conducted with both polar self propelled and a-polar Brownian disks and by comparing the dynamics of both systems in the same experimental conditions, we demonstrate without ambiguity that collective motion emerges from the interplay of self-propulsion and hard-core repulsion only [1]. Interestingly the alignment, which has no nematic origin, is effectively induced during the collisions because of the self propulsion. \\[4pt] [1] Phys. Rev. Lett 105 135702 (2010) [Preview Abstract] |
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