Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G11: Focus Session: Active Soft Matter II - Dynamical Response |
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Sponsoring Units: DPOLY GSNP DBIO Chair: Zvonimir Dogic, Brandeis University Room: 203 |
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G11.00001: Active nematics of flat and spherical surfaces Invited Speaker: Zvonimir Dogic The laws of equilibrium statistical mechanics impose severe constraints on the properties of conventional materials assembled from inanimate building blocks. Consequently, such materials cannot exhibit spontaneous motion or perform macroscopic work; i.e., a fluid in a beaker remains quiescent unless driven by external forces. Inspired by biological phenomena such as ciliary beating or \textit{Drosophila} cytoplasmic streaming our aim is to develop a new category of materials assembled from animate, energy-consuming building blocks. Starting from a few well-characterized biochemical components we assemble and study far-from-equilibrium analogs of conventional liquid crystals. Released from the constraints of equilibrium, this internally driven polymeric material exhibits a host of highly-sought after properties including appearance of steady-state streaming flows that are accompanied by the spontaneous unbinding and annihilations of motile defects as well as appearance and subsequent self-healing of fracture lines. Active liquid crystals can serve as a platform for developing novel material applications, testing fundamental theoretical models of far-from-equilibrium active matter and potentially shedding light on self-organization in living cells. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G11.00002: Visualization of the material response in an actomyosin network at the onset of internal motor activity Samantha Stam, Margaret Gardel The actomyosin cortex of living cells generates forces that drive structural rearrangements at the sub-cellular, cellular, and tissue length scales during cell migration, cell division, and tissue formation.~In the cortex, filaments of myosin II motors actively generate stresses on actin filament networks and bundles to form an active contractile material. However, how the spatial and temporal regulation of contractile deformation is affected both by local stresses and the material response is not understood.~For instance, the extent to which the stress-strain relationship within active networks may be understood with governing equations from continuum elasticity is unknown. Here, we directly measure strain fields within quasi-2D actin networks subjected to varying degrees of internal myosin activity. We observe evidence that both the motor-generated stress and its propagation are regulated by network properties such as cross-linking. Stresses propagate anisotropically and produce a non-trivial material response even at motor concentrations much lower than those required to observe robust contractile behavior. Our data yield insights into how cellular networks make use of varying microstructures to regulate motor-generated force and the resulting strain. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G11.00003: Quantifying actin wave modulation on periodic topography Can Guven, Meghan Driscoll, Xiaoyu Sun, Joshua Parker, John Fourkas, Anders Carlsson, Wolfgang Losert Actin is the essential builder of the cell cytoskeleton, whose dynamics are responsible for generating the necessary forces for the formation of protrusions. By exposing amoeboid cells to periodic topographical cues, we show that actin can be directionally guided via inducing preferential polymerization waves. To quantify the dynamics of these actin waves and their interaction with the substrate, we modify a technique from computer vision called ``optical flow." We obtain vectors that represent the apparent actin flow and cluster these vectors to obtain patches of newly polymerized actin, which represent actin waves. Using this technique, we compare experimental results, including speed distribution of waves and distance from the wave centroid to the closest ridge, with actin polymerization simulations. We hypothesize the modulation of the activity of nucleation promotion factors on ridges (elevated regions of the surface) as a potential mechanism for the wave-substrate coupling. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G11.00004: Characterizing tunable dynamics in an active gel Gil Henkin, Stephen DeCamp, Daniel Chen, Zvonimir Dogic We experimentally investigate dynamics of an active gel of bundled microtubules that is driven to far-from-equilibrium steady states by clusters of kinesin molecular motors. Upon the addition of ATP, the coordinated action of thousands of molecular motors drives this gel to an active, percolating state that persists for hours and is only limited by the stability of constituent proteins and the availability of the chemical fuel ATP. We extensively characterize how enhanced transport in emergent macroscopic flows depends on relevant molecular parameters, including ATP, motor, and depletant concentrations, microtubule concentration and length, as well as structure of the motor clusters. Our results show that the properties and dynamics of this active isotropic gel are highly tunable, suggesting that this is an ideal system for studying the behavior of active materials. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G11.00005: Tuning mechanical relaxation through the regulation of non-equilibrium actin assembly Patrick M. McCall, David R. Kovar, Margaret L. Gardel Two of the most fundamental differences between the biopolymer filamentous actin (F-actin) and more conventional synthetic polymers are its semiflexibility and intrinsically non-equilibrium (active) nature. While the consequences of semiflexiblity on the mechanics of F-actin-based materials have received much study, less is known about the role of non-equilibrium dynamics. A major roadblock to experimental progress in this regard is that the assembly dynamics of purified actin at steady-state are too slow for appreciable effects to be observed. Taking a cue from living cells, we address this problem by polymerizing actin in the presence of the actin regulatory proteins formin, profilin, and cofilin, which promote filament elongation, nucleotide exchange on monomers, and filament disassembly through severing, respectively, to increase the rates of these processes. The mechanics of the resulting entangled F-actin solution is then monitored at steady-state with passive particle tracking microrheology. At fixed formin and profilin concentrations, the self-diffusion time of 1-micron tracer particles drops by more than two orders of magnitude as the cofilin concentration is increased above a molar ratio threshold of 10{\%}. In addition, the elastic plateau gives way to anomalous scaling with a crossover time that shifts with cofilin concentration. Interestingly, these effects are not observed in the absence of formin, indicating filament treadmilling as the likely mechanism of the enhanced relaxation. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G11.00006: Non-equilibrium States of Active Filament Networks Robert A. Blackwell, Meredith D. Betterton, Oliver M. Sweezy, Matthew A. Glaser Active networks of filamentous proteins and crosslinking motor proteins play a critical role in many cellular processes. Among the most important active networks is the mitotic spindle, an assembly of microtubules and crosslinking motor proteins that forms during cell division and that ultimately separates chromosomes into two daughter cells. To evolve a better understanding of spindle formation, structure, and dynamics, we have developed course-grained models of active networks composed of filaments, modeled as hard spherocylinders, in diffusive equilibrium with a reservoir of crosslinking motors, modeled as Hookean springs that can adsorb to microtubules and translocate at finite velocity along the microtubule axis. We explore the phase diagram and other characteristics of this model in two and three dimensions as a function of filament packing fraction, and of crosslink concentration, velocity, and adsorption and desorption rates. We observe a variety of interesting emergent behaviors including sorting of filaments into polar domains, generation of extensile stress, and superdiffusive transport. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G11.00007: Dynamics and rheological applications of chiral nanostructures Arijit Ghosh, Ambarish Ghosh We have developed a system of magnetic chiral (helical) nanostructures that can be actuated in fluidic environments with rotating magnetic fields. These objects demonstrate interesting dynamical behavior, determined by the counteracting applied and viscous torques. Under certain experimental conditions, a bistable chaotic dynamics could be observed. This is surprising, since motion at low Reynolds' numbers is typically deterministic. We have developed an analytical theory to understand this dynamics, which has led to the development of a novel tool for micro-rheological measurements. We will demonstrate how the helical nanostructures can be used to map a complex rheological environment with micron scale spatial resolution, in a measurement time significantly shorter than all other currently used techniques. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G11.00008: Confocal Rheometry of Active Networks Daniel Chen, Stephen Decamp, Daniel Blair, Zvonimir Dogic While much is known about the rheological responses of passive biopolymer networks, we currently lack a conceptual framework to describe active networks under shear.~ To this end, we have engineered an active gel composed of microtubules, bidirectional kinesin motors, and molecular depletant that self-organizes into a highly dynamic network of bundles.~ The network continually remodels itself under ATP-driven cycles of extension, buckling, fracturing, and self-healing [1].~ In this talk I will present comprehensive confocal rheometry measurements elucidating the interplay between the network's dynamic morphology and its linear and non-linear rheological responses. [1] T. Sanchez, D.T.N. Chen, S.J. Decamp, M. Heymann, and Z. Dogic Nature 491 (7424), 431-434 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G11.00009: Chain-configuration dependent rheological properties in transient networks Michelle Sing, Zhen-Gang Wang, Gareth McKinley, Bradley Olsen Complex associative networks capable of shear thinning followed by recovery on the order of seconds are of interest as injectable biomaterials. However, there is a limited understanding of the molecular mechanisms that contribute to rheological properties such as the network's yield stress and rate of self-healing. Here we present a transient network theory for associative physical gels arising from the chemical kinetic form of the Smoluchowski Equation capable of modeling the full chain end-to-end distance distribution while tracking the fraction of looped, bridged, and free chain configurations in the gel. By varying the equilibrium association rate relative to the material relaxation time, we are able to track the evolution of loop and bridge chain fraction as the system undergoes stress instabilities. We have evidence that these instabilities result from non-monotonic trends in loop and bridge chain fraction when the end group association rate is high relative to the dissociation rate. This behavior provides insight into the complex kinetic interactions responsible for certain mechanical behaviors while serving as a valuable predictive tool for gel design. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G11.00010: Effects of filament rigidity in myosin II-induced actin network contractility and dynamics Kimberly Weirich, Margaret Gardel Cells change shape, deforming to move and divide. The dynamic protein scaffold that shapes the cell is the cortex, a disordered, thin network of actin filaments. Random, local stresses generated by myosin II in the network create cellular-scale deformations. Myosin induced buckling and severing of actin filaments has been shown to underlie the contractility of two-dimensional disordered actin networks. This non-linear elastic response of actin filaments is thought to be an essential symmetry breaking mechanism to produce robust contractility in disordered actomyosin networks. To test this idea, we explore the effects of an actin bundling protein fascin, a crosslinker which induces polarity specific bundling of actin filaments, to create a network of F-actin bundles. We investigate myosin-induced stresses in a network of randomly oriented actin filaments, confined to a thin sheet at a supported lipid bilayer surface through a crowding agent. We find fascin-bundled filaments are less prone to filament buckling and show increased filament sliding, causing the myosin activity to induce network reorganization rather than contraction. Thus, changes in the filament bending rigidity in motor-filament systems can drive the system between distinct states with unique dynamic and mechanical signatures. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G11.00011: Defect Dynamics in Active 2D Nematic Liquid Crystals Stephen DeCamp, Gabriel Redner, Michael Hagan, Zvonimir Dogic Active materials are assemblies of animate, energy-consuming objects that exhibit continuous dynamics. As such, they have properties that are dramatically different from those found in conventional materials made of inanimate objects. We present a 2D active nematic liquid crystal composed of bundled microtubules and kinesin motor proteins that exists in a dynamic steady-state far from equilibrium. The active nematic exhibits spontaneous binding and unbinding of charge $+$1/2 and -1/2 disclination defects as well as streaming of $+$1/2 defects. By tuning ATP concentration, we precisely control the amount of activity, a key parameter of the system. We characterize the dynamics of streaming defects on a large, flat, 2D interface using quantitative polarization light microscopy. We report fundamental characteristics of the active nematics such as defect velocities, defect creation and annihilation rates, and emergent length scales in the system. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G11.00012: Simulation Study of Defect Dynamics in an Extensile Active Nematic Gabriel Redner, Aparna Baskaran, Michael Hagan This talk will describe a novel particle-based simulation scheme for studying extensile active nematic liquid crystals. Motivated by recent experiments on bundled microtubles and molecular motors, we have implemented a minimal simulation model which simplifies the complex details of the experimental system while retaining the correct symmetries of alignment and activity. Our simulations exhibit the same basic phenomena as the experimental system, including the spontaneous generation, unbinding, and annihilation of defect pairs. Our flexible simulation approach allows us to investigate in detail the dynamics of defects interacting with other defects and with system boundaries, as well as to explore a broad region of parameter space. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G11.00013: Living liquid crystal: collective bacteria motion in anisotropic viscoelastic media Shuang Zhou, Andrey Sokolov, Oleg D. Lavrentovich, Igor S. Aranson By transducing energy stored in the environment to drive systematic movements, bio-mechanical hybrids can move and reconfigure their structure and properties in response to external stimuli. Here, we create a fundamentally new class of bio-mechanical hybrid -- living liquid crystals (LLCs), by combining two seemingly incompatible concepts, living swimming bacteria and inanimate but orientationally ordered lyotropic liquid crystal. The coupling between the activity-triggered flows and director reorientations results in a wealth of phenomena, including: (a) a characteristic length $\xi $ to describe the coupling between the orientation of LLC and the bacterial motion, (b) periodic stripe instabilities of the director in surface-anchored LLCs, (c) director pattern evolution into an array of disclinations with positive and negative topological charges as the surface anchoring is weakened or when the bacterial activity is enhanced. Our study provides an insight in understanding hierarchy of spatial scales in other active matter systems, as well as providing basis for devices with new functionalities, including specific responses to chemical agents, toxins, or photons. [Preview Abstract] |
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