Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S29: Active Matter and Liquid Crystals in Biological Systems IIIFocus
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Sponsoring Units: DSOFT DBIO GSNP DPOLY Chair: Katherine Copenhagen, Princeton University Room: 501 |
Thursday, March 5, 2020 11:15AM - 11:27AM |
S29.00001: Active folding and coiling in vivo Noah Mitchell, Dillon Cislo, Suraj Shankar, Zvonimir Dogic, Boris I Shraiman, Sebastian Streichan A common theme in biology is the assembly of cells into tubes, which in turn develop into specific shapes for specific functions. In a large class of organs including the gut, a simple tube transforms into a coil of compartments. This process is particularly striking in the Drosophila melanogaster midgut -- a tube that folds and then coils into a helical configuration in only two hours. Using a combination of light-sheet microscopy, genetics, and computer vision techniques, we extract the full 3D dynamics of this organ with sub-cellular resolution. We present a quantitative account of the dynamics of folding and coiling of the midgut that links cellular motion and deformation to the macroscopic shape change of the organ. |
Thursday, March 5, 2020 11:27AM - 11:39AM |
S29.00002: Dynamical self-consistent field-theory for active nematic liquid crystals: Nematic order and spatio-temporal structure in surface colonies of rod-like bacteria Drake Lee, Robert Wickham We present a dynamical self-consistent field theory for interacting, self-propelled rods that we use to study fascinating time-dependent, inhomogeneous structures observed in surface colonies of twitching Pseudomonas aeruginosa bacteria. We apply an extremal principle to an exact dynamical functional integral derived from the microscopic, many-body dynamics of self-propelled rods moving in two-dimensions. This reduces the problem to that of a single self-propelled rod moving in response to self-consistent, mean-force and torque fields. We address the thorny problem of calculating rod-rod interactions in this mean-field approximation. For uniform systems in two-dimensions, we observe that a continuous isotropic-nematic phase transition occurs at a critical rod concentration. The value of the critical concentration is independent of the magnitude of the self-propulsive force. We discuss our preliminary simulations of inhomogeneous, dynamical structures formed in motile bacteria surface colonies. |
Thursday, March 5, 2020 11:39AM - 11:51AM |
S29.00003: Growth and dynamics of active nematic droplets of Myxococcus xanthus bacteria Cassidy Yang, Joshua Shaevitz Myxococcus xanthus is a rod-shaped soil bacterium that exhibits various forms of emergent collective phenomena using only short-range interactions. When starved, they collectively bead from surfaces to form 3D droplet-like aggregates known as fruiting bodies that are comprised of hundreds of thousands of cells and are crucial for sporulation and survival. The combination of active cellular motility and local nematic interactions generates increased pressures that drive the dewetting process. Unlike passive fluids that form axisymmetric spherical cap-shaped droplets, we find that these aggregates break symmetry and are often elongated in shape with non-uniform contact angles. We characterize the growth and dynamics of these active nematic droplets and make progress towards understanding the role of motility in the formation of stable fruiting bodies through tracking sparsely labelled cells. |
Thursday, March 5, 2020 11:51AM - 12:03PM |
S29.00004: Solid-liquid transitions of deformable active particles Benjamin Loewe, Michael Chiang, Davide Marenduzzo, M Cristina Marchetti Experiments have established that confluent epithelial tissues exhibit solid-liquid transitions. Although epithelial tissues usually form a cell monolayer, there are situations in which that monolayer structure breaks, such as in cell extrusion and pseudo-stratified epithelia. To describe such situations, we have developed a multi-phase field model where cells are characterized by multiple scalar fields, and interact through steric repulsion. By reducing their deformability, these cells go from being highly-deformable with no overlap to almost-circular with high overlap, which can be thought of as a way to incorporate layering between cells. Using this model we have examined the interplay of cell deformability and cell motility in controlling the solid-liquid transition of ordered lattices. We find that by reducing cell deformability the melting transition changes from continuous to discontinuous: at finite overlap the system develops an intermittent region, alternating between crystalized and liquid states. Finally, by studying the formation of defect pairs in the intermittent region, we find that they correlate with spatial fluctuations of the cell-overlap, suggesting that cell extrusion is correlated with structural defects. |
Thursday, March 5, 2020 12:03PM - 12:39PM |
S29.00005: Uncovering the dynamic precursors to motor-driven contraction of active gels Invited Speaker: José Alvarado Cells and tissues have the remarkable ability to actively generate the forces required to change their shape. This active mechanical behavior is largely mediated by the actin cytoskeleton, a crosslinked network of actin filaments that is contracted by myosin motors. Experiments and active gel theories have established that the length scale over which gel contraction occurs is governed by a balance between molecular motor activity and crosslink density. By contrast, the dynamics that govern the contractile activity of the cytoskeleton remain poorly understood. Here we investigate the microscopic dynamics of reconstituted actin–myosin networks using real-space video microscopy and Fourier-space dynamic light scattering. Light scattering reveals different regimes of microscopic dynamics. We uncover two dynamical precursors that precede macroscopic gel contraction. One is characterized by a progressive acceleration of stress-induced rearrangements, while the other consists of sudden, heterogeneous rearrangements. Our findings suggest a qualitative analogy between self-driven rupture and collapse of active gels and the delayed rupture of passive gels observed in earlier studies of colloidal gels under external loads. |
Thursday, March 5, 2020 12:39PM - 12:51PM |
S29.00006: Microscopic simulations of a 3D active nematic composed of semiflexible polymers Matthew Peterson, Michael Hagan, Aparna Baskaran The field of active matter studies materials whose constituent particles can consume energy at the particle scale to produce motion. An active nematic has the additional constraint that these particles have nematic symmetry. Unlike a passive nematic, an active nematic can exhibit spontaneous creation and annihilation of topological defects. Recent work on 3D active nematics shows notable differences in the structure and dynamics of these defects compared to 2D systems—for example, defects tend to form as neutrally charged disclination loops. Here, we will use particle-based simulations to better understand the dynamical properties of dry active nematics in 3D, both in bulk and under confinement. |
Thursday, March 5, 2020 12:51PM - 1:03PM |
S29.00007: The structure and dynamics of microtubule bundles mediated by motor proteins Bezia Lemma, Linnea Lemma, Sebastian Fuerthauer, Michael John Shelley, Zvonimir Dogic, Daniel Needleman Mixtures of polar microtubule (MT) filaments and force-generating motor proteins self-organize by cross-linking and sliding MTs into dense networks of polar and apolar bundles. We can measure the geometric structure of these MT-based bundles using small-angle x-ray scattering and determine the polar structure of these materials by taking advantage of their chiral crystal lattice which produces constructive interference of second harmonic signal between polar-aligned MTs. A transition in the bundles' structure to a compressed square lattice corresponds to a change in the microtubules extensile sliding dynamics. These results are useful in understanding the dynamics of filamentous active materials. The quantified geometric packing of microtubule bundles can be incorporated into a framework of highly cross-linked active gel theories. |
Thursday, March 5, 2020 1:03PM - 1:15PM |
S29.00008: Microscopic to mesoscopic: what can modular molecular motors teach us about the nature of active stress? Steven Redford, Paul Ruijgrok, Jonathan Colen, Sasha Zemsky, Vincenzo Vitelli, Zev Bryant, Aaron Dinner, Margaret Gardel Coarse-grained hydrodynamic theories have proven invaluable to our understanding of active nematic liquid crystals. In general, these treatments succeed despite coarse graining out the microscopic details of the material, because they account for certain mesoscopic consequences of microscopic change. For example, changing the elastic constants in the nematic governing equations can account for microscopic differences in mesogen length and rigidity. However, no such treatment accounts for the microscopic nature of the stress itself. Here, we step towards rectifying this discrepancy by experimentally tuning the properties of stress-generating myosin motors in an actin based active liquid crystal. Within this system, we can begin to understand how systematically varying the microscopic properties of these motors affects the resultant active flow. |
Thursday, March 5, 2020 1:15PM - 1:27PM |
S29.00009: 2D Patterns of Active Stress ascribe 3D Deformations of Driven Actomyosin Networks Vikrant Yadav, Taeyoon Kim, Enrique De La Cruz, Michael Murrell During morphogenesis 2D active stresses in actomyosin networks can lead to 3D deformation of epithelial sheets. Though there is consensus on role of stress, how in-plane activity morphs epithelial sheets out of plane is not clearly understood. To understand this relation, we design an invitro actomyosin network with tunable mechanical stiffness and activity. The active stresses can be locally programmed into the 2D actomyosin sheets by photoactivation of myosin motors. We demonstrate that an interplay between mechanics of sheet and activity leads to controlled 3D deformations. We show that the extent of 3D deformation of network is proportional to activity, but varies non-monotonically with stiffness. By controlling the shape and duration of activation protocol, we can create 3D deformations of various shapes with positive, negative, or zero gaussian curvature. Through experiments and agent-based simulations we show that local inhomogeneities in stiffness of actomyosin sheet are responsible for activity-driven out of plane deformations. These results open an arena for designing bio-inspired smart active materials with programmable deformations. |
Thursday, March 5, 2020 1:27PM - 1:39PM |
S29.00010: Confinement effects on the phase behavior of collagen-like semiflexible polymers Russell Spencer, Bae-Yeun Ha Semiflexible polymers often self-assemble into aligned structures with emergent mechanical and optical properties. These properties depend on the particular molecular arrangement they form. Indeed, the main structural component of bone, skin and corneas is collagen, but their mechanical and optical properties are vastly different because of the way these molecules are arranged. In this talk, we discuss the phase behavior of collagen-like semi-flexible polymers in the presence or absence of confinement, focusing on isotropic, nematic and cholesteric phases. Using self-consistent field theory, we first investigate phase boundaries and locate regions where each arrangement is stable or metastable. We then consider confinement effects by introducing planar walls. Our results suggest that the presence of planar walls reduces allowed alignment directions to being parallel with the wall, allowing the ordering direction to be uniquely set by the geometry of the experimental setup. We discuss how they can be used to tailor materials. |
Thursday, March 5, 2020 1:39PM - 1:51PM |
S29.00011: Shape study of Spindle-like microtubule tactoids using experiment and computation Sumon Sahu, Lena Herbst, Ryan Quinn, Jennifer L Ross Mitotic spindles during metaphase are a fundamentally important puzzle of biophysics at the base of the question of how a cell divides its genetic material. Mitotic spindle self-assembly is inherently non-equilibrium with a vast parameter space. Factors that play a role as a system include microtubules, molecular motors, crosslinkers, and associated proteins, whose working principles are not yet clear. Recently, we reconstituted a minimal model system for creating microtubule-based spindle-like assemblies using microtubules and the plant-derived crosslinker, MAP65. Here we have extended our work to determine the important requirements for tactoid formation by exploring different crowders or viscosity agents to show that they do not play a significant role determining the formation or shape of the tactoids. Also, using a computational approach we have shown that similar steady state tactoids can evolve from growing filaments that follow a damped Langevin equation and Hookean type crosslinkers. We explored the phase space of this organization by varying initial conditions like filament number, orientation, and length as well as crosslinker number, binding dynamics. Using these two directions we are uncovering the principles behind spindle organization using tactoid as a model. |
Thursday, March 5, 2020 1:51PM - 2:03PM |
S29.00012: Shear-induced gelation of self-yielding active networks David Gagnon, Claudia Dessi, Zvonimir Dogic, Daniel Blair The activity of molecular motors reconfigures biopolymer networks and modifies their mechanical properties. Designing these active gels with tunable properties analogous to the cytoskeleton is a key prerequisite for creating biomimetic systems to study cellular behavior such as division and motility. Active gels form ephemeral networks with long-range but temporary active mechanical contacts. In this talk, I will describe how microscopic dynamics modify the macroscopic mechanical properties of extensile microtubule networks. Rheological measurements reveal a non-monotonic dependence on the applied shear rate. A simple phenomenological model, which describes the network as a collection of fluid-like and solid-like elements, quantitatively explains the shear-rate-dependent viscosity in terms of locally-measured activity-induced flows. Fast, active elements remodel the network and therefore do not transmit elastic stresses, while slow, temporarily crosslinked elements behave elastically until they break and reform under shear. |
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