Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session A10: Microswimmers IBio Fluids: External
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Chair: Amir Nourhani, The Pennsylvania State University Room: 503 |
Sunday, November 19, 2017 8:00AM - 8:13AM |
A10.00001: Micro-navigation in complex periodic environments Alexander Chamolly, Takuji Ishikawa, Eric Lauga Natural and artificial small-scale swimmers may often self-propel in environments subject to complex geometrical constraints. While most past theoretical work on low-Reynolds number locomotion addressed idealised geometrical situations, not much is known on the motion of swimmers in heterogeneous environments. We investigate theoretically and numerically the behaviour of a single spherical micro-swimmer located in an infinite, periodic body-centred cubic lattice consisting of rigid inert spheres of the same size as the swimmer. We uncover a surprising and complex phase diagram of qualitatively different trajectories depending on the lattice packing density and swimming actuation strength. These results are then rationalised using hydrodynamic theory. In particular we show that the far-field nature of the swimmer (pusher vs. puller) governs the behaviour even at high volume fractions. [Preview Abstract] |
Sunday, November 19, 2017 8:13AM - 8:26AM |
A10.00002: Computational Fluid Dynamics of Choanoflagellate Filter-Feeding Seyed Saeed Asadzadeh, Jens Walther, Lasse Tore Nielsen, Thomas Kiorboe, Julia Dolger, Anders Andersen Choanoflagellates are unicellular aquatic organisms with a single flagellum that drives a feeding current through a funnel-shaped collar filter on which bacteria-sized prey are caught. Using computational fluid dynamics (CFD) we model the beating flagellum and the complex filter flow of the choanoflagellate Diaphanoeca grandis. Our CFD simulations based on the current understanding of the morphology underestimate the experimentally observed clearance rate by more than an order of magnitude: The beating flagellum is simply unable to draw enough water through the fine filter. Our observations motivate us to suggest a radically different filtration mechanism that requires a flagellar vane (sheet), and addition of a wide vane in our CFD model allows us to correctly predict the observed clearance rate. [Preview Abstract] |
Sunday, November 19, 2017 8:26AM - 8:39AM |
A10.00003: Numerical Simulations Of Flagellated Micro-Swimmers Cecilia Rorai, Anton Markesteijn, Mihail Zaitstev, Sergey Karabasov We study flagellated microswimmers locomotion by representing the entire swimmer body. We discuss and contrast the accuracy and computational cost of different numerical approaches including the Resistive Force Theory, the Regularized Stokeslet Method and the Finite Element Method. We focus on how the accuracy of the methods in reproducing the swimming trajectories, velocities and flow field, compares to the sensitivity of these quantities to certain physical parameters, such as the body shape and the location of the center of mass. We discuss the opportunity and physical relevance of retaining inertia in our models. Finally, we present some preliminary results toward collective motion simulations. [Preview Abstract] |
Sunday, November 19, 2017 8:39AM - 8:52AM |
A10.00004: Swimming in a Brinkman porous medium at low Reynolds number Herve Nganguia, On Shun Pak Micro-organisms encounter heterogeneous viscous environments due to networks of obstacles embedded into viscous fluid media. In this talk we present a theoretical investigation of swimming in such a heterogeneous medium modeled by the Brinkman equation. We calculate analytically the flow field surrounding an idealized micro-swimmer, its propulsion speed as well as swimming efficiency. The analytical solutions allow us to probe the general characteristics of swimming in a heterogeneous viscous environment in comparison with the case in a purely viscous fluid. [Preview Abstract] |
Sunday, November 19, 2017 8:52AM - 9:05AM |
A10.00005: Numerical study of the motion of a flagellated swimmer inside a tube in the Stokes regime Ji Zhang, Yusheng Jiao, Xinliang Xu, Yang Ding Confined environments are common to micro-swimmers such bacteria and previous studies have shown that confinements such as a wall can influenced the trajectory of the micro-swimmers. Here we study whether some micro-swimmers can achieve a higher speed and energetic efficiency within a long tube comparing to the free-space case using a numerical model. The swimmer consists of an elliptical head and two helical flagella. To solve the governing Stokes equations inside an infinite tube, we combine the method of fundamental solution (MSF) and the method of Stokeslet. The geometry parameters, including shape and size of head and flagella, and relative spatial position of these components, are varied. Our results show that the geometry of the swimmer and the tube can greatly affect the speed of the micro-swimmer. For certain geometric parameters of the micro-swimmer, a greater confinement leads to a higher speed, which is consistent with the results from our robotic experiments. [Preview Abstract] |
Sunday, November 19, 2017 9:05AM - 9:18AM |
A10.00006: Slipping slender bodies and enhanced flagellar locomotion Yi Man, Eric Lauga In the biological world, many cells exploit slender appendages to swim, include numerous species of bacteria, algae and spermatozoa. A classical method to describe the flow field around such appendages is slender-body theory (SBT), which is often used to study flagellar motility in Newtonian fluids. However, biology environments are often rheologically complex due to the presence of polymers. These polymers generically phase-separate near rigid boundaries where low-viscosity fluid layers lead to effective slip on the surface. In this talk, we present an analytical derivation of SBT in the case where the no-slip boundary condition on the appendage is replaced by a Navier slip boundary condition. Our results demonstrate in particular a systematic reduction of the resistance coefficient of the slender filaments in their tangential direction, which leads to enhanced flagellar locomotion. [Preview Abstract] |
Sunday, November 19, 2017 9:18AM - 9:31AM |
A10.00007: Imaging the 3D flow around swimming Chlamydomonas reinhardtii using digital inline holographic microscopy Kyle Welch, Santosh Kumar, Jiarong Hong, Xiang Cheng Understanding the 3D flow induced by microswimmers is paramount to revealing how they interact with each other and their environment. While many studies have measured 2D projections of flow fields around single microorganisms, reliable 3D measurement remains elusive due to the difficulty in imaging fast 3D fluid flows at submicron spatial and millisecond temporal scales. Here, we present a precision measurement of the 3D flow field induced by motile planktonic algae cells, Chlamydomonas reinhardtii. We manually capture and hold stationary a single alga using a micropipette, while still allowing it to beat its flagella in the breastroke pattern characteristic to C. reinhardtii. The 3D flow field around the alga is then tracked by employing fast holographic imaging on 1 um tracer particles, which leads to a spatial resolution of ~100 nm along the optical axis and ~40 nm in the imaging plane normal to the optical axis. We image the flow around a single alga continuously through thousands of flagellar beat cycles and aggregate that data into a complete 3D flow field. Our study demonstrates the power of holography in imaging fast complex microscopic flow structures and provides crucial information for understanding the detailed locomotion of swimming microorganisms. [Preview Abstract] |
Sunday, November 19, 2017 9:31AM - 9:44AM |
A10.00008: Prey capture by freely swimming flagellates Anders Andersen, Julia Dolger, Lasse Tor Nielsen, Thomas Kiorboe Flagellates are unicellular microswimmers that propel themselves using one or several beating flagella. Here, we explore the dependence of swimming kinematics and prey clearance rate on flagellar arrangement and determine optimal flagellar arrangements and essential trade-offs. To describe near-cell flows around freely swimming flagellates we consider a model in which the cell is represented by a no-slip sphere and each flagellum by a point force. For uniflagellates pulled by a single flagellum the model suggests that a long flagellum favors fast swimming, whereas high clearance rate is favored by a very short flagellum. For biflagellates with both a longitudinal and a transversal flagellum we explore the helical swimming kinematics and the prey capture sites. We compare our predictions with observations of swimming kinematics, prey capture, and flows around common marine flagellates. [Preview Abstract] |
Sunday, November 19, 2017 9:44AM - 9:57AM |
A10.00009: Brenner’s work revisited - Stokes flow past a deformed sphere Amir Nourhani, Vincent H. Crespi, Paul E. Lammert We revisit Brenner’s seminal work on Stokes flow past a deformed sphere, providing an alternative "extrapolation operator" formulation which may be easier to apply. We introduce an extrapolation operator that extrapolates the boundary condition flow field on a spherical particle to any arbitrary point in space. The velocity field around a deformed sphere with arbitrary boundary conditions is obtained by extrapolation from a reference sphere with boundary conditions determined as a perturbation expansion in the deformation. The formalism is also compatible with non-perturbative numerical approaches. A significant potential application is to effects of geometry for microswimmers and nanomotors of non-spherical, even somewhat exotic, shape. [Preview Abstract] |
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