Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session D18: Biofluids: General II - Collective Behavior and Microswimmers |
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Chair: Alireza Karimi, University of Notre Dame Room: 306/307 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D18.00001: Impact of Viscoelasticity on the Coordinated Swimming of Motile Bacteria Alireza Karimi, Arezoo Ardekani The formation of bacterial communities is often associated with production of extracellular polymeric substances which impart viscoelastic behavior to the surrounding fluid. This phenomenon greatly affects the hydrodynamic interactions of swimming bacteria and the resulting chaotic dynamics. To investigate this process, we used a kinetic model developed to study the behavior of self-propelled particles in conjunction with Oldroyd-B constitutive equation and the Stokes equations. Using large-scale numerical simulations of the system, we analyzed the effect of the viscoelasticity on the coordinated behavior of the microorganisms. In addition, by varying the corresponding parameters of the problem such as Weissenberg number and viscosity ratio, we explored different flow regimes in order to gain insight regarding the characteristics of the flow patterns induced by the collective motion of motile bacteria. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D18.00002: Diffusion of passive particles in active suspensions Matthias Mussler, Salima Rafai, Thomas John, Philippe Peyla, Christian Wagner We study how an active suspension consisting of a definite volume fraction of the microswimmer \textit{Chlamydomonas Reinhardtii} modifies the Brownian movement of small to medium size microspheres. We present measurements and simulations of trajectories of microspheres with a diameter of 20 $\mu$m in suspensions of \textit{Chlamydomonas Reinhardtii}, a so called ``puller,'' and show that the mean squared displacement of such trajectories consist of parabolic and a linear part. The linear part is due to the hydrodynamic noise of the microswimmers while the parabolic part is a consequence of directed motion events that occur randomly, when a microsphere is transported by a microswimmer on a timescale that is in higher order of magnitude than the Brownian like hydrodynamic interaction. In addition, we theoretically describe this effect with a dimensional analysis that takes the force dipole model used to describe ``puller'' like \textit{Chlamydomonas Reinhardtii} into account. [Preview Abstract] |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D18.00003: A drag-based mechanism for vertical force production in the smallest flying insects Shannon Jones, Ryan Laurenza, Laura Miller Previous work has shown that the flight kinematics and aerodynamics of the smallest flying insects may be significantly different than that of their larger counterparts. These small insects, such as thrips and parasitoid wasps, are on the order of 1 mm in length and operate at a Reynolds number less than 10.~ Due to their small size and high wing beat frequency, quantitative data on the wing kinematics of the smallest insects is not available. As a result, there has been much debate and speculation about the flight strategies employed by these insects.~ With the challenges associated with generating lift at low Reynolds numbers, it could be beneficial for the smallest insects to use a drag-based motion to generate some or all of its vertical force, however this has not been rigorously investigated. We used computational fluid dynamics to investigate the feasibility of drag-based propulsion in the tiniest insects. We investigated the vertical force generated by an idealized drag-based vertical stroke over a range of Reynolds numbers from 1 to 150.~ We also compared this stroke to more conventional hovering stroke kinematics such as that of a fruit fly and dragonfly. [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D18.00004: Emergent structures and dynamics in suspensions of self-phoretic colloids Andrea Scagliarini, Ignacio Pagonabarraga Active fluids, such as suspensions of self-propelled particles , are a fascinating example of Soft Matter displaying complex collective behaviours which provide challenges in non-equilibrium Statistical Physics. The recent development of techniques to assemble miniaturized devices has led to a growing interest for micro and nanoscale engines that can perform autonomous motion (``microrobots''), as, for instance, self-phoretic colloids, for which the propulsion is induced by the generation of a chemical species in a reaction catalyzed at the particle surface. We perform a mesoscopic numerical study of suspensions of self-phoretic colloids. We show that, at changing the sign of the phoretic mobility (which accounts for the colloid-solute interactions), the system switches from a cluster phase to a state with slowed dynamics. We find that the cluster size distribution follows an exponential behaviour, with a characteristic size growing linearly with the colloid activity, while the density fluctuations grow as a power-law with an exponent depending on the cluster fractal dimension.We single out hydrodynamic interactions, showing that their effect is to work against cluster formation. For positive $\mu$, we observe that colloids tend to reach an ordered state on a triangular lattice. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D18.00005: Hydrodynamics of Choanoflagellate Feeding Anders Andersen, Lasse Tor Nielsen, Thomas Kiorboe Choanoflagellate filter feeding is a poorly understood process. Studies indicate that the pressure differences created by the beating of the flagellum are insufficient to produce an adequate water flow through the collar filter, the mechanism believed to ultimately transport food particles to the cell. The collar is composed of numerous microvilli arranged as a palisade, and the low porosity of the filter provides high resistance to the water flow. Additionally, ultrastructural studies often show signs of mucus-like substances in and around the collar, potentially further hampering water flow. We present high-speed video of live material showing the particle retention and the beating of the flagellum in the choanoflagellate species Diaphanoeca grandis. We use the observations as input to model the low Reynolds number fluid dynamics of the fluid force produced by the flagellum and the resulting feeding flow. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D18.00006: Localized structure of Euglena bioconvection Makoto Iima, Erika Shoji, Akinori Awazu, Hiraku Nishimori, Shunsuke Izumi Bioconvection of a suspension of Euglena gracilis, a photosensitive flagellate whose body length is approximately 50 micrometers, was experimentally studied. Under strong light intensity, Euglena has a negative phototaxis; they tend to go away from the light source. When the bright illumination is given from the bottom, a large scale spatio-temporal pattern is generated as a result of interaction between Euglena and surrounding flow. Recently, localized convection pattern had been reported, however, the generation process and interaction of the localized convection cells has not been analyzed. We performed experimental study to understand the localization mechanism, in particular, the onset of bioconvection and lateral localization behavior due to phototaxis. Experiments started from different initial condition suggests a bistability near the onset of the convection as binary fluid convection that also shows localized convection cells. Dynamics of localized convections cells, which is similar to the binary fluid convection case although the basic equations are not the same, is also reported. [Preview Abstract] |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D18.00007: Limit cycle dynamics in swimming systems Cyndee Finkel, Karl von Ellenrieder An experimental apparatus was constructed to model basic features expected in the flow about a freely swimming fish. A D-shaped cylinder is used to represent the body and an oscillating foil, the tail. The swimming system is suspended in a constant freestream flow. A closed loop PI controller is used to maintain a set point, stream-wise location. The system is released from multiple downstream and upstream locations and permitted to swim to the set point. The Strouhal number measured when the swimming system achieves a constant forward swimming speed is compared to values observed in nature. The results suggest that self-regulation passively selects the Strouhal number and that no other external sensory input is necessary for this to happen. This self-regulation is a result of a limit cycle process that stems from nonlinear periodic oscillations. Phase plane analyses are used to examine the synchronous conditions due to the coupling of the foil and wake vortices. It is shown that the phase locking indices depend on the Strouhal number and approach a frequency locking ratio of about $0.5$. The results suggest that Strouhal number selection in steady forward natural swimming is the result of a limit cycle process and not actively controlled by an organism. [Preview Abstract] |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D18.00008: Local fluid transport by planktonic swarms Monica Martinez-Ortiz, John Dabiri Energy transport in the ocean occurs through an intricate set of pathways mainly powered by physical phenomena. The hypothesis that vertical migrations of aquatic fauna may contribute to this process through the action of the induced drift mechanism has been investigated in recent years. Microscale measurements by Kunze et al (1), in Saanich Inlet have shown the presence of high kinetic energy dissipation rates in the vicinity of vertically migrating krill swarms. However, it remains uncertain if energy is being introduced at scales large enough to induce the transport of fluid across surfaces of equal density. Within this context, the present study aims to provide experimental insight of fluid transport by planktonic swarms. The vertical migration of Artemia salina is triggered and controlled by means of a system of stationary and translating luminescent signals. High speed flow visualizations elucidate the competing effects of upward drift by the passive sections of the organisms and downward flow induced by the appendages. The resulting fluid transport is assessed by using PIV at different stages of the migration. The kinetic energy spectrum is computed using velocity correlation functions to determine the length scales at which the animals introduce energy to the flow. [Preview Abstract] |
Sunday, November 24, 2013 3:59PM - 4:12PM |
D18.00009: Why do mayflies switch from rowing to flapping as they grow? Rodolphe Chabreyrie, Khaled Abdelaziz, Elias Balaras, Ken Kiger In order to maintain its metabolism, many species of mayfly nymph utilizes an oscillating array of wing-shaped gills to augment its extraction of dissolved oxygen from the surrounding water. As the nymph develops, the kinematics of these gills are observed to abruptly change from a rowing-like to flapping-like motion. In order to understand the role of this abrupt kinematic change, we consider a pure Lagrangian approach, looking at the mayfly as a stirring device. Using this Lagrangian approach we are able to provide the reason behind the observed kinematic transition during ontogeny. More precisely, recent and powerful tools from chaos theory are applied to in-sillico mayfly nymph experiments. In this talk, we show both qualitatively and quantitatively how the change of kinematics enables a better attraction, stirring and confinement of dissolved oxygen charged water within the near proximity of the gills surface. From the computational velocity field we reveal attracting barriers to transport, i.e. attracting Lagrangian coherent structures (LCS), that form the transport skeleton between and around the gills. In addition, we quantify how well advected particles and consequently dissolved oxygen is spread and mixed within the gills region. [Preview Abstract] |
Sunday, November 24, 2013 4:12PM - 4:25PM |
D18.00010: Simulation of collective behaviour in micro-scale swimmers: Effects of tumbling and rotary diffusion Deepak Krishnamurthy, Ganesh Subramanian Recent experiments have shown that suspensions of swimming micro-organisms are characterized by complex dynamics involving enhanced swimming speeds, large-scale correlated motions and enhanced tracer diffusion. Understanding this dynamics is of fundamental interest and also has relevance to biological systems. In this work we develop a particle-based computational model to study a suspension of hydrodynamically interacting rod-like swimmers with the relation between the swimming velocity and intrinsic stress being enforced from slender body theory. Such an \textit{a priori} specification reduces the computational cost since one now has a ``kinematic'' simulation with a fixed interaction law between swimmers; this does not restrict our study of the dynamics since the destabilizing mechanism has been attributed to the intrinsic (rather than the induced) stress field. Importantly, the model will include intrinsic de-correlation mechanisms found in bacteria such as rotary diffusion and tumbling whose effects have so far not been studied via simulations. Using this model we predict a box-size independent stability threshold based on the suspension concentration, tumble-time (duration between subsequent tumble events) and rotary diffusivity. Comparisons are made with the linear stability theory predictions by Subramanian {\&} Koch (JFM 2009). We demonstrate that the effect of tumbling and rotary diffusion is to stabilize the suspension. [Preview Abstract] |
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