Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session B36: Active Matter IIFocus
|
Hide Abstracts |
Sponsoring Units: GSOFT DBIO GSNP/DFD Chair: Robin Selinger, Kent University Room: 339 |
Monday, March 14, 2016 11:15AM - 11:27AM |
B36.00001: Material Flows in an Active Nematic Liquid Crystal Stephen DeCamp, Gabriel Redner, Aparna Baskaran, Michael Hagan, Zvonimir Dogic Active matter systems are composed of energy consuming constituent components which drive far-from-equilibrium dynamics. As such, active materials exhibit energetic states which would be unfavorable in passive, equilibrium materials. We study one such material; an active nematic liquid crystal which exists in a dynamical steady state where $+$/-1/2 defects are continuously generated and annihilated at a constant rate. The active nematic is composed of micron-sized microtubule filaments which are highly concentrated into a quasi-2D film that resides on an oil-water interface. Kinesin motor proteins drive inter-filament sliding which results in net extensile motion of the microtubule film. Notably, we find a mesophase in which motile $+$1/2 defects, acquire system-spanning orientational order. Currently, we are tracking material flows generated by the active stresses in the system to measure length scales at which energy is dissipated, and to measure the relation between internally generated flows and bend in the nematic field. [Preview Abstract] |
Monday, March 14, 2016 11:27AM - 11:39AM |
B36.00002: Antipolar ordering of topological charges in active liquid crystals Jorn Dunkel, Anand Oza Recent experiments demonstrated that ATP-driven microtubule-kinesin bundles can self-assemble into two-dimensional active liquid crystals that exhibit a rich creation and annihilation dynamics of topological defects, reminiscent of particle-pair production processes in quantum systems. This remarkable discovery has sparked considerable theoretical and experimental interest. Here, we present and validate a minimal continuum theory for this new class of active matter systems by merging universality ideas with the classical Landau-de Gennes theory. The resulting model agrees quantitatively with recently published data and, in particular, predicts a previously unexplained regime of antipolar order. Our analysis implies that active liquid crystals are governed by the same generic ordering principles that determine the non-equilibrium dynamics of dense bacterial suspensions and elastic bilayer materials. Moreover, the theory manifests a profound energetic analogy with strongly interacting quantum gases. Generally, our results suggest that complex nonequilibrium pattern-formation phenomena might be predictable from a few fundamental symmetric-breaking and scale-selection principles. [Preview Abstract] |
Monday, March 14, 2016 11:39AM - 11:51AM |
B36.00003: Points or vectors? The polar structure of disclinations in active and passive nematics Luca Giomi, Arthur Vromans Topological defects play a pivotal role in the physics of liquid crystals and represent one of the most prominent and well studied aspects of mesophases. While in two-dimensional nematics, disclinations are traditionally treated as point-like objects, recent experimental studies on active nematics have suggested that half-strength disclinations might in fact posses a polar structure. In this talk I will provide a precise definition of polarity for half-strength nematic disclinations, introduce a simple and robust method to calculate this quantity from experimental and numerical data and investigate how the orientational properties of active and passive half-strength disclinations affect their dynamics. [Preview Abstract] |
Monday, March 14, 2016 11:51AM - 12:03PM |
B36.00004: Theory and Experiments of Topologically Driven Flows in Nematic Suspensions Christopher Conklin, Jorge Vinals, Chenhui Peng, Yubing Guo, Sergij Shiyanovskii, Qi-Huo Wei, Oleg Lavrentovich We present theory, numerical solutions, and experiments of electric field driven flows in nematic liquid crystals (LC) in which a patterned molecular orientation acts as an electrolytic active medium. Surface patterning by photoalignment in a thin cell is used to create various alignments of a nematic liquid crystal film, that may include topological defects. The active patterned LC electrolyte converts electric field energy into LC flows and transport of embedded particles of any type (fluid, solid, gaseous) along predesigned trajectories, and without limitation on the electric nature (charge, polarizability) of these particles and interfaces. Flow is quadratic in the electric field which leads, even for an imposed AC field, to systematic flow velocities, including persistent vortices of controllable rotation speed and direction. The latter are essential for micro- and nanoscale mixing applications. [Preview Abstract] |
Monday, March 14, 2016 12:03PM - 12:15PM |
B36.00005: Controlling Defects and Flow in Active Nematic Suspensions Suraj Shankar, Pau Guillamat Bassedas, Jordi Ign\'es-Mullol, Francesc Sagu\'es, M. Cristina Marchetti Experiments on active nematics composed of cytoskeletal biopolymers activated by molecular motors have shown that in these systems topological defects drive self-sustained flows and the transition to spatio-temporal chaos. In active nematics, defects become dynamical entities and behave like self-propelled particles. In a freely suspended nematic layer the defect speed is controlled by the activity and the viscosity of the active fluid that is so far unknown. Experiments, however, are carried out on very thin nematic layers at an oil-water interface. Our collaborators in Barcelona have shown that increasing the viscosity of the oil can substantially slow down the defects and increase their number. Considering a model of an active nematic at an oil-water interface, we have calculated the defect speed as a function of oil viscosity and find that theory and experiments agree well when the oil viscosity is changed over four orders of magnitude. Importantly, by combining theory and experiments these results provide a parameter-free estimate for the interfacial viscosity of the active nematic layer, which has never been measured before. [Preview Abstract] |
Monday, March 14, 2016 12:15PM - 12:27PM |
B36.00006: Active nematics on the surface of a torus Perry Ellis, Ya-Wen Chang, Alberto Fernandez-Nieves Nematic materials on the surface of a sphere must have a net topological charge of $s = + 2$. In equilibrium nematics experiments have shown that this net topological charge can be realized with four $s = + 1/2$ defects, which also corresponds to the theoretically expected ground state configuration. Surprisingly, even though active nematics are continuously driven out of equilibrium by the internal energy of the nematogens, when confined to the surface of a sphere these materials can also realize this net topological charge with four $s = + 1/2$ defects. In contrast to the spherical confinement case, the situation for toroidal confinement has not been experimentally explored despite the existence of theory and simulation work examining the structure of ordered materials on the surface of a torus. Here, we experimentally realize an extensile active nematic confined to a toroidal surface and explore how the interplay between topology, activity, and nematic elasticity affect the structure and dynamics of the material. [Preview Abstract] |
Monday, March 14, 2016 12:27PM - 12:39PM |
B36.00007: Dynamics and Instabilities of an overdamped active nematic liquid crystal Elias Putzig, Aparna Baskaran Active nematics have been studied extensively in the context of suspensions of active particles, with a Stokes equation describing the flow of the surrounding fluid. Here we will present a continuum model of an overdamped (often termed 'dry') active nematic, where activity enters through self-induced flows. These flows represent the ability of the internal forces to convect, shear, or rotate the nematic order. The self-induced shear gives rise to an instability in the homogeneous ordered state which is analogous to that seen in active suspensions. The self-induced rotation gives rise to a new instability. A phase diagram from this model will be presented, and the phenomenology will be compared with what is seen in experimental and simulated active systems. [Preview Abstract] |
Monday, March 14, 2016 12:39PM - 12:51PM |
B36.00008: Viscoelastic and elastomeric active matter: linear instability and nonlinear dynamics Ewan J. Hemingway, M. E. Cates, M. C. Marchetti, S. M. Fielding We consider a continuum model of active viscoelastic matter, whereby a model of an active nematic liquid-crystal is coupled to a minimal model of polymer dynamics with a viscoelastic relaxation time $\tau_{c} $. To explore the resulting interplay between active and polymeric dynamics, we first generalise a linear stability analysis (from earlier studies without polymer) to derive criteria for the onset of spontaneous flow. Perhaps surprisingly, our results show that the spontaneous flow instability persists even for divergent polymer relaxation times. We explore the novel dynamical states to which these instabilities lead by means of nonlinear numerical simulations. This reveals oscillatory shear-banded states in 1D, and activity-driven turbulence in 2D, even in the limit $\tau_{c} \to \infty $. Adding polymer can also have calming effects, increasing the net throughput of spontaneous flow along a channel in a new type of "drag-reduction", an effect that may have implications for cytoplasmic streaming processes within the cell. [Preview Abstract] |
Monday, March 14, 2016 12:51PM - 1:03PM |
B36.00009: A Kinetic Model of Active Extensile Bundles Daniel Goldstein, Bulbul Chakraborty, Aparna Baskaran Recent experiments in active filament networks reveal interesting rheological properties (Dan Chen: APS March Meeting 2015 D49.00001). This system consumes ATP to produce an extensile motion in bundles of microtubules. This extension then leads to self generated stresses and spontaneous flows. We propose a minimal model where the activity is modeled by self-extending bundles that are part of a cross linked network. This network can reorganize itself through buckling of extending filaments and merging events that alter the topology of the network. We numerically simulate this minimal kinetic model and examine the emergent rheological properties and determine how stresses are generated by the extensile activity. We will present results that focus on the effects of confinement and network connectivity of the bundles on stress fluctuations and response of an active gel. [Preview Abstract] |
Monday, March 14, 2016 1:03PM - 1:15PM |
B36.00010: Shortening actin filaments cause force generation in actomyosin network to change from contractile to extensile Nitin Kumar, Margaret Gardel Motor proteins in conjunction with filamentous proteins convert biochemical energy into mechanical energy which serves a number of cellular processes including cell motility, force generation and intracellular cargo transport. In-vitro experiments suggest that the forces generated by kinesin motors on microtubule bundles are extensile in nature whereas myosin motors on actin filaments are contractile. It is not clear how qualitatively similar systems can show completely different behaviors in terms of the nature of force generation. In order to answer this question, we carry out in vitro experiments where we form quasi 2D filamentous actomyosin networks and vary the length of actin filaments by adding capping protein. We show that when filaments are much shorter than their typical persistence length (approximately 10 microns), the forces generated are extensile and we see active nematic defect propagation, as seen in the microtubule-kinesin system. Based on this observation, we claim that the rigidity of rods plays an important role in dictating the nature of force generation in such systems. In order to understand this transition, we selectively label individual filaments and find that longer filaments show considerable bending and buckling, making them difficult to slide and extend along their length. [Preview Abstract] |
Monday, March 14, 2016 1:15PM - 1:27PM |
B36.00011: Dynamics of Actively Driven Crosslinked Microtubule Networks Vikrant Yadav, Kasimira Stanhope, Arthur A. Evans, Jennifer L. Ross We have designed a model experiment to explore dynamics of crosslinked active microtubule clusters crosslinked with MAP65. Microtubule clusters are allowed to settle on a slide coated with kinesin-1 molecular motors, which move microtubules. We systematically tune either concentration of cross linkers bound to microtubule ($\rho_{c})$ or the global concentration of microtubules ($\rho_{MT})$. We quantified the shape of the cluster by measuring the standard deviation ($\sigma )$ of the cluster outline. At low $\rho_{MT\, }$or $\rho_{c}$ the network is in an expanding state. At higher $\rho_{MT\, }$or $\rho_{c}$ expansion slows down, reaches zero at a critical density, and become negative indicating contraction. Further increase of $\rho_{MT\, }$or $\rho_{c}$ halts any kind of dynamics. The $\rho_{MT}$-$\rho_{c}$ phase space shows distinct regions of extensile, contractile and static regimes. We model these results using active hydrodynamic theory. Microtubules are modeled as active rods whereas effect of crosslinkers is modeled using a collision term that prefers anti-parallel alignment of microtubules. A linearized analysis of hydrodynamic equation predicts existence of density driven expanding, contracting, and static phases for microtubule clusters. [Preview Abstract] |
Monday, March 14, 2016 1:27PM - 1:39PM |
B36.00012: Competing dynamic phases of active polymer networks Simon Freedman, Shiladitya Banerjee, Aaron R. Dinner Recent experiments on in-vitro reconstituted assemblies of F-actin, myosin-II motors, and cross-linking proteins show that tuning local network properties can changes the fundamental biomechanical behavior of the system. For example, by varying cross-linker density and actin bundle rigidity, one can switch between contractile networks useful for reshaping cells, polarity sorted networks ideal for directed molecular transport, and frustrated networks with robust structural properties. To efficiently investigate the dynamic phases of actomyosin networks, we developed a coarse grained non-equilibrium molecular dynamics simulation of model semiflexible filaments, molecular motors, and cross-linkers with phenomenologically defined interactions. The simulation's accuracy was verified by benchmarking the mechanical properties of its individual components and collective behavior against experimental results at the molecular and network scales. By adjusting the model's parameters, we can reproduce the qualitative phases observed in experiment and predict the protein characteristics where phase crossovers could occur in collective network dynamics. Our model provides a framework for understanding cells' multiple uses of actomyosin networks and their applicability in materials research. [Preview Abstract] |
Monday, March 14, 2016 1:39PM - 1:51PM |
B36.00013: Rapid non-equilibrium turnover fluidizes entangled F-actin solutions Patrick M. McCall, David R. Kovar, Margaret L. Gardel The actin cytoskeleton of living cells is a semiflexible polymer network which regulates cell division, motility, and morphogenesis by controlling cell shape. These complex shape-changing processes require both mechanical deformation and remodeling of the actin cytoskeleton. Molecular motors generate internal forces to drive deformation, while cytoskeletal remodeling is regulated by non-equilibrium polymer turnover. Although the mechanical properties of equilibrium actin filament (F-actin) networks are well-described by theories of semiflexible polymers, these theories do not incorporate the effects of non-equilibrium turnover. To address this experimentally, we developed a model system in which both the turnover rate and the length distribution of purified F-actin can be tuned independently at steady-state through the combined action of actin regulatory proteins. Specifically we tune the concentrations of cofilin, profilin, and formin to regulate F-actin severing, recycling, and nucleation, respectively. We find that the actin turnover rate can be tuned by cofilin up to 25-fold (31 $+$/- 2 subunits/sec/filament). Surprisingly, changes in turnover rate have no effect on the steady-state F-actin length distribution, which is instead set by formin concentration. Passive microrheology measurements show that increased turnover leads to striking fluidization in both entangled and crosslinked networks. Non-equilibrium turnover thus enables modulation of network mechanics, which impacts force transmission and material deformation. [Preview Abstract] |
Monday, March 14, 2016 1:51PM - 2:03PM |
B36.00014: Critical forces for actin filament buckling and force transmission influence transport in actomyosin networks Samantha Stam, Margaret Gardel Viscoelastic networks of biopolymers coordinate the motion of intracellular objects during transport. These networks have nonlinear mechanical properties due to events such as filament buckling or breaking of cross-links. The influence of such nonlinear properties on the time and length scales of transport is not understood. Here, we use in vitro networks of actin and the motor protein myosin II to clarify how intracellular forces regulate active diffusion. We observe two transitions in the mean-squared displacement of cross-linked actin with increasing motor concentration. The first is a sharp transition from initially subdiffusive to diffusive-like motion that requires filament buckling but does not cause net contraction of the network. Further increase of the motor density produces a second transition to network rupture and ballistic actin transport. This corresponds with an increase in the correlation of motion and thus may be caused when forces propagate far enough for global motion. We conclude that filament buckling and overall network contraction require different amounts of force and produce distinct transport properties. These nonlinear transitions may act as mechanical switches that can be turned on to produce observed motion within cells. [Preview Abstract] |
Monday, March 14, 2016 2:03PM - 2:15PM |
B36.00015: Detection of Non-Equilibrium Fluctuations in Active Gels Alexandru Bacanu, Chase Broedersz, Jannes Gladrow, Fred Mackintosh, Christoph Schmidt, Nikta Fakhri Active force generation at the molecular scale in cells can result in stochastic non-equilibrium dynamics on mesoscpopic scales. Molecular motors such as myosin can drive steady-state stress fluctuations in cytoskeletal networks. Here, we present a non-invasive technique to probe non-equilibrium fluctuations in an active gel using single-walled carbon nanotubes (SWNTs). SWNTs are semiflexible polymers with intrinsic fluorescence in the near infrared. Both thermal and active motor-induced forces in the network induce transverse fluctuations of SWNTs. We demonstrate that active driven shape fluctuations of the SWNTs exhibit dynamics that reflect the non-equilibrium activity, in particular the emergence of correlations between the bending modes. We discuss the observation of breaking of detailed balance in this configurational space of the SWNT probes. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700