Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session C35: Active Matter: Collective Phenomena in Living Systems IIFocus
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Sponsoring Units: DBIO GSOFT GSNP Chair: Yuhai Tu, IBM Research Room: 338 |
Monday, March 14, 2016 2:30PM - 2:42PM |
C35.00001: Using a stochastic field theory to understand group behavior in microswimmer suspensions Patrick Underhill, Yuzhou Qian, Peter Kramer Active suspensions of microswimmers appear both in natural biological systems (e.g. bacteria or algae) and in synthetic systems. Even without external forcing they are out of equilibrium, which gives rise to interesting properties in both small and large concentrations of the particles. These properties have been observed in experiments as well as simulation/modeling approaches. It is important to understand how hydrodynamic interactions between active swimmers cause and/or alter the suspension properties including enhanced transport and mixing. One of the most successful approaches has been a mean field theory. However, in some situations the mean field theory makes predictions that differ significantly from experiments and direct (agent or particle based) simulations. There are also some quantities that cannot be calculated by the mean field theory. In this talk, we will describe our new approach which uses a stochastic field to overcome the limitations of the mean field assumption. It allows us to calculate how interactions between organisms alter the correlations and mixing in conditions where the mean field theory cannot. [Preview Abstract] |
Monday, March 14, 2016 2:42PM - 2:54PM |
C35.00002: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 2:54PM - 3:06PM |
C35.00003: Geometry and mechanics of growing bacterial colonies. Zhihong You, Daniel Pearce, Anupam Sengupta, Luca Giomi Bacterial colonies are abundant on living and non-living surfaces, and are known to mediate a broad range of processes in ecology, medicine and industry. Although extensively researched – from single cells up to the population levels – a comprehensive biophysical picture, highlighting the cell-to-colony dynamics, is still lacking. Here, using numerical and analytical models, we study the mechanics of self-organization leading to the colony morphology of cells growing on a substrate with free boundary. We consider hard rods to mimic the growth of rod-shaped non-motile cells, and show that the colony, as a whole, does not form an ordered nematic phase, nor does it result in a purely disordered (isotropic) phase. Instead, different sizes of domains, in which cells are highly aligned at specific orientations, are found. The distribution of the domain sizes follows an exponential relation – indicating the existence of a characteristic length scale that determines the domain size relative to that of the colony. A continuum theory, based on the hydrodynamics of liquid crystals, is built to account for these phenomena, and is applied to describe the buckling transition from a planar to three-dimensional (3D) colony. The theory supports preliminary experiments conducted with different strains of rod shaped bacterial cells, and reveals that the buckling transition can be regulated by varying the cell stiffness and aspect ratio. This work proposes that, in addition to biochemical pathways, the spatio-temporal organization in microbial colonies is significantly tuned by the biomechanical and geometric properties of the microbes in consideration. [Preview Abstract] |
Monday, March 14, 2016 3:06PM - 3:18PM |
C35.00004: Cell-cell interactions impacts on the rate of swarm expansion and the edge shape of a colony swarming {\it Pseudomonas aeruginosa} Aboutaleb Amiri, Giordano Tierra, Zhiliang Xu, Joshua Shrout, Mark Alber Collective motion has been observed by several bacterial species including the pathogenic bacterium {\it P. aeruginosa}. A flagellum at the pole is known to generate a self-propulsion motion. However, the role of type IV pili (TFP), distributed on the cell membrane, during swarming needs to be investigated in more details. In this work we introduce a model that combines the hydrodynamic and biophysical interactions in order to study the impact of the TFP interactions on swarming behavior of the colony. The model describes the motion and interactions of rod-shaped self propelled bacteria inside a thin liquid film. It also includes the equations describing the production and diffusion of surfactant rhamnolipids that is responsible for extraction of water from substrate, and Marangoni driven expansion of the thin liquid film by altering the surface tension. We show that TFP interactions are responsible for slower expansion rate of colonies of TFP deficient mutants compared to wild type. Experimental observations were used to calibrate the model and verify the model assumptions and predictions. [Preview Abstract] |
Monday, March 14, 2016 3:18PM - 3:30PM |
C35.00005: The 3-D spatial structure of multicellular aggregates can give them a competition-dependent growth advantage in early biofilm development Vernita Gordon, Kasper Kragh, Jaime Hutchison, Gavin Melaugh, Chris Rodesney, Aled Roberts, Yasuhiko Irie, Peter Jensen, Stephen Diggle, Rosalind Allen, Thomas Bjarnsholt Biofilms are structured communities of sessile microbes. Traditional models of biofilm development begin with single bacteria seeding a surface. However, biofilms can also be seeded by multicellular aggregates. How the three-dimensional structure of aggregates impacts the initiation and development of biofilms is not known. Here we use a combination of experiments and simulations to determine the impact of the seeding structure. We find that whether aggregates or single cells grow better depends on the density of single cells initially seeded. The density of single cells, which we take as our measure of the level of competition, impacts per-cell access to growth resource. The overall biomass accumulation arising from an aggregate is a combination of slow growth in the resource-limited interior, and faster growth on the sides and top. When competition is low, aggregates are disadvantaged, compared with single cells. However, when competition is high, aggregates are fitter than single cells, because the cells at the top of the aggregates have better access to growth resources than do high-density single cells on the surface. [Preview Abstract] |
Monday, March 14, 2016 3:30PM - 3:42PM |
C35.00006: Mechanical signaling coordinates the embryonic heart Kevin Chiou, Jason Rocks, Benjamin Prosser, Dennis Discher, Andrea Liu The heart is an active material which relies on robust signaling mechanisms between cells in order to produce well-timed, coordinated beats. Heart tissue is composed primarily of active heart muscle cells (cardiomyocytes) embedded in a passive extracellular matrix. During a heartbeat, cardiomyocyte contractions are coordinated across the heart to form a wavefront that propagates through the tissue to pump blood. In the adult heart, this contractile wave is coordinated via intercellular electrical signaling.Here we present theoretical and experimental evidence for {\emph mechanical} coordination of embryonic heartbeats. We model cardiomyocytes as mechanically excitable Eshelby inclusions embedded in an overdamped elastic-fluid biphasic medium. For physiological parameters, this model replicates recent experimental measurements of the contractile wavefront which are not captured by electrical signaling models. We additionally challenge our model by pharmacologically blocking gap junctions, inhibiting electrical signaling between myocytes. We find that while adult hearts stop beating almost immediately after gap junctions are blocked, embryonic hearts continue beating even at significantly higher concentrations, providing strong support for a mechanical signaling mechanism. [Preview Abstract] |
Monday, March 14, 2016 3:42PM - 3:54PM |
C35.00007: Jamming and Localization of Interacting Run-and-Tumble Particles Richard Blythe, Martin Evans, Alexander Slowman Certain species of bacteria, notably \textit{Escherichia coli}, exhibit a characteristic run-and-tumble motion comprising a sequence of straight-line runs at constant velocity interspersed with tumble events that randomize the direction of motion. In a many-body setting, this nonequilibrium dynamics can generate the phenomenon of motility-induced phase separation, which is also seen for a wide variety of self-propelled particles more generally. Whilst the propensity of self-propelled particles to phase separate is understood at a mesoscopic level, the origin of this behaviour in the inelastic collisions between particles implied by the microscopic dynamics is not. Here we present exact results for run-and-tumble particles in one dimension that reveal a richly-structured stationary state that comprises a superposition of three distinct physical states whose relative weights vary with the run and tumble rates, namely a jammed state, a localized state and a delocalized state. [Preview Abstract] |
Monday, March 14, 2016 3:54PM - 4:06PM |
C35.00008: Wing attachment position of fruit fly minimizes flight cost Robert Noest, Jane Wang Flight is energetically costly which means insects need to find ways to reduce their energy expenditure during sustained flight. Previous work has shown that insect muscles can recover some of the energy used for producing flapping motion. Moreover the form of flapping motions are efficient for generating the required force to balance the weight. In this talk, we show that one of the morphological parameters, the wing attachment point on a fly, is suitably located to further reduce the cost for flight, while allowing the fly to be close to stable. We investigate why this is the case and attempt to find a general rule for the optimal location of the wing hinge. Our analysis is based on computations of flapping free flight together with the Floquet stability analysis of periodic flight for descending, hovering and ascending cases. [Preview Abstract] |
Monday, March 14, 2016 4:06PM - 4:18PM |
C35.00009: Noise regulation and symmetry breaking during vertebrate body elongation. Thierry Emonet, Dipjyoti Das, Scott A. Holley Elongation of the vertebrate body axis is driven by collective cell migration and cell proliferation at the posteriorly advancing embryonic tailbud. Within the Zebrafish tailbud an ordered stream of cells symmetrically bifurcates to form the left and right halves of the presomitic mesoderm. Maintaining bilateral symmetry during this process is critical to avoid catastrophic spine deformation. Using direct comparison between experimental data and a simple model of cell migration we identified the dynamic regulation of the noise in the direction of motion of individual cells as a critical factor in maintaining symmetric cell flow. Genetic perturbations that reduced noise led to body axis deformation whereas an increase in noise led to retarded elongation as predicted by our model. [Preview Abstract] |
Monday, March 14, 2016 4:18PM - 4:30PM |
C35.00010: Spontaneous Planar Chiral Symmetry Breaking in Cells Jeremy Hadidjojo, David Lubensky Recent progress in animal development has highlighted the central role played by planar cell polarity (PCP) in epithelial tissue morphogenesis. Through PCP, cells have the ability to collectively polarize in the plane of the epithelium by localizing morphogenetic proteins along a certain axis. This allows direction-dependent modulation of tissue mechanical properties that can translate into the formation of complex, non-rotationally invariant shapes. Recent experimental observations$^{\mathrm{[1]}}$ show that cells, in addition to being planar-polarized, can also spontaneously develop planar chirality, perhaps in the effort of making yet more complex shapes that are reflection non-invariant. In this talk we will present our work in characterizing general mechanisms that can lead to spontaneous chiral symmetry breaking in cells. We decompose interfacial concentration of polarity proteins in a hexagonal cell packing into irreducible representations. We find that in the case of polar concentration distributions, a chiral state can only be reached from a secondary instability after the cells are polarized. However in the case of nematic distributions, we show that a finite-amplitude (subcritical, or ``first-order'') nematic transition can send the system from disorder directly to a chiral state. In addition, we find that perturbing the system by stretching the hexagonal packing enables direct (supercritical, or ``second-order'') chiral transition in the nematic case. Finally, we do a Landau expansion to study competition between stretch-induced chirality and the tendency towards a non-chiral state in packings that have retained the full 6-fold symmetry. [1] K. Taniguchi \textit{et al}., Science (2011) [Preview Abstract] |
Monday, March 14, 2016 4:30PM - 4:42PM |
C35.00011: Epithelial Proliferation on Curved Toroidal Surfaces. Ya-Wen Chang, Ricardo Cruz, Alexandros Fragkopoulos, Samantha Marquez, Andres Garcia, Alberto Fernandez-Nieves Cellular environment influences a multitude of cellular functions by providing chemical and physical signals that modulate cell behavior, dynamics, development, and eventually survival. In strongly interacting epithelial cells, cells coordinate their behavior to respond to mechanical constraints in 2D. Local differences in tissue tension has also been shown to impact cell reproduction within an epithelial-cell sheet. Much less is known about how cells respond to out-of-plane curvatures. Here, we describe the proliferation of MDCK on toroidal hydrogel substrates, which unlike spheres or planes, have regions of both positive and negative Gaussian curvature. Additionally, the range of curvatures can be controlled by varying the size and aspect ratio of the torus, allowing us to quantify the relation between substrate curvature and cell proliferation. [Preview Abstract] |
Monday, March 14, 2016 4:42PM - 4:54PM |
C35.00012: ABSTRACT WITHDRAWN |
Monday, March 14, 2016 4:54PM - 5:06PM |
C35.00013: Synthetic electrophysiology: optically controlled oscillators in an engineered bioelectric tissue Harold McNamara, Hongkang Zhang, Christopher Werley, Adam Cohen Multicellular electrical dynamics underlie crucial physiological functions, but the complexity of natural bioelectricity can obscure the relation of individual components (proteins, cells) to emergent system-level dynamics. Here we introduce optopatch-spiking HEK(OS-HEK) tissue, a minimal synthetic bioelectric tissue with 4 transgenic components that supports optical initiation of propagating electrical waves as well direct optical voltage readout. In conjunction with a home-built inverted microscope capable of patterned illumination, we use this tissue to probe the biophysical attributes of this excitable bioelectric medium, including dispersion relations, curvature-dependent wavefront propagation, electrotonic coupling, and effects of boundaries. We then used chemical patterning to define cellular circuits that support controllable oscillations and which retain memory for more than 2 hours (corresponding to 10$^{\mathrm{4}}$ oscillations), constituting a substrate for binary bioelectric data storage. Finally, we use optical patterning of boundary conditions in a physically homogeneous tissue to design dynamically reconfigurable oscillators. [Preview Abstract] |
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