Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T15: Focus Session: Active Soft Matter IV- Locomotion and Collective Behavior |
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Sponsoring Units: DPOLY GSNP DBIO Chair: Alfredo Alexander-Katz, Massachusetts Institute of Technology Room: 304 |
Thursday, March 6, 2014 11:15AM - 11:51AM |
T15.00001: to be determined by you Invited Speaker: Davide Marenduzzo |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T15.00002: Dynein's C-terminal Domain Plays a Novel Role in Regulating Force Generation Arne Gennerich, Matthew Nicholas, Sibylle Brenner, Caitlin Lazar, Sarah Weil, Richard Vallee, Peter Hook Cytoplasmic dynein is a microtubule motor involved in a wide range of low and high force requiring functions in metazoans. In contrast, yeast dynein is involved in a single, nonessential function, nuclear positioning. Interestingly, the single-molecule function of yeast dynein is also unique: whereas mammalian dyneins generate forces of 1-2 pN, S. cerevisiae dynein stalls at 5-7 pN. The basis for this functional difference is unknown. However, the major structural difference between mammalian and yeast dyneins is a $\sim $30 kDa C-terminal extension (CT) present in higher eukaryotic dyneins, but missing in yeast. To test whether the CT accounts for the differences in function, we produced recombinant rat dynein motor domains (MD) with (WT-MD) and without ($\Delta $CT-MD) the CT, using baculovirus expression. To define motor function, we performed single-molecule optical trapping studies. Dimerized WT-MD stalls at $\sim $1 pN and detaches from microtubules after brief stalls, in agreement with previous studies on native mammalian dynein. In sharp contrast, but similar to yeast dynein, $\Delta $CT-MD stalls at $\sim $6 pN, with stall durations up to minutes. These results identify the CT as a new regulatory element for controlling dynein force generation. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T15.00003: Active Jamming Jeremie Palacci, Stefano Sacanna, David Pine, Paul Chaikin Self-propelled micro-particles are intrinsically~out-of-equilibrium. This renders their physics far richer than that of passive colloids while relaxing some thermodynamical constraints and give rise to the emergence of complex phenomena e.g. collective behavior, swarming\textellipsis I will present the effect of a few active particles in a dense 2D layer of passive colloids. Surprising effect arise from the presence of the self-propelled particles which considerably modify the dynamics of the system. The addition of self propelled particles into materials open new perspectives in the design and the properties of new materials [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T15.00004: Colloidal Dancers: Designing networks of DNA-functionalized colloids for non-random walks Emily W. Gehrels, W. Benjamin Rogers, Zorana Zeravcic, Vinothan N. Manoharan We present experimental developments of a system of DNA-functionalized colloidal particles with the goal of creating directed motion (`dancing') along patterned substrates in response to temperature cycling. We take advantage of toehold exchange in the design of the DNA sequences that mediate the colloidal interactions to produce broadened, flat, or even re-entrant binding and unbinding transitions between the particles and substrate. Using this new freedom of design, we devise systems where, by thermal ratcheting, we can externally control the direction of motion and sequence of steps of the colloidal dancer. In comparison to DNA-based walkers, which move autonomously and whose motion is controlled by the substrate, our colloidal dancers respond to external driving, and their motion can be controlled in situ. Our use of DNA-functionalized colloidal particles instead of pure DNA systems also enables walking on the mesoscale in contrast to the molecular length scales previously demonstrated, allowing for the future prospect of directed transport over larger distances. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T15.00005: Cell crawling on filamentous tracks Jorge Lopez, Jennifer Schwarz, Moumita Das Recent experiments suggest that the migration of some cells in three dimensions has strong resemblance to one-dimensional migration. Motivated by this observation, we simulate a one-dimensional model cell made of beads and springs that moves on a tense semiflexible filamentous track. Physical parameters, such as the spring constants and friction coefficients, are calculated using effective theories. We investigate the mechanical feedback between the model cell and this track, as mediated by the active myosin-driven contractility and the catch/slip bond behavior of the focal adhesions, as the model cell crawls. We then compare our calculations of cell speed and the amount of deformation in the track with experiments. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T15.00006: Hydrodynamics and control of microbial locomotion Jorn Dunkel, Vasily Kantsler, Marco Polin, Hugo Wioland, Raymond Goldstein Interactions between swimming cells, surfaces and fluid flow are essential to many microbiological processes, from the formation of biofilms to the fertilization of human egg cells. Yet, relatively little remains known quantitatively about the physical mechanisms that govern the response of bacteria, algae and sperm cells to flow velocity gradients and solid surfaces. A better understanding of cell-surface and cell-flow interactions promises new biological insights and may advance microfluidic techniques for controlling microbial and sperm locomotion, with potential applications in diagnostics and therapeutic protein synthesis. Here, we report new experimental measurements that quantify surface interactions of bacteria, unicellular green algae and mammalian spermatozoa. These experiments show that the subtle interplay of hydrodynamics and surface interactions can stabilize collective bacterial motion, that direct ciliary contact interactions dominate surface scattering of eukaryotic biflagellate algae, and that rheotaxis combined with steric surface interactions provides a robust long-range navigation mechanism for sperm cells. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T15.00007: Enhancement of microbial motility due to advection-dependent nutrient absorption Carlos A. Condat, Mario E. Di Salvo In their classical work, Berg and Purcell [Biophys. J. 20, 193 (1977)] concluded that the motion of a small microorganism would not significantly increase its nutrient uptake rate, if the nutrient consisted of high diffusivity particles. As a result, it has been generally assumed that nutrient transport to small microorganisms such as bacteria is dominated by molecular diffusion and that swimming and feeding currents play a negligible role. On the other hand, recent studies have found that flagellar motion may increase advection-mediated uptake. We formulate a model to investigate the hypothesis that fast-moving microbes may enhance their swimming speed by taking advantage of advection to increase nutrient absorption. Surprisingly, using realistic parameter values for bacteria and algae, we find that even modest increases in nutrient absorption may lead to a significant increase of the microbial speed. We also show that, optimally, the rate of effective energy transfer to the microbial propulsion system should be proportional to the speed for slow motion, while it should be proportional to a power of the speed close to two for fast motion. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T15.00008: Collective behavior of chemotactic colloids: clusters, asters and oscillations Suropriya Saha, Ramin Golestanian, Sriram Ramaswamy Catalytic colloidal swimmers are a particularly promising example of systems that emulate properties of living matter, such as motility, gradient-sensing, signaling and replication. Here we present a comprehensive theoretical description of dynamics of an individual patterned catalytic colloid, leading controllably to chemotactic or anti-chemotactic behavior. We find that both the positional and the orientational degrees of freedom of the active colloids can exhibit condensation, signaling formation of clusters and asters. The kinetics of catalysis introduces a natural control parameter for the range of the interaction mediated by the diffusing chemical species. For various regimes in parameter space in the long-ranged limit our system displays precise analogs to gravitational collapse, plasma oscillations and electrostatic screening. We present prescriptions for how to tune the surface properties of the colloids during fabrication to achieve each type of behavior. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T15.00009: Collective motion in populations of colloidal robots Denis Bartolo, Antoine Bricard, Jean-Baptiste Caussin, Olivier Dauchot, Nicolas Desreumaux Could the behavior of bacteria swarms, fish schools, and bird flocks be understood within a unified framework? Can one ignore the very details of the interaction mechanisms at the individual level to elucidate how strikingly similar collective motion emerges at the group level in this broad range of motile systems? These seemingly provocative questions have triggered significant advance in the physics and the biology, communities over the last decade. In the physics language these systems, made of motile individuals, can all be though as different realizations of ``active matter.'' In this talk, I will show how to gain more insight into this vivid field using self-propelled colloids as a proxy for motile organism. I will show how to motorize colloidal particles capable of sensing the orientation of their neighbors. Then, I will demonstrate that these archetypal populations display spontaneous transitions to swarming motion, and to global directed motion with very few density and orientation fluctuations. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T15.00010: Emergent collective phenomena in a mixture of hard shapes through active rotation Michael Engel, Nguyen Nguyen, Daphne Klotsa, Sharon Glotzer We investigate collective phenomena with rotationally driven spinners of concave shape. Each spinner experiences a constant internal torque in either a clockwise or counterclockwise direction. Although the spinners are modeled as hard, otherwise non-interacting rigid bodies, their active motion induces an effective interaction that favors rotation in the same direction. With increasing density and activity, phase separation occurs via spinodal decomposition, as well as self-organization into rotating crystals. We observe the emergence of cooperative, super-diffusive motion along interfaces, which can transport inactive test particles. Our results demonstrate novel phase behavior of actively rotated particles that is not possible with linear propulsion or in non-driven, equilibrium systems of identical hard particles. Reference: arXiv:1308.2219 [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T15.00011: The flocking-laning transition in systems of self-propelled rods Hui-Shun Kuan, Robert Blackwell, Matthew A. Glaser, Meredith D. Betterton Collective motion occurs in a wide range of active systems, from flocks of birds to actin filaments in motility assays. In systems of self-propelled high-aspect ratio rods in two dimensions, flocking and laning phases can occur. We use Brownian dynamics simulation to study the collective motion of self-propelled rods in 2D for aspect ratios 20 and 40, packing fraction from 0.3 to 0.9, and Peclet number from 0 to 8. The flocking phase is globally isotropic, highly inhomogeneous, and exhibits high-density polar clusters. The laning phase has global nematic and local polar order and is relatively homogeneous. We study the transition from laning to flocking and show that this can be regarded as a transition from a fluid to a locally jammed state based on measurements of the contact number distribution, stress autocorrelation function, and structure factor autocorrelation function. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T15.00012: Emergence of collective motion in a model of interacting passive Brownian particles Victor Dossetti, Francisco J. Sevilla, Alexandro Heiblum-Robles In this work, we show that the state of a system of passive Brownian (non-self-propelled) particles interacting only through a social-like force (velocity alignment in this case), goes from stationary phases in thermal equilibrium with no net flux of particles, to far-from-equilibrium phases exhibiting collective motion. The mechanism that leads to the instability of the equilibrium phases relies on the competition between two time scales, namely, the mean collision time of a Brownian particle in a thermal bath and the time it takes for a particle to orient its direction of motion along the direction of motion of its neighbors. [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T15.00013: Flocking at a distance in active granular matter Harsh Soni, Nitin Kumar, Sriram Ramaswamy, Ajay Sood Flocking, the self-organised motion of vast numbers of living creatures in a single direction, relies on organisms sensing each other's presence, orientation and direction of movement. We have attempted to emulate these properties in experiments of fore-aft asymmetric particles energised by a vertically vibrated horizontal surface, and validate and extend our results using computer simulations and a simple hydrodynamic theory. In these studies the asymmetric rods communicate their orientation and directed motion over several rod lengths through a medium of spherical beads. This results in a phase transition from an isotropic state to a coherently moving flock at exceptionally low rod concentrations, an observation reinforced by large-scale numerical simulations. Our findings include a phase diagram in the plane of rod and bead concentrations, power-law spatial correlations upon approaching the phase boundary, and insights into the underlying mechanisms. [Preview Abstract] |
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