Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session G31: Biological Fluid Dynamics: Micro-Swimmer General II |
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Chair: Paulo Arratia, University of Pennsylvania Room: 613 |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G31.00001: Clogging of microswimmers at a constriction. Philippe Peyla, Marvin Brun-Cosme-Bruny, Salima Rafai, Andre Foertsch, Walter Zimmermann We study the clogging of a suspension of photosensitive microswimmers [\textit{Chlamydomonas Reinhardtii }(CR)] moving to a constriction in a microfluidic device. Swimming cells are fleeing light and accumulate at a gate that is twice larger than a CR. We study the statistics of times between two successive egresses. Our results fall in the general framework of clogging obtained for panicking pedestrians at a gate or granular materials at the exit of a silo: the survival function obeys a power law decrease with times. Our results also show a faster-is-slower effect: when cells are faster, clogging is increased. This phenomenon - very well known in crowd evacuation during a panic - unveils the role of tangential friction between cells at the constriction where unusual densities are reached. Our experimental observations are supported by lattice Boltzmann simulations. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G31.00002: Bacterial magneto-convection Albane Thery, Lucas Le Nagard, Jean-Christophe Ono-dit-Biot, Cecile Fradin, Kari Dalnoki-Veress, Eric Lauga Magnetotactic bacteria are prokaryotic microswimmers that synthetize magnetosomes, which are chains of nano-magnets in their cytoplasm. Under external magnetic fields these cells are subject to magnetic torques and thus passively align along magnetic lines, providing a simple control mechanism. We use microfluidics experiments to show that collective motion arises from an initial uniform distribution of magnetotactic bacteria under confinement when an external magnetic field is applied. Dense suspensions of magnetotactic bacteria of strain AMB1 are driven toward a glass capillary wall by a magnetic field perpendicular to the channel walls. Their initial random spacing on the surface becomes unstable due to attractive hydrodynamic interactions between swimmers. These interactions result in bacterial magneto-convection: bacterial plumes perpendicular to the wall emerge spontaneously and develop into self-sustained convection cells. The plumes grow and merge and their dynamics is studied experimentally by measuring their wavelength and the flow they generate. Using a numerical model based on hydrodynamic singularities, we are able to capture quantitatively the instability observed in the cell suspension and reproduce the flow field as well as the long-time clustering dynamics. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G31.00003: Phase transitions in the system of active and passive microswimmers Naveen Kumar Agrawal, Pallab Sinha Mahapatra We study the steady-state phases and their transition in a system of active and passive microswimmers. The active and passive microswimmers are initially, randomly distributed in a fluidic medium inside a square enclosure. The active microswimmers move by a constant magnitude self-propulsion force. Whereas, the passive microswimmers have no self-propulsion force, and they move by force exerted by the fluid and the other neighboring microswimmers. A microswimmer's interactions with other microswimmers and the fluidic medium govern the direction of the exerted thrust on it. We have used a discrete particle model to solve the governing equations. Here, the hydrodynamic interaction is modeled as Stokes drag. The phase transition depends on the coordination coefficients (identified by a parameter $\chi )$ of the microswimmers, initial states of the microswimmers, and the fraction of active microswimmers present in the system ($\rho )$. At low $\chi $, the microswimmers exhibit a random motion. For the higher $\chi $ values, the phase transits from random motion to the milling phase, where microswimmers rotate around the core. Milling motions with a hollow core are also observed. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G31.00004: Correlations in microswimmer suspensions Viktor Skultety, Alexander Morozov Recent years witnessed a significant interest in physical, biological and engineering properties of self-propelled particles, such as bacteria or synthetic microswimmers. The main distinction of this 'active matter' from its passive counterpart is the ability to extract energy from the environment and convert it into directed motion. One of the most striking consequences of this distinction is the appearance of collective motion in self-propelled particles suspended in a fluid observed in recent experiments and simulations: at low densities particles move around in an uncorrelated fashion, while at higher densities they organise into jets and vortices comprising many individual swimmers. Here, we present a novel kinetic theory that predicts the existence of strong correlations even below the transition to collective motion. We calculate the velocity-velocity correlation functions and the effective diffusivity of passive tracers, and reveal their non-trivial density dependence. The theory is in quantitative agreement with our recent Lattice-Boltzmann simulations (J. Stenhammar et al., Phys. Rev. Lett. 119, 028005 (2017)) and captures the asymmetry between pusher and puller swimmers below the transition to collective motion. [Preview Abstract] |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G31.00005: Auto-phoretic nanorods driven up the wall by gravity Quentin Brosseau, Florencio Balboa Usabiaga, Enkeleida Lushi, Yang Wu, Leif Ristroph, Mike Ward, Mike Shelley, Jun Zhang Gravitaxis is the directed upward motion of micro-organisms against gravity, and is observed for a few ciliated organisms like~\textit{Chlamydomonas},~\textit{Euglenas}~or~\textit{Paramecium}. Lacking a dedicated sensor, their gravitactic response relies on bottom-heaviness or shape anisotropy to induce a bias in their swimming direction. Here we study the gravitaxis of heavy self-electrophoretic Janus nanorods that move upwards on a steeply inclined substrate. Comparisons in experiments and simulations between homogeneous and bottom-heavy nanorods reveal two mechanisms contributing to the gravitactic response of the latter: a buoyancy torque and hydrodynamic interactions with the wall. We show that lubrication forces induce an effective fore-aft asymmetry on nanorods that reinforces the orientation bias to move up the steep wall against gravity. [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G31.00006: Transport and Dynamics of Swimming Microorganisms in Time-Periodic Flows Ranjiangshang Ran, Boyang Qin, Brendan Blackwell, Paulo Arratia Microorganisms often need to navigate through complex flow environments to successfully feed and reproduce. Examples include algae in oceans and lakes, bacteria in gastrointestinal tracts, and during the production of food and vaccines. Here, we experimentally investigate the transport and mixing of swimming E. coli in two-dimensional time-periodic flows using particle tracking velocimetry and dye mixing experiments. Mixing is assessed by computing stretching fields and finite-time Lyapunov exponents (FTLE) from experimentally measured velocity fields. Velocimetry data shows that bacteria lowers the peak velocity and vorticity of the flow. This result is a function of bacteria concentration and flow geometry; that is, flow separatrices hinder mixing compared to passive particles likely due to bacteria trapping. Stretching fields show that bacteria shorten the average wavelength of Lagrangian coherent structures (LCSs). Overall, mixing can increase or decrease by the addition of active particles (i.e. swimming bacteria) but there seems to be a non-trivial dependence on Reynolds number, path length, flow geometry, and bacteria volume fraction. [Preview Abstract] |
Sunday, November 24, 2019 5:06PM - 5:19PM |
G31.00007: Route to bacterial swarming Xiang Cheng, Yi Peng, Zhengyang Liu Collective motions of active fluids such as bird flocks, fish schools and bacterial swarms demonstrate the intriguing emergent behaviors of nonequilibrium systems. While moving independently at low density, active entities in an active fluids move collectively with its neighbors at high density, exhibiting strong orientational order at a scale orders of magnitude larger than the size of individual entities. Although such a disorder-order nonequilibrium phase transition has been previously studied, the detailed kinetics of this transition has not been systematically explored in experiments. Here, using light-controlled E. coli, whose locomotion can be reversibly controlled by light, we experimentally study the kinetic pathway of the swarming transition in 3D bacterial suspensions. The phase diagram of bacterial swarming as functions of bacterial concentration, the velocity of active swimmers and the number fraction of active swimmers are systematically mapped. Moreover, we identify different kinetic pathways for the swarming transition depending on the control parameters. Our results reveal the route to the emergent bacterial swarming and provide new insights into the nonequilibrium phase transition in active fluids. [Preview Abstract] |
Sunday, November 24, 2019 5:19PM - 5:32PM |
G31.00008: Hydrodynamic interactions of Chlamydomonas with a solid surface Abel-John Buchner, Koen Muller, Daniel Tam Motile unicellular organisms swim through complex environments and often interact with solid surfaces. Their swimming is influenced by the proximity to solid substrates, through hydrodynamic and steric interactions. These interactions directly influence the cell population density distribution and the residence time in the vicinity of the surface, in turn modulating the probability of cell-surface adhesion and subsequent surface colonisation. The extent to which hydrodynamic forces influence cell-wall interactions remains unclear and previous experimental studies have often been limited to two-dimensional flow cells, which confine the trajectories of the swimming cells. Here, we investigate the interaction of free-swimming cells with surfaces in an otherwise unconstrained three-dimensional flow chamber. Our tracking experiments focus on the model ``puller'' organism Chlamydomonas reinhardtii. Swimming cells are recorded simultaneously by four separate cameras and triangulated in three-dimensions. Kinematic statistics are calculated from approximately 30,000 swimming tracks. Our results provide evidence of the existence of a long-range hydrodynamic interaction, which induces orbiting behaviour in the near-surface region. [Preview Abstract] |
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