Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session P53: Swimming, Migration and Porous Media Flows 
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Sponsoring Units: DFD GSOFT DBIO Room: Hilton Baltimore Holiday Ballroom 4 
Wednesday, March 16, 2016 2:30PM  2:42PM 
P53.00001: The intestine is a blender Patricia Yang, Morgan LaMarca, Victoria Kravets, David Hu According to the U.S. Department of Health and Human Services, digestive disease affects 60 to 70 million people and costs over 140 billion annually. Despite the significance of the gastrointestinal tract to human health, the physics of digestion remains poorly understood. In this study, we ask a simple question: what sets the frequency of intestinal contractions? We measure the frequency of intestinal contractions in rats, as a function of distance down the intestine. We find that intestines contract radially ten times faster than longitudinally. This motion promotes mixing and, in turn, absorption of food products by the intestinal wall. We calculate viscous dissipation in the intestinal fluid to rationalize the relationship between frequency of intestinal contraction and the viscosity of the intestinal contents. Our findings may help to understand the evolution of the intestine as an ideal mixer. [Preview Abstract] 
Wednesday, March 16, 2016 2:42PM  2:54PM 
P53.00002: Confinement of Single Microswimmers in Circular Microfluidic Chambers Tanya Ostapenko, Thomas Boeddeker, Christian Kreis, Fabian Schwarzendahl, Marco G. Mazza, Oliver Baeumchen The characteristics of active fluids, such as suspensions of biological microswimmers, may not only originate from the mutual interactions between the constituents, but also from interactions with interfaces and confining walls. In fact, the natural habitats of many living organisms are complex geometric environments, rather than bulk situations. The influence of interfaces on the dynamics was recognized as an important factor, and there are differences in the way that pushertype swimmers (e.g. \textit{E. coli}) and pullertype swimmers (e.g. \textit{C. reinhardtii}) behave close to flat interfaces. Using experiments and simulations, we report on the dynamics of single pullertype swimmers in 2D circular microfluidic chambers. We find that the radial probability distribution of trajectories displays a characteristic wall hugging effect, where swimmers remain trapped at a concave interface for decreasing chamber size. For trajectories in the vicinity of the concave wall, an alignment of the local swimming direction with the local wall tangent is observed. In contrast, the swimmers tend to scatter off convex interfaces with short interaction times. Based on geometric arguments involving the swimmer's persistence length, we explain this entrapment effect at concave interfaces. [Preview Abstract] 
Wednesday, March 16, 2016 2:54PM  3:06PM 
P53.00003: Mirrorsymmetry breakings in human sperm rheotaxis Norbert Stoop, Anton Bukatin, Igor Kukhtevich, Joern Dunkel, Vasily Kantsler Rheotaxis, the directed response to fluid velocity gradients, has been shown to facilitate stable upstreamswimming of mammalian sperm cells along solid surfaces, suggesting a robust mechanism for longdistance navigation during fertilization. However, the dynamics by which a human sperm orients itself w.r.t. ambient flows is poorly understood. Here, we combine microfluidic experiments with mathematical modeling and 3D flagellar beat reconstruction to quantify the response of individual sperm cells in timevarying flow fields. Singlecell tracking reveals two kinematically distinct swimming states that entail opposite turning behaviors under flow reversal. We constrain an effective 2D model for the turning dynamics through systematic largescale parameter scans, and find good quantitative agreement with experiments. We present comprehensive 3D data demonstrating the rolling dynamics of freely swimming sperm cells around their longitudinal axis. Contrary to current beliefs, this analysis uncovers ambidextrous flagellar waveforms and shows that the cell’s turning direction is is not defined by the rolling direction. Instead, the different rheotactic turning behaviors are linked to a broken mirrorsymmetry in the midpiece section, likely arising from a buckling instability. [Preview Abstract] 
Wednesday, March 16, 2016 3:06PM  3:18PM 
P53.00004: Microorganism billiards in closed plane curves Madison Krieger Recent experiments and numerical simulations have demonstrated that many species of microorganisms reflect aspecularly from a solid surface  due to steric and hydrodynamic interactions with the wall, their outgoing angle is fixed and independent of the angle of incidence. Motivated by these results, we discuss theory and computation of the ``aspecular billiard'', a modification of the classical billiard in which the outgoing angle is constant. We restrict our attention to closed plane curves, focusing on three canonical examples: the ellipse, the Bunimovich stadium, and the Sinai billiard. These systems can have a rich array of orbits, and the Lyapunov exponent is shown to be dependent on the billiard geometry and the outgoing angle. We apply these results to the design of tunable passive sorting mechanisms. [Preview Abstract] 
Wednesday, March 16, 2016 3:18PM  3:30PM 
P53.00005: \textit{Helicobacter pylori} displays spiral trajectories while swimming like a corkscrew in solutions. Maira A. Constantino, Joseph M. Hardcastle, Rama Bansil, Mehdi Jabbarzadeh, Henry C. Fu \textit{Helicobacter pylori} is a helical shaped bacterium that causes gastritis, ulcers and gastric cancer in humans and other animals. In order to colonize the harsh acidic environment of the stomach \textit{H. pylori} has evolved a unique biochemical mechanism to go across the viscoelastic gellike gastric mucus layer. Many studies have been conducted on the swimming of \textit{H. pylori} in viscous media. However a yet unanswered question is if the helical cell shape influences bacterial swimming dynamics or confers any advantage when swimming in viscous solution. We will present measurements of \textit{H. pylori} trajectories displaying corkscrew motion while swimming in solution obtained by tracking single cells using 2dimensional phase contrast imaging at high magnification and fast frame rates and simultaneously imaging their shape. We observe a linear relationship between swimming speed and rotation rate. The experimental trajectories show good agreement with trajectories calculated using a regularized Stokeslet method to model the low Reynolds number swimming behavior. [Preview Abstract] 
Wednesday, March 16, 2016 3:30PM  3:42PM 
P53.00006: Investigating wake patterns and propulsive frequencies of a flat plate under pitching motion. Joseph MOUBOGHA MOUBOGHA, Jacques Andre ASTOLFI Fundamental mechanisms of swimming are explored using a simple geometry device  flat plate  in purepitching motion in a hydrodynamic tunnel. The experiments are carried out at different Reynolds numbers based on the plate length c. Pitching motion is generated for reduced frequencies k between 0 and 2 and for an angular amplitude of 10 deg. Velocity fields are obtained in the wake of the plate using Particle Image Velocimetry and measurements of drag coefficients are estimated from mean velocity profiles. This study confirms the occurrence of a threshold oscillation frequency beyond which the plate enters a propulsive regime and the wake features organized structures. In this case an inversion of the typical Karman vortex street is observed. The evolution of mean transverse velocity profiles in the wake of the plate shows that the usual wake profile with velocity deficit  plate with drag  can be transformed into a jet  plate with thrust  above a certain reduced frequency. [Preview Abstract] 
Wednesday, March 16, 2016 3:42PM  3:54PM 
P53.00007: Continuousflow Electrokinetic Particle Separation in a Bifurcating Microchannel. Di Li, Xinyu Lu, Xiangchun Xuan Separating particles from a heterogeneous mixture is important and necessary in many engineering and biomedical applications. Electrokinetic flowbased continuous particle separation has so far been realized primarily by the use of particle dielectrophoresis induced in constricted and/or curved microchannels. We demonstrate in this talk that particles can be continuously separated by size when passing through a bifurcating microchannel. This sheathless labelfree separation relies on the wallinduced electrical lift force that acts to focus particles to the center of the mainbranch and deflect them to sizedependent flow paths in the two sidebranches. We also develop a numerical model to predict and understand this separation. [Preview Abstract] 
(Author Not Attending)

P53.00008: Predatorprey model for the selforganization of stochastic oscillators in dual populations Sara Moradi, Johan Anderson, Ozgur D. Gürcan A predatorprey model of dual populations with stochastic oscillators is presented. A linear crosscoupling between the two populations is introduced that follows the coupling between the motions of a Wilberforce pendulum in two dimensions: one in the longitudinal and the other in torsional plain. Within each population a Kuramoto type competition between the phases is assumed. Thus, the synchronization state of the whole system is controlled by these two types of competitions. The results of the numerical simulations show that by adding the linear crosscoupling interactions predatorprey oscillations between the two populations appear which results in selfregulation of the system by a transfer of synchrony between the two populations. The model represents several important features of the dynamical interplay between the drift wave and zonal flow turbulence in magnetically confined plasmas, and a novel interpretation of the coupled dynamics of drift wavezonal flow turbulence using synchronization of stochastic oscillator is discussed. [Preview Abstract] 
Wednesday, March 16, 2016 4:06PM  4:18PM 
P53.00009: Exploring the Spatiotemporal Dynamics of Covariant Lyapunov Vectors for Chaotic Convection Mu Xu, Mark Paul Covariant Lyapunov vectors provide access to fundamental features of chaos in highdimensional systems that are driven farfromequilibrium. We explore the spatiotemporal dynamics of covariant Lyapunov vectors for chaotic RayleighB\'enard convection to provide new physical insights. We use the covariant Lyapunov vectors to quantify the transition from hyperbolic to nonhyperbolic dynamics, to determine the degree of Oseledec splitting exhibited by the dynamics, and to shed light upon upon the tangled nature of the Lyapunov vectors. In this talk, we will explore the spatiotemporal dynamics of the Lyapunov vectors and their relation with the chaotic pattern dynamics of the flow field. Our results suggest that the Lyapunov vectors contain two distinct spatiotemporal features consisting of highly localized regions near defect structures and a spatially distributed checkerboard pattern. We will explore the connection between these features and the ideas of physical and spurious modes that may compose the dynamics. [Preview Abstract] 
Wednesday, March 16, 2016 4:18PM  4:30PM 
P53.00010: LatticeBoltzmann Simulation of Tablet Disintegration Jiaolong Jiang, Ning Sun, Dilip Gersappe Using the latticeBoltzmann method, we developed a 2D model to study the tablet disintegration involving the swelling and wicking mechanisms. The surface area and disintegration profile of each component were obtained by tracking the tablet structure in the simulation. Compared to pure wicking, the total surface area is larger for swelling and wicking, which indicates that the swelling force breaks the neighboring bonds. The disintegration profiles show that the tablet disintegrates faster than pure wicking, and there are more wetted active pharmaceutical ingredient particles distributed on smaller clusters. Our results indicate how the porosity would affect the disintegration process by changing the wetting area of the tablet as well as by changing the swelling force propagation. [Preview Abstract] 
Wednesday, March 16, 2016 4:30PM  4:42PM 
P53.00011: The role of the capillary force in the liquid distribution in porous media Bojan Markicevic The dynamics of the liquid spreading in porous media occupied by gas is investigated numerically using the capillary network models. In the numerical solution, the flow at the free interface is fully resolved from the force balance at each pore along the interface allowing for local flows to fill or empty the pores. The flow is transient and the interface shape is determined at each time step. The liquid/solid interactions are investigated for whole range from fully wetting to fully nonwetting cases, and the spread of neutral fluid is also solved. For the neutral fluid, the interface irregularity are caused by pore varying volume with the interface of specific thickness separating fully saturated and gas occupied parts of porous medium. For the capillary interactions present, the interface thickness increases and due to the gas entrapment by spreading liquid, the saturation profiles develop in the direction of the liquid flow. The profiles depend on the capillary force as liquid spreads along the paths consisting of smaller pores for wetting, and larger pores for nonwetting interactions. Finally, the influence of the capillary force is counteracted by viscous force, where for faster flows, the saturation profiles vanish and the interface of limited thickness develops. [Preview Abstract] 
Wednesday, March 16, 2016 4:42PM  4:54PM 
P53.00012: Numerical modeling and simulation of flow through porous fabric surface Zheng Gao, Xiaolin Li We designed a numerical scheme to model the permeability of the fabric surface in an incompressible fluid by coupling the projection method with the Ghost Fluid Method in the front tracking framework. The pressure jump condition is obtained by adding a source term to the Poisson's equation in the projection step without modifications on its coefficients. The numerical results suggest that this approach has the ability to reproduce the relationship between pressure drop and relative velocity observed in the experiments. We use this algorithm to study the effects of porosity on the drag force and stability of parachutes during its inflation and deceleration. [Preview Abstract] 
Wednesday, March 16, 2016 4:54PM  5:06PM 
P53.00013: A Numerical Study of Shear Flow in Partially Vegetated Open Channels Jingfang Qu, S Chen, Jie Yu, Xiaolin Li Shear flow at the interface between a porous layer and an open conduit is a problem of fundamental importance to problems ranging from natural to engineered flows. Such shear flows are known to be unstable, inducing waves and coherent vortices via KelvinHelmholtz instability. These coherent flow structures can strongly enhance the exchange of scalar variables and vector variable such as momentum in and out of the canopy, hence playing an important role in controlling environmental quality of these system. We developed a numerical model using finite difference method for flow in open channel occupied by a vegetation canopy. We apply the method to simulate the shear flow and compare with the experimental study by White and Nepf in 2007. Preliminary comparisons with the experimental data show good agreements. [Preview Abstract] 
Wednesday, March 16, 2016 5:06PM  5:18PM 
P53.00014: Finite Amplitude Analysis of Mixed Convection in a Vertical Annulus Filled with Porous Medium. Premananda Bera, moumita bhowmik Using the weakly nonlinear stability analysis, we have investigated the stability of stably stratified mixed convective flow in a vertical annulus filled with fluid saturated metallic foam with porosity 0.97. Since the curvature parameter $(C)$plays a vital role to describe the geometry, therefore special attention has been given to understand the effect of $C$on the flow by considering three different values of $C$(0.001, 0.6, and 10). The nonlinear results are presented for the fluid as water (Prandtl number $(\Pr )$equal to 7) and a fixed Reynolds number $Re$equal to 500. In the entire weekly nonlinear analysis, only super critical bifurcation has been predicted at and beyond the critical Rayleigh number $(Ra)$.The equilibrium amplitude ($\vert A\vert )$ increases beyond the critical point and decreases on reducing the gap between the concentric cylinders. Due to nonlinear interaction, a substantial enhancement in heat transfer rate is also observed from the basic state beyond the bifurcation point, i.e. Nusselt number predicted by nonlinear analysis is much more than those predicted by fully developed basic state. [Preview Abstract] 
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