Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session FK: Suspensions II |
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Chair: Andrea Prosperetti, Johns Hopkins University Room: Hilton Chicago Joliet |
Monday, November 21, 2005 8:00AM - 8:13AM |
FK.00001: The stress in a non-uniform suspension of heavy particles Quan Zhang, Andrea Prosperetti The ensemble averaged stress system in a non-uniform suspension of equal spheres subject to external forces or torques is considered. It is found that, as a consequence of the spatial non-uniformity of the system, new terms arise both in the symmetric and antisymmetric parts of the bulk stress. In particular, a new antisymmetric contribution is found even in the absence of external torques. All these terms depend on the particle volume fraction but are independent of the particle size. At low concentrations these results are established by a new formulation of the renormalization technique while, at finite concentrations, a combination of ensemble averaging and numerical simulation is used. [Preview Abstract] |
Monday, November 21, 2005 8:13AM - 8:26AM |
FK.00002: Decay of large scale fluctuations in sedimenting suspensions Jonathan Luke For sufficiently short times and on sufficiently large length scales, the initial dynamics of a sedimenting suspension are that of a viscous fluid of variable density. For an initially well-stirred suspension, the variations in the density produce velocity fluctuations with a variance proportional to the container diameter. These fluctuations become vanishing small as time increases. The dynamics are quadratic and have features including an infinite collection of constants of the motion, a monotonically decreasing energy functional and a linear manifold of stationary solutions with a linear submanifold of stable stationary states. The implications of these features for quantifying the decay of velocity fluctuations are discussed. In particular, the transient phenomenon of fluctuation enhanced sedimentation is characterized. [Preview Abstract] |
Monday, November 21, 2005 8:26AM - 8:39AM |
FK.00003: Velocity fluctuations of initially-stratified sedimenting spheres Peter Mucha, Shang-You Tee, Michael Brenner, David Weitz The study of velocity fluctuations in the sedimentation of spheres is complicated by the time evolution of the underlying particle distribution, both at the microscale and in the bulk. We perform a series of experiments and simulations to isolate the effect of an initial, stable stratification of the particle density. The directly observed dependence of velocity fluctuations on stratification, in agreement with a previously-obtained scaling theory, confirms the importance of even very small stratifications in controlling velocity fluctuations in sedimentation. [Preview Abstract] |
Monday, November 21, 2005 8:39AM - 8:52AM |
FK.00004: Particle migration of a Brownian suspension in simple and chaotic pressure-driven microchannel flows James Gilchrist, Changbao Gao We investigate the flow and concentration profiles of particle suspensions in microfluidic channels. In steady pressure-driven flows, self-organization occurs due to particle migration, typically driving particles away from the walls toward the center of the channel despite the diffusive Brownian motion of the particles. In channels whose geometry induces flow in the transverse direction to the pressure gradient, competition between particle self-organization and mixing due to advection results in concentration profiles where the underlying 3D flow templates pattern formation. Using confocal laser scanning microscopy, we directly image monosized microspheres flowing through 50 x 50 micron 40 mm long straight channels with both smooth and herringbone-ridged boundary conditions and measure the 3D spatial concentration profile. [Preview Abstract] |
Monday, November 21, 2005 8:52AM - 9:05AM |
FK.00005: The Effect of a Translocating Cylindrical Particle on the Ionic Current through a Nanopore Hui Liu, Shizhi Qian, Haim Bau The effect of a translocating particle on the ionic current through a pore has been used for particle counting, biosensing, and DNA sequencing. Using a continuum model consisting of Nernst-Planck, Poisson, and Navier-Stokes equations, we compute the ionic current and the particle's velocity as functions of the particle's and pore's dimensions, surface charges, electric field intensity, electrolyte concentration, and particle's location. When the electrolyte concnetration is high and the particle's surface charge low, the particle's presence reduces the ionic current. When the electrolyte concentration is low, the particle's presence enhances the ionic current. In some cases, the ionic current is enhanced during part of the particle's trajectory and suppressed during another part. Our theoretical predictions qualitatively agree with experimental data and predictions obtained with Molecular Dynamics simulations pertaining to the translocation of DNA molecules in nanopores. [Preview Abstract] |
Monday, November 21, 2005 9:05AM - 9:18AM |
FK.00006: The Latest on Segregation-Banding in Suspensions: From Benchtop Experiments to a New Large-Scale Rotating-Cylinder Facility Peter J. Thomas Segregation-banding in suspensions was first described in Refs. 1 and 2, for flow in a partially-filled horizontal rotating cylinder, and in Ref. 3, for flow in a horizontal Taylor- Couette system. Here new experimental results are reported that reveal how the phenomenon, in horizontal rotating cylinders, is affected by some of the governing experimental parameters. It is possible to suppress banding for certain particle-fluid combinations, indicating that surface tension is important to the initiation of the process. The phenomenon becomes increasingly non-stationary as the cylinder diameter is reduced from about 10cm to 3cm. Banding patterns in a 10cm-cylinder appear quasi-stationary. However, the bands in the 3cm-cylinder merge on time scales of 100 hours. Our research program is presently extended to very large rotating cylinders (length:4m, diam.:1m) to study scaling issues in particle-laden flows. Our new large-scale rotating-cylinder rig, currently under construction, is briefly introduced here for the first time.\newline [1] Boote and Thomas, Phys. Fluids {\bf 11}, p.2020, 1999. \newline [2] Thomas {\it et al.}, Phys. Fluids {\bf 13}, p.2720, 2001.\newline [3] Tirumkudulu {\it et al.}, Phys. Fluids {\bf 11}, p.507, 1999. [Preview Abstract] |
Monday, November 21, 2005 9:18AM - 9:31AM |
FK.00007: Destabilization of a cloud of particles falling in a viscous fluid Bloen Metzger, Maria Ekiel-Jezewska, Elisabeth Guazzelli Surprisingly, a spherical cloud of particles settling in a fluid at low Reynolds number does not remain spherical. The cloud evolves into a torus which subsequently breaks up into two or more droplets in a repeating cascade. This is investigated both experimentally and numerically. [Preview Abstract] |
Monday, November 21, 2005 9:31AM - 9:44AM |
FK.00008: Experimental evidence of particle-size sorting due to preferential concentration in polydisperse turbulent suspensions Jeff Parsons A series of laboratory experiments of a stratified particle-laden mixed layer have documented an enrichment of particles with Stokes number approximately equal one being removed from the mixed layer. Due to the particular geometry used in the experiments, plumes of preferentially concentrated particles were isolated and measured. Enrichment of particles that had Stokes numbers close to one was not well correlated with the particle concentration or the dissipation rate, but was present in all twelve of the experiments performed. [Preview Abstract] |
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