Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session H17: Biofluids: Micro-swimming Theory I |
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Chair: Juan Rodrigo Velez-Cordero, University of California, San Diego Room: 28C |
Monday, November 19, 2012 10:30AM - 10:43AM |
H17.00001: Unsteady swimming of small organisms Shiyan Wang, Arezoo Ardekani Small planktonic organisms ubiquitously display unsteady or impulsive motion to attack a prey or escape a predator in natural environments. Despite this, the role of unsteady hydrodynamic forces such as history and added mass forces on the low Reynolds number propulsion of small organisms is poorly understood. In this paper, we derive the fundamental equation of motion for an organism swimming by the means of surface distortion in a nonuniform flow at a low Reynolds number regime. We show that the history and added mass forces, that where traditionally neglected in the literature for small swimming organisms, cannot be neglected as the Stokes number increases above unity. For example, these unsteady inertial forces are of the same order as quasi-steady Stokes forces for \textit{Paramecium. }Finally, we quantify the effects of convective inertial forces in the limit of small, but nonzero, Reynolds number regime. [Preview Abstract] |
Monday, November 19, 2012 10:43AM - 10:56AM |
H17.00002: Advective effects on the propulsion of phoretic micro-swimmers Sebastien Michelin, Denis Bartolo This work focuses on the dynamics of self-propelled spherical particles that can exchange a solute with the surrounding fluid. The propulsion mechanism is based on the interaction of this solute with the surface of the ``swimming" particle: concentration gradients along the particle's surface result in a slip velocity distribution, and the particle effectively behaves like an artificial squirmer. In the well-studied diffusive limit, the solute concentration is decoupled from the Stokes flow problem, and the particle's velocity can be directly computed from the distribution of solute flux on the boundary. In this presentation, we identify instead the effect of the solute advection on the propulsive properties of such phoretic micro-swimmers by considering the fully-coupled non-linear problem for the solute concentration and velocity fields around the particle. [Preview Abstract] |
Monday, November 19, 2012 10:56AM - 11:09AM |
H17.00003: Achiral rigid magnetically actuated swimmers Henry Fu, U. Kei Cheang, Farshad Meshkati, MinJun Kim So far, many magnetically actuated artificial microswimmers have relied on either swimmer flexibility or chiral geometry to overcome constraints on swimming strategies at low Reynolds numbers and achieve propulsion. However, being either flexible or chiral is not a necessary condition for propulsion of microswimmers rotated by external fields. We analyze achiral, rigid swimming using experiment, numerical simulation, and symmetry analysis. Achiral rigid swimming is demonstrated with planar colloidal structures constructed of magnetic beads and rotated by a spatially uniform magnetic field. This swimming is numerically modeled using a boundary element method. Finally, symmetry analysis is used to generically determine which combinations of achiral rigid geometry and magnetic moment can achieve propulsion. For planar colloidal microswimmers the dipole moment must not be perpendicular to a symmetry plane in order to swim. [Preview Abstract] |
Monday, November 19, 2012 11:09AM - 11:22AM |
H17.00004: Propulsion in a generalized Newtonian fluid Juan Rodrigo V\'elez-Cordero, Eric Lauga The two-dimensional dynamics of an undulating surface has been used as a simplified model to study the transport of fluid by the movement of cilia carpets (so-called envelope model). The collective motion of cilia is idealized as a surface that displaces waves in one direction and whose material points (tips of the cilia) perform a combination of normal and tangential motion with respect to the mean plane. We calculate the mean pumping velocity and rate of work done by an undulating surface in a Generalized Newtonian fluid modeled by the Carreau equation. The influence of the variable viscosity appears only to fourth order in the wave parameter, Ak, where A and k are the wave amplitude and wavenumber respectively. The non-Newtonian effects appear only if both modes of motion, normal plus tangential, are active. The mean rate of work always diminishes for different combinations of normal and tangential motion if the fluid is shear-thinning. Surprisingly, this is not similar for the mean velocity, which for certain motion patterns increases if the fluid is shear-thinning, but for others increases if it is shear-thickening. [Preview Abstract] |
Monday, November 19, 2012 11:22AM - 11:35AM |
H17.00005: Diffusion of torqued active particles Mario Sandoval, Eric Lauga Motivated by swimming microorganisms whose trajectories are affected by the presence of an external torque, we calculate the diffusivity of an active particle subject to an external torque and in a fluctuating environment. The analytical results are compared with Brownian dynamics simulations showing excellent agreement between theory and numerical experiments. [Preview Abstract] |
Monday, November 19, 2012 11:35AM - 11:48AM |
H17.00006: Nematode swimming and turning: locomotion of {\it C.\ Elegans} in bulk fluid and thin fluid layers Alejandro Bilbao, Venkat Padmanabhan, Kendra Rumbaugh, Siva Vanapalli, Jerzy Blawzdziewicz A millimeter-long nematode {\it C.\ Elegans} propels itself by performing sinous undulations, and it turns by assuming strongly curved $\Omega$-shaped body postures. All these stereotyped motions can accurately be described in terms of piecewise-harmonic body curvature, which propagates backwards along the nematode length [PLoS ONE, 7: e40121 (2012)]. We combine our piecewise-harmonic-curvature description with accurate hydrodynamic bead-chain models to investigate swimming efficiency and maneuverability of the nematode in bulk fluid and in a thin fluid layer. We find that the nematode swims faster and maneuvers more efficiently under confinement, because of a larger transverse hydrodynamic resistance. However, the optimal swimming gate is only weakly affected. [Preview Abstract] |
Monday, November 19, 2012 11:48AM - 12:01PM |
H17.00007: Simulation of micro-organisms swimming near ciliated surfaces Henry Shum, Anurag Tripathi, Julia Yeomans, Anna Balazs Ciliated tissues can be found lining the respiratory tract and Fallopian tubes in mammals. The main function of the cilia is to sweep objects such as the ovum, dirt or bacteria in a directed manner. The self-cleaning action of these tissues would be a desirable property for surfaces that are continually submerged and prone to biofouling. We therefore investigate the effect of artificially driven cilia on swimming organisms. In this study we use a 3-D immersed boundary approach, with the fluid flow solved by the lattice Boltzmann method and the immersed objects modeled as elastic structures. Two types of objects are considered: (i) cilia, which are driven by an external field, and (ii) bacteria, which are self-motile and propelled by a rotating helical flagellum. Placing a bacterial cell in the vicinity of a surface covered by an array of actuated cilia yields a rich system to explore. Of particular interest is the possibility of guiding the motion of bacteria towards, along or away from the ciliated surface. [Preview Abstract] |
Monday, November 19, 2012 12:01PM - 12:14PM |
H17.00008: Chemotaxis of crawling and swimming \textit{Caenorhabditis Elegans} Amar Patel, Alejandro Bilbao, Venkat Padmanabhan, Zeina Khan, Andrew Armstrong, Kendra Rumbaugh, Siva Vanapalli, Jerzy Blawzdziewicz A soil-dwelling nematode \textit{Caenorhabditis Elegans} efficiently navigates through complex environments, responding to chemical signals to find food or avoid danger. According to previous studies, the nematode uses both gradual-turn and run-and-tumble strategies to move in the direction of the increasing concentration of chemical attractants. We show that both these chemotaxis strategies can be described using our kinematic model [PLoS ONE, 7: e40121 (2012)] in which harmonic-curvature modes represent elementary nematode movements. In our chemotaxis model, the statistics of mode changes is governed by the time history of the chemoattractant concentration at the position of the nematode head. We present results for both nematodes crawling without transverse slip and for swimming nematodes. [Preview Abstract] |
Monday, November 19, 2012 12:14PM - 12:27PM |
H17.00009: The Fidelity of Adaptive Phototaxis Idan Tuval, Knut Drescher, Raymond Goldstein Along the evolutionary path from single cells to multicellular organisms with a central nervous system are species of intermediate complexity that move in ways suggesting high-level coordination, yet have none. Instead, organisms of this type possess many autonomous cells endowed with programs that have evolved to achieve concerted responses to environmental stimuli. Here experiment and theory are used to develop a quantitative understanding of how cells of such organisms coordinate to achieve phototaxis, by using the colonial alga Volvox carteri as a model. It is shown that the surface somatic cells act as individuals but are orchestrated by their relative position in the spherical extracellular matrix and their common photoresponse function to achieve colony-level coordination. Analysis of models that range from the minimal to the biologically faithful shows that, because the flagellar beating displays an adaptive down-regulation in response to light, the colony needs to spin around its swimming direction and that the response kinetics and natural spinning frequency of the colony appear to be mutually tuned to give the maximum photoresponse. These models further predict that the phototactic ability decreases dramatically when the colony does not spin at its natural frequency, a result confirmed by phototaxis assays in which colony rotation was slowed by increasing the fluid viscosity. [Preview Abstract] |
Monday, November 19, 2012 12:27PM - 12:40PM |
H17.00010: Granular resistive force theory explains the neuromechanical phase lag during sand-swimming Yang Ding, Sarah Sharpe, Daniel Goldman Undulatory locomotion is a common gait used by a diversity of animals in a range of environments. This mode of locomotion is characterized by the propagation of a traveling wave of body bending, which propels the animal in the opposite direction of the wave. Previous studies of undulatory locomotion in fluids, on land, and even within sand revealed that the wave of muscle activation progresses faster than the traveling wave of curvature. This leads to an increasing phase lag between activation and curvature at more posterior segments, known as the neuromechanical phase lag. In this study, we compare biological measurements of phase lag during the sand-swimming of the sandfish lizard to predictions of a simple model of undulatory swimming that consists of prescribed kinematics and granular resistive forces. The neuromechanical phase lag measured using electromyography (EMG) quantitatively matches the predicted phase lag between the local body curvature and torque exerted by granular resistive forces. Two effects are responsible for the phase lag in this system: the yaw motion of the body and different integration length over a traveling force pattern for different positions along the body. [Preview Abstract] |
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