Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session NF: Biofluids XIII |
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Chair: Eleanor Norris, Loughborough University Room: Salt Palace Convention Center 151 G |
Tuesday, November 20, 2007 11:35AM - 11:48AM |
NF.00001: Unidirectional superhydrophobic surfaces Manu Prakash, John Bush It has long been known that the hairy, waxy cuticle of water-walking insects renders them water-repellent; they thus exhibit high static contact angles. We have recently demonstrated that by the virtue of the geometry and flexibility of the hair, the integument is also directionally anisotropic and so plays a key propulsive role. We here report our attempts to design and implement an analogous synthetic surface that exhibits unidirectional adhesion. The surface effectively acts like a fluidic-diode; allowing contact lines to advance in only one direction. When vibrated randomly, drops suspended on the surface advance in only one direction. Applications in valve-less pumps and drop transport in microfluidic devices are discussed. [Preview Abstract] |
Tuesday, November 20, 2007 11:48AM - 12:01PM |
NF.00002: Capillary self-assembly of floating bodies Sunghwan Jung, Paul Thompson, John Bush We study the self-assembly of bodies supported on the water surface by surface tension. Attractive and repulsive capillary forces exist between menisci of, respectively, the same and opposite signs. In nature, floating objects (e.g. mosquito larvae) thus interact through capillary forces to form coherent packings on the water surface. We here present the results of an experimental investigation of such capillary pattern formation. Thin elliptical metal sheets were designed to have variable shape, flexibility and mass distribution. On the water surface, mono-, bi-, or tri-polar menisci could thus be achieved. The influence of the form of the menisci on the packings arising from the interaction of multiple floaters is explored. Biological applications are discussed. [Preview Abstract] |
Tuesday, November 20, 2007 12:01PM - 12:14PM |
NF.00003: Coupling of a protein-laden air/water interface to a shearing bulk flow. Ali Azadani, Juan Lopez, Amir Hirsa Understanding the structure of proteins and other bio-molecules such as RNA is essential to the development of pharmaceuticals. Detailed structural data is obtained by techniques that rely on crystallization. 2D protein crystallization on lipid monolayers at a quiescent air/water interface is now a well-established process, but it only operates under a very restricted set of conditions and on a very slow time scale. We have recently been able to significantly extend the conditions under which the proteins will crystallize as well as speed up the process by subjecting the interface to a shearing flow. Here, using experimental measurements and computational simulations, we investigate the coupling between a protein-laden film and the bulk flow that provides the interfacial shear. This flow in a stationary open cylinder is driven by the constant rotation of the floor. Despite the huge range of length scales involved, a good description of the resultant interfacial velocity field has been obtained using a fairly standard macroscale Newtonian interface model, albeit with variable surface shear viscosity to provide a macroscale description of the molecular scale processes. [Preview Abstract] |
Tuesday, November 20, 2007 12:14PM - 12:27PM |
NF.00004: Collective Self-Propelled Motion Of Microcapsules O. Berk Usta, Alexander Alexeev, Anna C. Balazs We study the collective motion of two capsules on a substrate, using a coupling of lattice-Boltzmann method for fluid flow and lattice-spring method for simulation of elastic solids. One of the capsules acts as a seeder of nanoparticles which can reduce or increase the adhesive properties of the surface. The release, of nanoparticles, is modeled as a random diffusive process. Since this process is symmetric, for the case of a single particle, either no motion or/and a random direction is expected depending on the sequence of the random numbers and the strength of the perturbation due to adhesion gradients. However, with the addition of an empty microcapsule, the symmetry is broken. In the first case, where nanoparticles reduce surface adhesion, the second capsule moves on an adhesion gradient created by the seeding capsule and in turn moves the seeder capsule thorugh hydrodynamic interactions. Eventually both capsules can sit on an adhesion gradient and sustain their motion as long as the first capsule can spread nanoparticles. We identify the parameters and conditions for the motion to be sustained. We also study the inverse problem where the nanoparticles increase the surface adhesion. In this scenario, a capsule can signal a distant capsule to move towards it. [Preview Abstract] |
Tuesday, November 20, 2007 12:27PM - 12:40PM |
NF.00005: A Thin-film Approach to Bacterial Swarming Eleanor Norris, John Ward Swarming is a term used to describe the rapid spread of bacterial colonies on a moist semi-solid substrate. The phenomenon is cell density dependent and usually occurs in response to low nutrient levels. Swarming plays an important part in many bacterial infections, including wound infections and septicaemia as well as lung infections in, for example, cystic fibrosis patients. We aim to develop an understanding of the processes involved in bacterial swarming and our approach to the mathematical modelling is motivated by experimental observations. The equations describing the biological mechanisms determining the behaviour of the bacteria are coupled with the standard thin-film reduction of the Navier-Stokes equation. The initial results of this modelling will be presented, along with a comparison of these results with the available experimental data. [Preview Abstract] |
Tuesday, November 20, 2007 12:40PM - 12:53PM |
NF.00006: Instabilities and dynamics in suspensions of self-locomoting rods Michael Shelley, David Saintillan Suspensions of swimming microorganisms are characterized by complex dynamics involving strong fluctuations and large-scale correlated motions. These motions, which result from the many-body interactions between the particles, are biologically relevant as they impact mean particle transport, mixing and diffusion, with possible consequences for nutrient uptake. In this work, we use numerical simulations to investigate aspects of the dynamics and microstructure in suspensions of interacting self-locomoting rods at low Reynolds number. We propose a detailed model that accounts for hydrodynamic interactions based on slender-body theory and encompasses both biological and non-biological locomotion mechanisms. In agreement with previous predictions, we demonstrate that aligned suspensions of swimming particles are unstable as a result of hydrodynamic fluctuations. In spite of this instability, we demonstrate that a local orientational ordering persists over short length scales and has a significant impact on the mean swimming speed. Consequences of large-scale orientational disorder for particle dispersion are discussed and explained in the context of generalized Taylor dispersion theory. Dynamics in thin liquid films are also presented, and are characterized by a strong particle migration towards the interfaces. [Preview Abstract] |
Tuesday, November 20, 2007 12:53PM - 1:06PM |
NF.00007: Pattern formation and instabilities in active suspensions: Kinetic theory and continuum simulations David Saintillan, Michael Shelley We use kinetic theory and non-linear continuum simulations to study collective dynamics in suspensions of self-propelled particles. The dynamics are modeled using a continuity equation for the particle configurations, coupled to a mean-field description of the flow arising from the stress exerted by the particles on the fluid. Based on this model, we first investigate the stability of both aligned and isotropic suspensions. In aligned suspensions, an instability is shown to always occur at finite wavelengths, a result that generalizes previous predictions by Simha and Ramaswamy (2002). In isotropic suspensions, an instability for the active particle stress is also found to exist, in which shear stresses are eigenmodes and grow exponentially at long scales. Non-linear effects in these systems are also investigated using numerical simulations in two-dimensions. The results of the stability analysis are confirmed, and the long-time non-linear behavior is shown to be characterized by the formation of strong density fluctuations, which appear to be driven by the active stress instability. [Preview Abstract] |
Tuesday, November 20, 2007 1:06PM - 1:19PM |
NF.00008: Instability of a Rotating Elastic Filament due to Viscous Stresses Bian Qian, Kenneth S. Breuer The deformation of an elastic filament due to viscous stresses at low Reynolds number is important in the dynamics of flexible biological filaments such as bacterial flagella and DNA. Several theoretical and numerical works have reported that a twist-induced \textit{writhing} instability or initial asymmetry with respect to the rotational axis may induce a shape bifurcation from a straight rotating elastic filament to a helical state. We present experimental results obtained using a macroscopic flexible rope rotating in a low-Re tank. The three-dimensional rope shape and torque-speed relationship are measured and characterized as functions of filament elasticity, fluid viscosity and system geometry. Transitions between different filament mode shapes, and the effect of the mode shape on the torque are quantified. In addition, the roles of bending and torsional stiffness is discussed. [Preview Abstract] |
Tuesday, November 20, 2007 1:19PM - 1:32PM |
NF.00009: Flow induced protein nucleation: Insulin oligomerization under shear. Andrew Dexter, Ali Azadani, Mirco Sorci, Georges Belfort, Amir Hirsa A large number of diseases are associated with protein aggregation and misfolding, such as Alzheimer's, Parkinson's and human prion diseases such as Creutzveld-Jakob disease. Characteristic of these diseases is the presence of amyloid fibrils and their precursors, oligomers and protofibrils. Considerable evidence exists that a shearing flow strongly influences amyloid formation both \textit{in vitro} and \textit{in vivo.} Furthermore, the stability of protein-based pharmaceuticals is essential for conventional therapeutic preparations and drug delivery systems. By studying the nucleation and growth of insulin fibrils in a well-defined flow system, we expect to identify the flow conditions that impact protein aggregation kinetics and which lead to protein destabilization. The present flow system consists of an annular region bounded by stationary inner and outer cylinders and is driven by rotation of the floor. Preliminary results indicate that a continuous shearing flow can accelerate the aggregation process. The interfacial shear viscosity was found to drastically increase during aggregation and appears to be a useful parameter to probe protein oligomerization and the effects of flow. [Preview Abstract] |
Tuesday, November 20, 2007 1:32PM - 1:45PM |
NF.00010: Energy required to lift a water strider's hydrophobic leg out of water Duck-Gyu Lee, Dominic Vella, Ho-Young Kim Although it is well-known that a water strider's legs are superhydrophobic, the reason for their strong water repellency is still a matter of debate. Cylindrical objects floating on water can support similar maximum load as long as the contact angle is over 90 degrees, which discards the hypothesis of the role of superhydrophobicity in static floating. Here we show that the energy required to lift a water strider's cylindrical leg out of water strongly depends on the wettability. Our theory and experiments reveal that the energy saving achieved by the superhydrophobicity of the legs reaches over 90\% as compared with the legs with the contact angle of 90 degrees. This implies that a water strider can jump out of water in emergency with a great ease owing to its legs' strong water repellency. [Preview Abstract] |
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