Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session Q22: Biological Fluid Dynamics: Locomotion  Microswimmers 
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Chair: On Shun Pak, Santa Clara University Room: Georgia World Congress Center B310 
Tuesday, November 20, 2018 12:50PM  1:03PM 
Q22.00001: Boundary behaviours of Leishmania mexicana: a hydrodynamic simulation study Benjamin Walker, Richard Wheeler, Kenta Ishimoto, Eamonn Gaffney The cause of a neglected human tropical disease, the microswimmer Leishmania mexicana exhibits complex surface interactions with the sandfly vector midgut. Previously unstudied, the hydrodynamics of this largebodied monoflagellate puller may not be deduced from previous studies of lowReynolds number swimming using the timereversibility of Stokes flow. Thus we aim to elucidate the boundary behaviours of such a swimmer, and begin by identifying a planar mode of the beating flagellum from digital videomicroscopy. Swimmer behaviour is then studied in detail via the boundary element method, utilising timeaveraged phase plane analysis to study virtual Leishmania promastigotes in the presence of a planar boundary. From this analysis we identify a remarkable morphologydependent mechanism of boundary approach, a potential mechanism driving tipfirst epithelial attachment seen in L. mexicana in vivo. 
Tuesday, November 20, 2018 1:03PM  1:16PM 
Q22.00002: Is hydrodynamic interaction enough for green algae to bounce back from a wall? Mehdi Mirzakhanloo, MohammadReza Alam The primary mechanism behind the interaction of swimming microorganisms with solid boundaries has been a matter of dispute. Based on series of visual observations on the behavior of green alga Chlamydomonas reinhardtii (widely known as a paradigm for pullertype swimmers), it has been believed that the scattering process is mainly governed by contact forces rather than hydrodynamic interactions. Here, via investigation of a model microswimmer designed in such a way that its flow field closely resembles the complex oscillatory flow of a C. reinhardtii cell, we show that the scattering from a wall can be purely hydrodynamic and that no mechanical force is needed for sensing and escaping the boundary. 
Tuesday, November 20, 2018 1:16PM  1:29PM 
Q22.00003: Threedimensional imaging and force mode analysis of microflows induced by swimming Chlamydomonas reinhardtii Kyle J Welch, Santosh Kumar, Bokai Zhang, Xinliang Xu, Jiarong Hong, Xiang Cheng Understanding the fluid flow induced by microswimmers is paramount to revealing how they interact with each other and their environment. Here, we present a threedimensional (3D) measurement and characterization of the flow field induced by motile planktonic algal cells, Chlamydomonas reinhardtii. A single alga is captured and held stationary by a micropipette, which beats its flagella in a characteristic breastroke pattern. We track the 3D flow field around the alga by employing fast holographic imaging on 1 um tracer particles, which leads to a nominal spatial resolution of ~ 54 nm along the optical axis and ~ 44 nm in the imaging plane. The method allows us to image the flow around a single alga continuously over thousands of flagellar beat cycles and show timeaveraged and phasebinned 3D flow fields. We analyze these 3D flow fields and determine the dominant force modes of the flagellar motion of C. reinhardtii. Our study demonstrates the power of holography in imaging detailed microscopic flows and provides crucial information for understanding the detailed locomotion of swimming microorganisms. 
Tuesday, November 20, 2018 1:29PM  1:42PM 
Q22.00004: The (Anti) Gravity Machine: Behavioral Imaging at Subcellular Resolution of Marine Plankton Undergoing VerticalMigration Deepak Krishnamurthy, Francois Benoit du Rey, Hongquan Li, Pierre Cambournac, Elgin Korkmazhan, Manu Prakash Marine plankton exhibit a Diel Vertical Migration with vertical displacement scales from several tens to hundreds of meters. Even at the scale of small phytoplankton and zooplankton (100 μm to a few mm) the interaction of this vertical swimming behavior with hydrodynamics affects large scale distribution of populations in the ocean and is thus an important component of understanding ocean ecology. However, concurrently observing organismal physiology and behavior is challenging due to the vast separation of scales involved. Resolving physiological processes involves subcellular (micron) resolution while tracking freely swimming organisms implies vertical displacements of several meters. We present a simple solution to this problem in the form of a “hydrodynamic treadmill” incorporated into a tabletop tracking microscope. We use this method to study the behavior of freely swimming marine plankton, including the larvae of P. miniata (Bat Star), O. spiculata (Brittle Star), S. purpuratus (Sea Urchin) and D. excentricus (Sand Dollar). Our studies reveal a rich space of dynamic behavioral states including continuous swimming, hovering and feeding. We further use our method to study problems such as the growth dynamics and ecology of falling marine snow. 
Tuesday, November 20, 2018 1:42PM  1:55PM 
Q22.00005: Flow around a squirmer in a shearthinning fluid Kyle Pietrzyk, Herve Nganguia, Charu Datt, Lailai Zhu, Gwynn Elfring, On Shun Pak Microorganisms often swim in biological fluids displaying shearthinning rheology. Although previous analyses exploited the reciprocal theorem to calculate the propulsion speed of a squirmer in a shearthinning fluid, this approach does not provide information on the flow surrounding the squirmer. In this work, we fill in this missing information by calculating the nonNewtonian correction to the flow in the small Carreau number limit analytically. Our detailed flow analysis allows us to physically interpret how shearthinning rheology alters swimming performance at low Reynolds numbers. 
Tuesday, November 20, 2018 1:55PM  2:08PM 
Q22.00006: Swimming in a twofluid model Patrick J. McCurdy, Herve Nganguia, On Shun Pak In this talk, we consider a twofluid model to investigate the motion of a microswimmer in heterogeneous environments. We extend previous analyses to account for the effect of porosity and study its influence on the propulsion speed and the surrounding flow. Our exact solution allows us to characterize the flow fields and their spatial decay analytically. The setup of the theoretical model can be relevant to different biological scenarios, including a swimming cell surrounded by an extracellular matrix, and a bacterium enclosed by a degelled region during its invasion into mucus layers protecting epithelial cells. 
Tuesday, November 20, 2018 2:08PM  2:21PM 
Q22.00007: Coupling between swimming and feeding efficiencies of ellipsoidal squirmers in a nutrientdependent viscous flow Patrick Eastham, Kourosh Shoele Ciliary locomotion is a wildly used method of transportation employed by bacteria and other singlecelled organisms. Due to the Stokes limit that these creatures move in, their thrust is entirely made by viscous stresses. Recently we analytically studied the effect of a weakdependence of viscosity on the dynamic nutrient field surrounding a mobile spherical squirmer and presented that the optimal stroke modes for the variableviscosity environment can be different from those in the constantviscosity environment, where nutrient and viscosity are uncoupled. The current work extends these findings to the case where the fluid viscosity is fully dependent on the nutrient. This dependence creates feedback between the nutrient field and the fluid flow with intriguing consequences. Using the Finite Element method, the effects of nutrient Peclet number, stroke modes, and form of nutrientviscosity dependence are investigated. The changes in the speed and feeding efficiencies achieved by the model bacteria compared to the constantviscosity case are shown and we discuss how the shape of the swimmer affects the observed feedback between its swimming and feeding performances. 
Tuesday, November 20, 2018 2:21PM  2:34PM 
Q22.00008: Polygonal swimming via coordinated flagellar beat switching in microswimmer Euglena gracilis Alan Cheng Hou Tsang, Amy T. Lam, Ingmar H. RiedelKruse Biological microswimmers exhibit versatile strategies for sensing and navigating their environment, e.g., runandtumble and curvature modulation. Here we report a striking behavior of Euglena gracilis, where Euglena cells swim in polygonal trajectories due to a sudden increase in light intensity. While smoothly curved trajectories are common for microswimmers, such quantized ones have not been reported previously. This polygonal behavior emerges from periodic switching between the flagellar beating patterns of helical swimming and spinning behaviors. We develop and experimentally validate a biophysical model that captures all three behavioral states. Coordinated switching between these behaviors selects for ballistic, superdiffusive, diffusive, or subdiffusive motion (including tuning the effective diffusion constant over several orders of magnitude), thereby enabling navigation in spatially structured light fields, e.g., edge avoidance and gradient descent. This feedbackcontrol links multiple system scales (flagellar beats, cellular behaviors, phototaxis strategies) with implications for other natural and synthetic microswimmers. 
Tuesday, November 20, 2018 2:34PM  2:47PM 
Q22.00009: Nonlocal hydrodynamic interactions between undulating flagella Kirsty Y. Wan, Raymond E Goldstein The emergence of synchronized or coordinated behaviours is a welldocumented phenomenon pertaining to interacting pairs, bundles, or groups of slender beating filaments such as arrays of airway epithelial cilia, or the flagella of the spherical alga Volvox carteri. More recent experimental and theoretical work highlight the key role of flagellar waveform compliance in mediating measured synchronization regimes. Conceptual, lowdimensional models which consider elastohydrodynamic interactions between nearby flagella often assume large interflagellar separations, which are not always realistic and cannot readily capture all of the observed dynamics. Here we visualize and quantify symmetrybreaking processes in novel synchronization modes of algal flagella during the physiological "shock response", demonstrating the extent to which nonlocal respresentations of hydrodynamic interactions between extended microfilaments are necessary in the context of asymptoticallyclose interflagellar distances.

Tuesday, November 20, 2018 2:47PM  3:00PM 
Q22.00010: Proper Orthogonal Decomposition of flagellar motion in sperm to reconstruct and compare typical beat patterns Ashwin Nandagiri, Avinash Gaikwad, Moira O'Bryan, Julio Soria, Ranganathan Prabhakar, Sameer Jadhav Internallydriven flagella in sperm beat with a complex waveform. We extract flagellar beating patterns from highspeed, highresolution videos of flagellar beating in tethered mouse sperm. Proper Orthogonal Decomposition is used to resolve beat patterns into major shape modes. Typical beat patterns are constructed by averaging the timedependent amplitudes of the major shape modes. The Slender Body Theory is applied to obtain typical surface traction and hydrodynamic power distributions across sperm flagella. It is observed that sperm from wildtype mice and CRISP2deficient mutants have similar major shape modes but differ in their amplitudes. The power distribution curves are thus different for the two species indicating a change in the location of the maximal power dissipation due to the mutation. 
Tuesday, November 20, 2018 3:00PM  3:13PM 
Q22.00011: Taming elastoelectrohydrodynamic instability for lowReynoldsnumber propulsion Lailai Zhu, Howard A. Stone Several microorganisms propel themselves by propagating oscillatory bending waves along their slender appendages, flagella and cilia. This selforganised oscillation results from the sliding forces powered by the dynein motor proteins. Such a sophisticated strategy may not be easily engineered to drive synthetic microrobots. An oscillatory electric or magnetic field is commonly used to wiggle a polarized body with a flexible slender structure along which undulatory waves are propagated. By employing the electrohydrodynamic instability to actuate the undulation of an elastic filament, we propose a new strategy to drive the lowReynoldsnumber synthetic swimmers based on a timeindependent uniform electric field. We solve the coupled electrohydrodynamic equations and Euler–Bernoulli equations for the elastic filament numerically. We demonstrate that in certain regimes, instability occurs through a pitchfork or Hopf bifurcation; in the latter case, the filament undergoes selforganised undulation resulting in locomotion. We perform a linear stability analysis incorporating elastohydrodynamic models and the predicted critical elastic numbers corresponding to the onset of instability agree well with the numerical counterparts. 
Tuesday, November 20, 2018 3:13PM  3:26PM 
Q22.00012: Stability of Helical Microswimmers Under Confinement Hakan O Caldag, Serhat Yesilyurt Artificial microswimmers are prospective agents for various microfluidic and medical applications. Many swimming scenarios involve confinement such as arteries or microchannels. Confinement affects swimming trajectories and velocities both for artificial and natural swimmers. Typically, helical swimmers which are rotated by an external torque follow helical trajectories in the pushermode while they follow the centerline of the channel in the pullermode depending on the geometry of the swimmer and the channel. In order to study the effects of geometric parameters on the trajectories of swimmers inside channels, kinematics of the swimmer is coupled with a CFD model, which is used to obtain linear and angular velocities of the timevarying system subject to magnetic torques and contact forces. The force dipole created by the rotating swimmer contributes to pushermode instability. The level of instability is measured by the nondimensional radius of the helical trajectories. The effects of geometric and physical parameters on stability are reported to improve the understanding on stability of the swimmers under confinement which is expected to be crucial for controlled navigation. 
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