Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session EJ: Bio-Fluids: General I |
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Chair: Petia Vlahovska, Dartmouth College Room: 102A |
Sunday, November 23, 2008 4:10PM - 4:23PM |
EJ.00001: Microfluidic Separation of Chiral Particles Marcos, Henry Fu, Thomas Powers, Roman Stocker We present a combined theoretical and experimental investigation of the fluid mechanics of a helix exposed to a shear flow. In addition to classic Jeffery orbits, Resistive Force Theory predicts a drift of the helix across streamlines, perpendicular to the shear plane. The direction of the drift is determined by the direction of the shear and the chirality of the helix. We verify this prediction experimentally using microfluidics, by exposing Leptospira biflexa, a non-motile strain of helical-shaped bacteria, to a plane parabolic flow. As the shear in the top and bottom halves of the microchannel has opposite sign, we predict and observe the bacteria in these two regions to drift in opposite directions. The magnitude of the separation is in good quantitative agreement with theory. This setup can be used to separate microscale chiral objects. [Preview Abstract] |
Sunday, November 23, 2008 4:23PM - 4:36PM |
EJ.00002: Gyrotactic trapping: a bifurcation in vertical motility triggers formation of thin phytoplankton layers William Durham, John Kessler, Roman Stocker Characterized by spikes in cell concentration orders of magnitude above ambient, thin layers of phytoplankton are recurrent features of the coastal ocean, yet the mechanism of their formation remains unclear. We propose that cell motility in combination with depth-variable fluid shear can form thin layers via a mechanism we call ``gyrotactic trapping.'' The swimming direction of many motile phytoplankton is set by a balance between a gravitational torque (caused by cell asymmetry) and a viscous torque (caused by shear). Local peaks in shear disrupt this torque balance, producing a gradient in vertical cell flux and leading to intense cell accumulation. We tested gyrotactic trapping using two species of phytoplankton exposed to a lid-driven cavity flow, observing strong thin layers for both. The experimental layers closely matched results from individual-based simulations. Furthermore, an advection-diffusion model reveals that gyrotactic trapping can generate thin phytoplankton layers under realistic conditions in the ocean, where vertical distances are on the order of meters and layers are subject to dissipation by turbulence. [Preview Abstract] |
Sunday, November 23, 2008 4:36PM - 4:49PM |
EJ.00003: Predation by the Dwarf Seahorse on Copepods: Quantifying Motion and Flows Using 3D High Speed Digital Holographic Cinematography - When Seahorses Attack! Brad Gemmell, Jian Sheng, Ed Buskey Copepods are an important planktonic food source for most of the world's fish species. This high predation pressure has led copepods to evolve an extremely effective escape response, with reaction times to hydrodynamic disturbances of less than 4 ms and escape speeds of over 500 body lengths per second. Using 3D high speed digital holographic cinematography (up to 2000 frames per second) we elucidate the role of entrainment flow fields generated by a natural visual predator, the dwarf seahorse (\textit{Hippocampus zosterae}) during attacks on its prey, \textit{Acartia tonsa}. Using phytoplankton as a tracer, we recorded and reconstructed 3D flow fields around the head of the seahorse and its prey during both successful and unsuccessful attacks to better understand how some attacks lead to capture with little or no detection from the copepod while others result in failed attacks. Attacks start with a slow approach to minimize the hydro-mechanical disturbance which is used by copepods to detect the approach of a potential predator. Successful attacks result in the seahorse using its pipette-like mouth to create suction faster than the copepod's response latency. As these characteristic scales of entrainment increase, a successful escape becomes more likely. [Preview Abstract] |
Sunday, November 23, 2008 4:49PM - 5:02PM |
EJ.00004: Control volume based hydrocephalus research Benjamin Cohen, Abram Voorhees, Timothy Wei Hydrocephalus is a disease involving excess amounts of cerebral spinal fluid (CSF) in the brain. Recent research has shown correlations to pulsatility of blood flow through the brain. However, the problem to date has presented as too complex for much more than statistical analysis and understanding. This talk will highlight progress on developing a fundamental control volume approach to studying hydrocephalus. The specific goals are to select physiologically control volume(s), develop conservation equations along with the experimental capabilities to accurately quantify terms in those equations. To this end, an \textit{in vitro} phantom is used as a simplified model of the human brain. The phantom's design consists of a rigid container filled with a compressible gel. The gel has a hollow spherical cavity representing a ventricle and a cylindrical passage representing the aquaducts. A computer controlled piston pump supplies pulsatile volume fluctuations into and out of the flow phantom. MRI is used to measure fluid velocity, and volume change as functions of time. Independent pressure measurements and flow rate measurements are used to calibrate the MRI data. These data are used as a framework for future work with live patients. [Preview Abstract] |
Sunday, November 23, 2008 5:02PM - 5:15PM |
EJ.00005: Transition to organized behavior on suspensions of concentrated bacteria Sujoy Ganguly, Luis Cisneros, John Kessler, Raymond Goldstein Concentrated populations of the swimming bacterium Bacillus subtilis develop a collective phase, the Zooming BioNematic, that exhibits large-scale coherence analogous to the molecular alignment of nematic liquid crystals. Bacterial suspensions were prepared in order to experimentally measure the transition to organized behavior as a function of the cell number concentration. PIV analysis was used to obtain cell velocities and define an order parameter in order to characterize the dynamics of the system. [Preview Abstract] |
Sunday, November 23, 2008 5:15PM - 5:28PM |
EJ.00006: The low Reynolds hydrodynamics of bent rods precessing above flat planes Roberto Camassa, Elizabeth Bouzarth, Pavel Chtcheprov, David Marron, Richard McLaughlin, Jonathan Toledo, Leandra Vicci, Longhua Zhao We examine the role of bend in rods precessing upright cones above flat planes in Newtonian fluids at low Reynolds. We experimentally document that the effect of bend in the rod is the creation of a novel set of nested tori on which fluid particles live: for straight rods, the tori degenerate into points in a Poincare section, while any amount of bend breaks symmetry and creates these tori. We present slender body asymptotic models which predict quantitatively and qualitatively similar behavior. [Preview Abstract] |
Sunday, November 23, 2008 5:28PM - 5:41PM |
EJ.00007: Effects of bulk and free surface shear flows on amyloid fibril formation David Posada, Mirco Sorci, Georges Belfort, Amir Hirsa Amyloid diseases such as Alzheimer's and Huntington's, among others, are characterized by the conversion of monomers to oligomers (precursors) and then to amyloid fibrils. Besides factors such as concentration, pH, and ionic strength, evidence exists that shearing flow strongly influences amyloid formation in vitro. Also, during fibrillation in the presence of either gas or solid surfaces, both the polarity and roughness of the surfaces play a significant role in the kinetics of the fibrillation process. By studying the nucleation and growth of a model system (insulin fibrils) in a well-defined flow field, we can identify the flow and interfacial conditions that impact protein aggregation kinetics. The present flow system consists of an annular region, bounded by stationary inner and outer cylinders and driven by rotation of the floor, with either a hydrophobic (air) or hydrophilic (solid) interface. We show both the combined and separated effects of shear and interfacial hydrophobicity on the fibrillation process, and the use of interfacial shear viscosity as a parameter for quantifying the oligomerization process. [Preview Abstract] |
Sunday, November 23, 2008 5:41PM - 5:54PM |
EJ.00008: A quantitative study of the adhesive locomotion of terrestrial gastropods Janice Lai, Robert Shepherd, Juan Carlos del Alamo, Juan C. Lasheras The locomotion of terrestrial gastropods exhibits unique characteristics which allow these soft-body animals to crawl while adhering to steep surfaces. Gastropods move by gliding over a ventral foot lubricated by a thin layer of mucus. They generate trains of pedal waves through periodic muscle contractions in the central portion of the ventral foot, producing a forward traction, while the rim of the foot glides over the substrate. We analyzed the kinematics and dynamics of locomotion by conducting two sets of experiments. In the first set, we used digital image processing techniques to correlate the frequency and wavelength of the pedal waves to the migration velocity. In the second set, we computed the spatial and temporal evolution of the traction forces transmitted across the thin lubricating layer from measurements of the deformation of an elastic substrate of known properties and calculate the mechanical work used for crawling. We found that the pedal waves accelerate as they move forward along the ventral foot producing a breaking in symmetry which could contribute to the generation of a net traction force. [Preview Abstract] |
Sunday, November 23, 2008 5:54PM - 6:07PM |
EJ.00009: The locomotion of marine and terrestrial gastropods: can the acceleration of the ventral pedal waves contribute to the generation of net propulsive forces? Juan C. del Alamo, Javier Rodroguez-Rodriguez, Janice Lai, Juan C. Lasheras Marine and terrestrial gastropods move by gliding over a ventral foot that is lubricated by secreted mucus (terrestrial) or simply by water (marine). The rim of the ventral foot generates suction forces that keep the animal adhered to the substrate. The central part of the foot produces a net propulsive force by generating trains of pedal waves through periodic muscle contractions. Recent experiments show that, in some gastropods, these pedal waves become faster and longer as they move forward, suggesting a mechanism for the generation of net propulsive forces by building a pressure difference across consecutive waves. We have investigated the efficiency of this mechanism through a theoretical analysis of a two-dimensional lubrication layer between a train of waves of slowly varying length and speed, and a flat, rigid, impermeable surface. The inhomogeneity of the speed and length of the pedal waves has been modeled through multiple-scale asymptotics. We have considered a Newtonian fluid to separate the effect of this inhomogeneity from the viscoelastic propulsion reported in previous works. [Preview Abstract] |
Sunday, November 23, 2008 6:07PM - 6:20PM |
EJ.00010: ABSTRACT WITHDRAWN |
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