Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session R21: Bio: Microswimmers |
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Chair: Eva Kanso, University of Southern California Room: D139-140 |
Tuesday, November 22, 2016 1:30PM - 1:43PM |
R21.00001: Precession in Stokes flow: spin and revolution of a bacterial flagellum Takuji Ishikawa, Yoichiro Sawano, Hiromichi Wakebe, Yuichi Inoue, Akihiko Ishijima, Yuji Shimogonya The bacterial flagellar motor is an ion-driven rotary machine in the cell envelope of bacteria. When we performed a bead assay, in which the cell body was affixed to a glass surface to observe the rotation of a truncated flagellum via the positioning of a 250 nm-diameter gold nanoparticle, we often observed that the filament motion consisted of two types of rotation: spin and revolution, which resulted in precession. Since the mechanism of flagella precession was unknown, we investigated it using numerical simulations. The results show that the precession occurred due to hydrodynamic interactions between the flagellum and the wall in the Stokes flow regime. We also developed a simple theory of the precession, which validity was confirmed by comparing with the simulation. The theory could be utilized to predict both the filament tilt angle and motor torque from experimental flagellar precession data. The knowledge obtained is important in understanding mechanical properties of the bacterial motor and hook. [Preview Abstract] |
Tuesday, November 22, 2016 1:43PM - 1:56PM |
R21.00002: Hydrodynamics of freely swimming flagellates Julia Dolger, Lasse Tor Nielsen, Thomas Kiorboe, Tomas Bohr, Anders Andersen Flagellates are a diverse group of unicellular organisms forming an important part of the marine ecosystem. The arrangement of flagella around the cell serves as a key trait optimizing and compromising essential functions. With micro-particle image velocimetry we observed time-resolved near-cell flows around freely swimming flagellates, and we developed an analytical model based on the Stokes flow around a solid sphere propelled by a variable number of differently placed, temporally varying point forces, each representing one flagellum. The model allows us to reproduce the observed flow patterns and swimming dynamics, and to extract quantities such as swimming velocities and prey clearance rates as well as flow disturbances revealing the organism to flow-sensing predators. Our results point to optimal flagellar arrangements and beat patterns, and essential trade-offs. For biflagellates with two symmetrically arranged flagella we contrasted two species using undulatory and ciliary beat patterns, respectively, and found breast-stroke type beat patterns with equatorial power strokes to be favorable for fast as well as quiet swimming. [Preview Abstract] |
Tuesday, November 22, 2016 1:56PM - 2:09PM |
R21.00003: Emergence of multiple synchronization modes in hydrodynamically-coupled cilia Hanliang Guo, Eva Kanso Motile cilia and flagella exhibit different phase coordinations. For example, closely swimming spermatozoa are observed to synchronize together; bi-flagellates \textit{Chlamydomonas} regulate the flagella in a ``breast-stroke'' fashion; cilia on the surface of \textit{Paramecium} beat in a fixed phase lag in an orchestrated wave like fashion. Experimental evidence suggests that phase coordinations can be achieved solely via hydrodynamical interactions. However, the exact mechanisms behind it remain illusive. Here, adapting a ``geometric switch'' model, we observe different synchronization modes in pairs of hydrodynamically-coupled cilia by changing physical parameters such as the strength of the cilia internal motor and the separation distance between cilia. Interestingly, we find regions in the parameter space where the coupled cilia reach stable phase coordinations and regions where the phase coordinations are sensitive to perturbations. We also find that leaning into the fluid reduces the sensitivity to perturbations, and produces stable phase coordination that is neither in-phase nor anti-phase, which could explain the origin of metachronal waves in large cilia populations. [Preview Abstract] |
Tuesday, November 22, 2016 2:09PM - 2:22PM |
R21.00004: Hydrodynamic interaction between two helical swimmers Alejandro Ruiz Esparza, Francisco Godinez, Eric Lauga, Roberto Zenit Many motile bacteria, such as E. coli, possess several helical flagellar filaments that bundle together to form a coherent helical element for propulsion. In order to understand the process of bundling, we study the interaction between two identical helical magnetic swimmers that self propel in a highly viscous Newtonian fluid due to the rotation of an external magnetic field. Our experiments reveal that hydrodynamic interactions lead to nontrivial collective and relative effects, both in translation and rotation. We will present our experimental results and discuss the physical mechanisms responsible for our observations. [Preview Abstract] |
Tuesday, November 22, 2016 2:22PM - 2:35PM |
R21.00005: Stokesian swimming of a helical swimmer across an interface Francisco Godinez, Armando Ramos, Roberto Zenit Microorganisms swim in flows dominated by viscous effects but in many instances the motion occurs across heterogeneous environments where the fluid properties may vary. To our knowledge, the effect of such in-homogeneity has not been addressed in depth. We conduct experiments in which a magnetic self-propelled helical swimmer displaces across the interface between two immiscible density stratified fluids. As the swimmer crosses the interface, at a fixed rotation rate, its speed is reduced and a certain volume of the lower fluid is dragged across. We quantify the drift volume and the change of swimming speed for different swimming speeds and different fluid combinations. We relate the reduction of the swimming speed with the interfacial tension of the interface. We also compare the measurements of the drift volume with some recent calculations found in the literature. [Preview Abstract] |
Tuesday, November 22, 2016 2:35PM - 2:48PM |
R21.00006: Instabilities of a rotating helical rod Yunyoung Park, William Ko, Yongsam Kim, Sookkyung Lim Bacteria such as {\it{Escherichia coli}} and {\it{Vibrio alginolyticus}} have helical flagellar filament. By rotating a motor, which is located at the bottom end of the flagellar filament embedded in the cell body, CCW or CW, they swim forward or backward. We model a left-handed helix by the Kirchhoff rod theory and use regularized Stokes formulation to study an interaction between the surrounding fluid and the flagellar filament. We perform numerical studies focusing on relations between physical parameters and critical angular frequency of the motor, which separates overwhiring from twirling. We are also interested in the buckling instability of the hook, which is very flexible elastic rod. By measuring buckling angle, which is an angle between rotational axis and helical axis, we observe the effects of physical parameters on buckling of the hook. [Preview Abstract] |
Tuesday, November 22, 2016 2:48PM - 3:01PM |
R21.00007: The swimming speed of a confined rotating helix in creeping flow Veronica Angeles, Roberto Zenit Recent theoretical and numerical studies have shown that the swimming speed of a rotating helix confined in a tube or between walls is higher that the unconfined case, for the same helix properties (helix geometry and rotation speed). We conduct experiments using a magnetic self-propelled force-free robot placed in between two walls or inside a cylinder. We vary the degree of confinement and measure the translation speed for different helix geometries and rotation speeds. We do find an increase of the swimming speeds, which is in good agreement with the predictions of a wall-corrected resistive-force theory. However, since the torque also increases as a result of confinement, the experiments are restricted by the available magnetic torque. Therefore, the increase in swimming speed is only observed for low confinement levels. [Preview Abstract] |
Tuesday, November 22, 2016 3:01PM - 3:14PM |
R21.00008: A numerical study of the effects of fluid rheology and stroke kinematics on flagellar swimming in complex fluids Chuanbin Li, Robert Guy, Becca Thomases It is observed in experiments that as the fluid rheology is changed, Chlamydomonas reinhardtii exhibits changes in both flagellar kinematics and the swimming speed. To understand this phenomenon, we develop a computational model of the swimmer, using flagellar strokes fit from experimental data. We conduct numerical simulations by changing strokes and fluid rheology independently to dissect the effects of these two factors. We discover that stroke patterns extracted from viscoelastic fluids generate much lower stress and have higher efficiency at the cost of lower swimming speed. We also discover that higher fluid elasticity hinders swimming for a fixed stroke pattern. [Preview Abstract] |
Tuesday, November 22, 2016 3:14PM - 3:27PM |
R21.00009: Propulsion by Helical Strips in Circular Channels Serhat Yesilyurt, Ebru Demir Progress in manufacturing techniques avails the production of artificial micro swimmers (AMS) in various shapes and sizes. There are numerous studies on the generation of efficient locomotion by means of helical tails with circular cross-sections. This work focuses on locomotion with helical strips in circular channels. A CFD model is used to analyze the effects of geometric parameters and the radius of the channel on swimming velocity of infinite helical-strips in circular channels. Results show that there is an optimum wavelength that depends on thickness to channel radius ratio, suggesting that these parameters need to be optimized simultaneously. With constant torque, thinner strips swim faster, whereas under constant angular velocity application, thicker strips (in radial direction) prevail. As width approaches the wavelength, velocity decreases under both conditions, unless a magnetically coated tail is simulated, for which width has an optimum value. Increasing channel radius to helix amplitude ratio increases the velocity up to a maximum and after a slight drop, saturation occurs as bulk swimming conditions are approached. [Preview Abstract] |
Tuesday, November 22, 2016 3:27PM - 3:40PM |
R21.00010: Trajectories of Artificial Microswimmers with Helical Tails Inside Circular Channels. Serhat Yesilyurt, Hakan Caldag Trajectories are obtained for millimeter-scale artificial microswimmers inside circular channels filled with glycerol. Rotating magnetic field is applied to propel 3D-printed swimmers with helical tails and permanent magnetic heads. Experiments are recorded with a high-speed camera and processed with contrast-based image processing tools to extract 3D trajectories and orientations of the swimmers. Swimmers pushed by the tail exhibit a helical trajectory at all times while straight trajectories are observed when the length to diameter ratio is very high for pulled ones. Long tails are pointed towards the channel's centerline and short ones are pointed towards the wall. Weak Poiseuille flow is found to alter the swimming speed and suppress the step-out behavior. Flow from tail side increases the instability of swimmers. Experimental observations are validated with snapshot and dynamic models that use CFD to obtain average and time-dependent velocities and trajectories of the swimmer. Lastly, modulation of the rotating magnetic field tilts the swimmer in desired directions or halts the swimmer propulsion without stopping the rotation of the swimmer. [Preview Abstract] |
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