Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V61: Physics and Hydrodynamics of Micro-swimmers' SuspensionsFocus
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Sponsoring Units: GSOFT Chair: Arnold Mathijssen, Stanford University Room: BCEC 258B |
Thursday, March 7, 2019 2:30PM - 2:42PM |
V61.00001: Swimming bacteria in a Poiseuille flow : the quest for active Betherton-Jeffery trajectories Gaspard Junot, Thierry Darnige, Nuris Figueroa Morales, Anke Lindner, Harold Auradou, Eric Clement Using a 3D Lagrangian tracking technique, we determine the swimming trajectories of E.coli in a Poiseuille flow. We identify many trajectories displaying the qualitative features of an “active Bretherton-Jeffery” model featuring an axi-symmetric self-propelled ellipsoid swimming in a flow. By deriving closed-forms for two phase-portraits involving one spatial and two angular coordinates as well as, the mean angular shift displayed by the families of cycloid trajectories, we show that for a smooth swimmer mutant, a quantitative mapping with the actual experimental trajectories can be made. Furthermore we show that from cycloid trajectories, the effective aspect ratio of the swimmer can be extracted. This analysis also allows to highlight the crucial influence of Brownian rotation noise, on the swimmer trajectories. |
Thursday, March 7, 2019 2:42PM - 2:54PM |
V61.00002: Characteristics of collective and individual motions of swimming Phytophthora zoospores Quang Tran, Philippe Thomen, Céline Cohen, Eric Galiana, Xavier Noblin Phytophthora diseases cause big threats to agriculture and eco-systems. Spreading is based on rapid dispersion of biflagellate swimming zoospores that once having reached a host initiate plant infection. Understanding their swimming mechanism and their interactions against gradients and surrounding environments becomes important. In this study, we develop a microfluidic system to investigate collective and individual motions of Phytophthora parasitica zoospores, a species that infects a broad range of host plants. Our system has the ability to generate a chemical gradient diffusing to a group of swimming zoospores and observe their swimming motions as well as the changes of the gradient at the same time. Our results show that a group of P. parasitica react differently against different doses of potassium chloride gradients: Low concentration of potassium reduces their speed and lures them away or initiates auto-aggregation, while high concentration (>3mM) causes them to change their swimming pattern to circulating around or stop moving. Moreover, observing a single zoospore swimming in water, we achieved the characteristics of its beating flagella. The correlation between zoospore velocity and its flagella motions helps us explain their reactions against the potassium gradients. |
Thursday, March 7, 2019 2:54PM - 3:06PM |
V61.00003: Dancing to the swimmers' beat: Loopy Lévy flights enhance tracer diffusion in active suspensions Kiyoshi Kanazawa, Tomohiko Sano, Andrea Cairoli, Adrian Baule The diffusion process followed by a tracer in a medium out of equilibrium typically exhibits anomalous diffusion that cannot be modelled by Brownian motion. Prototypical active media are suspensions of swimming microorganisms like algae and bacteria, where the tracer is dragged by the hydrodynamic flow generated by the swimmers. Several experiments have characterised the tracer diffusion in dilute conditions by a greatly enhanced diffusion coefficient, non-Gaussian tails of the displacement statistics, and crossover scaling phenomena. Despite the abundant experimental results, there is so far no comprehensive theory that can describe all these features. Here we present a theoretical framework of the enhanced tracer diffusion from first-principles, by coarse-graining the microscopic tracer-swimmer interactions as a coloured Lévy Poisson process. This theory not only provides the toolkit necessary to characterise theoretically the tracer diffusion but also paves the way to the study of its stochastic thermodynamics. |
Thursday, March 7, 2019 3:06PM - 3:18PM |
V61.00004: Active colloidal particles in emulsion droplets: A model system for the cytoplasm Viva Horowitz, Zachary C. Chambers, Irep Gözen, Thomas G. Dimiduk, Vinothan N Manoharan In living cells, molecular motors create activity that enhances the diffusion of particles throughout the cytoplasm, and not just ones attached to the motors. We demonstrate initial steps toward creating artificial cells that mimic this phenomenon. Our system consists of active, Pt-coated Janus particles and passive tracers confined to emulsion droplets. We track the motion of both the active particles and passive tracers in a hydrogen peroxide solution, which serves as the fuel to drive the motion. We first show that correcting for bulk translational and rotational motion of the droplets induced by bubble formation is necessary to accurately track the particles. After drift correction, we find that the active particles show enhanced diffusion in the interior of the droplets and are not captured by the droplet interface. At the particle and hydrogen peroxide concentrations we use, we observe little coupling between the active and passive particles. We discuss the possible reasons for lack of coupling and describe ways to improve the system to more effectively mimic cytoplasmic activity. |
Thursday, March 7, 2019 3:18PM - 3:54PM |
V61.00005: Spontaneous chiral symmetry breaking in active fluids Invited Speaker: Jorn Dunkel Recent experiments show that bacterial and other active suspensions in confined geometries can self-organize into persistent flow structures that exhibit spontaneously broken mirror symmetry. I will discuss how this effect can be used to realize states of collective ferro- and anti-ferromagnetic order in bacterial spin lattices. To describe actively driven solvent flows within a minimal theoretical framework, we consider generalized Navier-Stokes (GNS) equations that combine a generic linear instability mechanism with a conventional advective nonlinearity. This phenomenological model is analytically tractable and captures a number of experimental observations. Triad analysis and numerical simulations of the GNS system predict that 3D active flows can realize chiral Beltrami vector fields that support inverse energy transport from smaller to larger scales, in contrast to classical 3D turbulence. |
Thursday, March 7, 2019 3:54PM - 4:06PM |
V61.00006: Dial-a-plume: Active Control of Localised Photo-Bio-Convection Marco Polin, Jorge Arrieta, Ramon Saleta-Piersanti, Idan Tuval Microorganismal motility is often characterised by complex responses to environmental physico-chemical stimuli. Although the biological basis of these responses is often not well understood, their exploitation already promises novel avenues to directly control the motion of living active matter at both the individual and collective level. Here we leverage the phototactic ability of the model microalga Chlamydomonas reinhardtii to precisely control the timing and position of localised cell photo-accumulation, leading to the controlled development of isolated bioconvective plumes. This novel form of photo-bio-convection allows a precise, fast and reconfigurable control of the spatio-temporal dynamics of the instability and the ensuing global recirculation, which can be activated and stopped in real time. A simple continuum model accounts for the phototactic response of the suspension and demonstrates how the spatio-temporal dynamics of the illumination field can be used as a simple external switch to produce efficient bio-mixing. |
Thursday, March 7, 2019 4:06PM - 4:18PM |
V61.00007: Percolation theory of intercellular communication through hydrodynamic trigger waves Arnold Mathijssen, Josh Culver, Saad Bhamla, Manu Prakash Here, akin to a chain reaction, we present the discovery of hydrodynamic trigger waves in cellular communities of the protist Spirostomum ambiguum, propagating hundreds of times faster than their swimming speed. These cells can contract their body by 50% within milliseconds, equivalent to 14g-forces. A single contraction (transmitter) generates long-ranged vortex flows that trigger neighbouring cells, in turn. We further present a microfluidic device to measure the sensitivity to hydrodynamic signals (receiver). Using percolation theory, a phase transition is revealed that requires a critical cell density to sustain communication. Our results suggest that this signalling could help organise cohabiting communities over large distances, comparable to quorum sensing. Moreover, as contractions release toxins, synchronised discharges could also facilitate the repulsion of large predators, or conversely immobilise large prey. We postulate that beyond protists numerous other freshwater and marine organisms could coordinate with variations of hydrodynamic trigger waves. |
Thursday, March 7, 2019 4:18PM - 4:30PM |
V61.00008: Gravity induced structures of microswimmers on a surface Zaiyi Shen, Alois Würger, Juho Lintuvuori Microswimmers can self-organize to a variety of collective states. The underlying forces, however, are not well characterized. Besides the electric, magnetic or chemical surface forces, hydrodynamic interactions are believed to play important role on the collective motion of active colloids. In typical experiments of artificial self-propelled colloids, the particles are observed to sediment. We study microswimmer suspensions by lattice Boltzmann simulations, using a squirmer model. We show that hydrodynamic interactions, together with a gravity-like field, lead to tunable collective behaviors of self-propelled spheres near a surface. For example chiral spinners, swarming clusters and living crystals are observed. We rationalize the formation of these structures, in detail, based on hydrodynamic interactions between a pair of swimmers. These interactions depend crucially of the swimming mechanism, pusher or puller, and can be tuned such that they are either directional, leading to the formation of small chiral spinners or mutually attractive, creating large hydrodynamically bound motile aggregates. |
Thursday, March 7, 2019 4:30PM - 4:42PM |
V61.00009: Emergence of phytoplankton patchiness at small scales in mild turbulence Rebekka Breier, Cristian C Lalescu, Michael Wilczek, Marco G. Mazza Phytoplankton often encounter turbulence in their habitat. The spatial distribution of motile phytoplankton cells exhibits patchiness at distances of decimeter to millimeter scale for numerous species with different motility strategies. The explanation of this general phenomenon remains challenging. We combine particle simulations and continuum theory to study the emergence of patchiness in motile microorganisms in three dimensions, by including hydrodynamic cell-cell interactions, which grow more relevant as the density in the patches increases. By addressing the combined effects of motility, cell-cell interaction and turbulent flow conditions, we uncover a general mechanism: the coupling of cell-cell interactions to the turbulent dynamics favors the formation of dense patches. |
Thursday, March 7, 2019 4:42PM - 5:18PM |
V61.00010: Non-Gaussian limit fluctuations in active swimmer suspensions Invited Speaker: Daisuke Mizuno Hydrodynamic fluctuations in suspensions of swimming microorganisms (Chlamydomonas and E-coli) exhibit heavily-tailed distribution which is not Gauss nor Levy, for which both the classical and extended central limiting theories do not apply. In this study, the physical limit distribution, instead of mathematical ones, was derived in an analytical form by summing the general power-law interactions from field sources (here, swimming microorganisms) randomly distributed in general spatial dimensions. The origin of the non-Gaussianity is not just the power-law decay of hydrodynamic fields, but the summing procedure of the fields, which we refer to as the physical limit operation [1]. |
Thursday, March 7, 2019 5:18PM - 5:30PM |
V61.00011: Bacterial diodes: Rectified transport of swimming cells in porous media flow Nicolas Waisbord, Jeffrey Guasto Directed motility enables swimming microbes to navigate their porous habitats for resources, where self-propulsion competes with fluid flow to affect processes ranging from disease transmission and bioremediation. Despite this broad importance, how directed motility affects the self-transport and dispersion of microswimmers in flow through constricted pores remains unknown. Focusing on magnetotactic bacteria in a microfluidic porous medium, we show that upstream oriented cells, directed by a magnetic field, are localized and trapped in vortical orbits at a constriction. Vortical cell localization results in three distinct regimes of rectified bacterial conductivity through a throat, akin to a ‘bacterial diode’, whereby cells swim upstream, become trapped within a pore, or are advected downstream with increasing flow speed. Langevin simulations reveal that the trapping regime results in near-complete transport suppression, while ephemoral trapping in the downstream regime enhances dispersion. We also show that vortical cell localization persists in three-dimensional flow through a packed microfluidic bed, emphasizing the relevance of this phenomenon in realistic hydraulic networks. |
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