Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H51: Emergent Self-organization in Active Matter IFocus
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Sponsoring Units: DBIO Chair: Jennifer Ross, University of Massachusetts, Amherst Room: LACC 511C |
Tuesday, March 6, 2018 2:30PM - 3:06PM |
H51.00001: Using light to study localized phase separation in living cells Invited Speaker: Dan Bracha Liquid-liquid phase separation driven by intrinsically disordered proteins has been shown to play a key role in the assembly of diverse intracellular compartments. However, the biophysical principles by which phase separation can be precisely localized within subregions of the cell are still largely unclear, particularly for low-abundance proteins. Our lab has developed several optogenetic platforms designed for spatiotemporally-controlling phase separation in living cells. We use this technology to demonstrate how localized phase separation can be induced under intracellular protein concentrations insufficient for global phase separation. The rapid responsiveness and quantitative tunability of these systems can further be used for mapping full intracellular phase diagrams of disordered proteins, including identifying various phase transition modes, thus making it a powerful tool for elucidating phase behavior in cells. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H51.00002: Edges Impose Planar Alignment in Nematic Monolayers by Directing Cell Elongation and Migration Nathan Bade, Randall Kamien, Richard Assoian, Kathleen Stebe In confluent monolayers, elongated, adherent cells can exhibit hallmarks of active nematic systems. We show that edges impose planar anchoring at the periphery of nematic monolayers by changing cell shape and guiding migration. Despite being unconfined, isolated cells near edges elongate and preferentially migrate along the edge. By this mechanism, cells propagate nematic alignment into bulk monolayers. We find that adhesive boundaries continue to influence cell alignment and enhance migration even after cells have formed confluent monolayers. At corners, conflicting boundary cues drive topological defect formation in predictable patterns that depend on the local migration of cells at boundaries. By developing an understanding of how edges serve as anchoring conditions for nematic monolayers, we will gain control over the patterns that emerge in these tissues. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H51.00003: Active matter systems can exhibit coexisting patterns of competing symmetries Timo Krüger, Lorenz Huber, Ryo Suzuki, Andreas Bausch, Erwin Frey Active matter systems have the ability to produce a far greater variety of ordered patterns than conventional thermal equilibrium systems. In particular, transitions between disordered phases and either polar or nematically ordered phases have been predicted and observed in two-dimensional active systems. However, coexistence between phases of different types of order has not been reported. Here, we demonstrate the emergence of dynamic coexistence of ordered states exhibiting nematic and polar symmetry, both in large scale agent-based simulations as well as experimentally in an actomyosin motility assay that consists of actin filaments propelled by immobilized molecular motors. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H51.00004: Active systems learning at the microscale Santiago Muinos Landin, Keyan Ghazi-Zahedi, Frank Cichos Living organisms are able to sense and process information about the environment they live in. They are also able to update this information in order to contruct solutions for real life problems such as finding food or avoiding danger. This active adaption process that in the long run drives the evolution of species is the result of a short time scale evolution of the knowledge of an organism that we know as learning. At the microscale the learning is hampered by stochasticity given that the intrinsic Brownian noise makes critical to build a feedback between stimulus and action. Here, we present a system based on a self-themophoretic microswimmer that allows the application of artifical intelligence algorithms at the microscale. Using reinforcement learning we show that even under noise conditions a system is able to learn how to optimize a simple navigation task. We study the influence of noise and the situation where multiple agents can share information to carry out specific tasks. This way we show how adaptation and intelligent collective behavior can be studied in artificial microswimmers systems. |
Tuesday, March 6, 2018 3:42PM - 4:18PM |
H51.00005: TBD Invited Speaker: Nikta Fakhri This abstract not available. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H51.00006: Hydrodynamics-mediated trapping of micro-swimmers near drops Arezoo Ardekani, Nikhil Desai, Vaseem Shaik The swimming characteristics and dynamics of a model micro-swimmer (force dipole) near a clean, and a surfactant covered drop, are investigated. We report the critical trapping radius, the basin of attraction, and the trapping time distribution, of deterministic and stochastic swimmers, as a function of the swimmer’s dipole strength, the viscosity ratio, and the dimensionless surface viscosity. We find that addition of surfactant greatly reduces the critical trapping radius for low values of swimmer dipole strength, viscosity ratio, and dimensionless surface viscosity. The basin of attraction though, remains at O(1) swimmer body length for all combinations of viscosity ratio and dimensionless surface viscosity. A dynamical system analysis, for deterministic swimmers, reveals the existence of saddle points in the phase-space, for all cases where swimmers can get trapped. Although swimmer escape is possible via diffusive motion, we find that even in this case, addition of surfactant increases the interface-retention times by ~ 5-25%. It is seen that all effects of surfactant addition saturate rapidly with increase in the surface viscosity. |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H51.00007: Influence of fast advective flows on pattern formation in Dictyostelium discoideum Azam Gholami, Torsten Eckstein, Estefania Vidal, Vladimir Zykov, Albert Bae, Eberhard Bodenschatz A classic example of self-organized patterns in nature is found in the social amobae Dictyostelium discoideum. We present experiments on pattern formation of Dictyostelium discoideum in the presence of fast advective flows. We observe wave trains that propagate both parallel and perpendicular to the flow direction. While the transverse wave velocity and the wave period are constant, the downstream wave propagation velocity scales linearly with the imposed flow velocity. We show that the acquired wave shape is highly dependent on nucleation point of the wave as well as on the strength of the advective flow. We compare the experimental results with numerical simulations performed using a Reaction-Diffusion model and found excellent agreement. Additionally we show that a Reaction-Diffusion model with fast dynamic assumptions does not reproduce the observed results. These results are relevant to understand the process of pattern formation and aggregation of Dictyostelium discoideum that may experience fluid flows in its natural habitat. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H51.00008: Instability-triggered Oscillations of Active Microfilament Feng Ling, Hanliang Guo, Eva Kanso Many biophysical and physiological processes involve intricate dynamics of active microfilaments submerged in complex fluids. |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H51.00009: Basal coupling leads to coordinated beating of microfilaments Hanliang Guo, Kirsty Wan, Janna Nawroth, Eva Kanso Cilia and flagella often beat in synchrony. More interestingly, they also switch between different synchronization modes. However, the biological and physical mechanisms responsible for such switching remain elusive. While experimental and theoretical evidence suggests that the phase coordination can be a result of hydrodynamical coupling, recent findings show that basal coupling plays an important role in such coordination. Here, we isolate hydrodynamic and basal coupling in a mathematical model of two microfilaments driven at their bases. To isolate basal from hydrodynamical coupling, we connect the microfilaments at their bases via an linear elastic spring and account for fluid drag using local resistive force theory. We find that the coordination of the filaments is strongly affected by the stiffness of the basal spring and the driving moment. Specifically, we observe a transition from in-phase to anti-phase coordination as the spring stiffness increases. We compare these results to synchronization via hydrodynamic interactions and comment on their relevance to understanding cilia coordination reported in experimental observations of algae cells such as \textit{Chlamydomonas}. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H51.00010: The Flow of Flexible and Rigid Blood Cells in 100 μm Glass Capillaries Christopher Brown, Alexey Aprelev, Frank Ferrone Sickle cell disease causes normally flexible red blood cells to become rigid when oxygen is removed, occasionally forming long sickle shapes. We have been studying the flow of blood drawn into 100 μm diameter glass capillaries by surface tension as a diagnostic for sickle cell disease. The flow of both oxygenated and deoxygenated cells though the horizontal capillaries is well described by the Lucas Washburn (LW) equation over most of the 3 cm capillary length, despite the fact that blood is not a simple fluid and is known to exhibit margination. The flow of the rigid deoxygenated cells is substantially slower than flexible cells, oxy or deoxy. If the LW equation is used to infer an effective viscosity, we find the viscosity of both rigid and flexible cells can be described by a master equation, η=η0(1+(φ/φ*)2.38), where φ* is unique to the size and rigidity of the cells. For deoxygenated cells, we also observe the formation of a dense phase that begins immediately behind the advancing meniscus and grows with time. As the dense phase grows, the flow begins to lag behind LW behavior. |
Tuesday, March 6, 2018 5:18PM - 5:30PM |
H51.00011: Generating Large Scale Flocks of Sperm in Viscoelastic Fluid Chih-Kuan Tung, Jelani Lyles, Soon Hong Cheong, MingMing Wu, Susan Suarez Sperm collective swimming in viscoelastic fluid provides a biologically relevant model system to study the behavior of active matter. To study the statistical mechanics of the flocking of sperm, it will be important to study the variation of sperm orientation within a large flock. While early experiments show that increasing cell density increases the average flock size, packing the field of view (570×426 µm) with sperm was not sufficient to generate large flocks. On the other hand, by transiently aligning sperm orientation with a flow, we were able to observe flock sizes close to the height of the field of view (across 435 µm or 240 cells) forming after the flow was turned off. This suggests that the sperm flock sizes depend on the history of the flock orientation. Furthermore, alignment due to cell-cell interactions through viscoelastic medium is not enough to overcome the vigorous swimming of sperm and align two flocks with different orientations, yet enough to prevent rotational diffusivity from efficiently breaking down the large flocks. We will also discuss the orientation variation within a large flock. |
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