Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session QV: Suspensions II |
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Chair: John Hinch, Cambridge University Room: Hyatt Regency Long Beach Regency B |
Tuesday, November 23, 2010 12:50PM - 1:03PM |
QV.00001: Mixing and microstructure in sedimenting suspensions Elisabeth Guazzelli, Laurence Bergougnoux We present experiments concerning the statistics of particle microstructure in sedimenting suspensions. We explore the role of initial mixing on the microstructure and its impact on velocity fluctuations. [Preview Abstract] |
Tuesday, November 23, 2010 1:03PM - 1:16PM |
QV.00002: Microrheology of colloidal dispersions Roseanna Zia, John Brady Dilute and concentrated colloidal dispersions are studied via nonlinear microrheology in the presence of excluded volume and hydrodynamic interactions via Stokesian dynamics simulation. In nonlinear microrheology, the motion of a Brownian probe is tracked as it is driven by an external force through the suspension. Mean probe motion defines the microviscosity by application of Stokes' drag law; probe fluctuations due to collisions give rise to force-induced diffusion. Together, these two quantities define the particle stress, from which the normal stress differences and osmotic pressure are obtained. Rheological properties depend strongly on the deformed microstructure, which in turn depends on the strength with which it is driven from equilibrium by the probe, as given by the Peclet number - the strength of probe forcing compared to thermal forces: Pe=Fb/kT, where kT is the thermal energy and b the bath particle size. Hydrodynamic interactions strongly influence this structure, modulating the force-induced diffusion and giving rise to force-thickening at large Pe, thus altering the effects of viscous stress at Pe$>>$1. [Preview Abstract] |
Tuesday, November 23, 2010 1:16PM - 1:29PM |
QV.00003: Plugging of microchannels by spherical particles Eric Climent, Constant Agbangla, Patrice Bacchin We investigate by means of numerical simulations the dynamic formation of 3D structures of microparticle aggregates blocking the flow through microchannels. Both the geometries of a straight channel and a sudden reduction of section are analyzed. We use the Force Coupling Method (Climent {\&} Maxey, 2010) to handle simultaneously multi-body hydrodynamic interactions of a confined flowing suspension together with particle/particle and particle/wall surface interactions leading to the adhesion and aggregation of particles. The basic idea of the Force Coupling Method relies on multipole expansion of velocity perturbations induced by the presence of particles in the flow. Simulation results show that varying the magnitude of DLVO interparticle and particle/wall interactions leads to distinct scenarios of pores clogging. We investigate the kinetics of the microchannel occlusion (corresponding to a temporal decrease of the bulk permeability of the channel). We identify the nature of the fouling mechanism: deposition, interception, bridging {\ldots} (see the papers of Sharp {\&} Adrian (2005), Ramachandran {\&} Fogler (1999) and Marshal, 2007). [Preview Abstract] |
Tuesday, November 23, 2010 1:29PM - 1:42PM |
QV.00004: A Quantitative Study of Bulk Stresses in Nonlinear Microrheology Ryan DePuit, Todd Squires We investigate the nonlinear microrheology of a simple model system - a spherical probe translating through a dilute suspension of rigid rods - to elucidate a variety of issues inherent in the interpretation of nonlinear microrheology. We have developed a computational system to quantitatively examine the issues present in interpretation of nonlinear microrheology, as originally discussed by Squires (Langmuir, 2008). Following recent work emphasizing the importance of the microstructural behavior in the bulk (Sriram et. al, 2009), we focus our attention on the bulk microstructural deformation, and examine the significance of its (Lagrangian) transient nature, as well as the consequences of the mixed and inhomogeneous flows inherent to nonlinear microrheology. From this quantitative study, we pose solutions for the current theoretical issues facing nonlinear microrheology in interpretation and comparison of the microviscosity with the shear viscosity from traditional bulk rheometry. [Preview Abstract] |
Tuesday, November 23, 2010 1:42PM - 1:55PM |
QV.00005: Long-time self-diffusivity of a catalytic partcle in a dilute suspension Sergey Shklyaev, John F. Brady, Ubaldo M. Cordova--Figueroa Active microrheology, which studies local changes in a microstructure of a suspension near a forced particle and a feedback of this redistribution on the particle motion, is of keen interest. Implementation of this concept to the chemically active particles is a promising field of research. We consider a long-time self-diffusivity of a catalytic (probe) particle dragged by an external force through a dilute suspension comprising reactant and product particles. The former decay at the contact with the probe particle producing $s$ product particles. Neglecting by the hydrodynamic interaction, we derive the boundary value problem which governs the microstructures of the both types of suspended particles. Distortion of the microstructure due to both the motion of the probe and the chemical reaction leads to change in the tensor of long-time self-diffusivity. The problem is considered analytically in a several limiting cases and numerically otherwise. In the absence of advection contributions ensuing from reactant and product partcles are completely different. The first one is negative and tends to zero for the fast reaction, whereas the second one is not of fixed sign and remains finite in the mentioned limiting case. Advection amplifies both the contributions, the increase is more pronounced for the longitudinal component of the diffusivity tensor. [Preview Abstract] |
Tuesday, November 23, 2010 1:55PM - 2:08PM |
QV.00006: In Situ Observations of Electric-Field Induced Nanoparticle Aggregation T.J. Woehl, N.D. Browning, W.D. Ristenpart Nanoparticles have been widely observed to aggregate laterally on electrodes in response to applied electric fields. The mechanism driving this behavior, however, is unclear. Several groups have interpreted the aggregation in terms of electrohydrodynamic or electroosmotic fluid motion, but little corroborating evidence has been presented. Notably, work to date has relied on \emph{post situ} observations using electron microscopy. Here we present a fluorescence microscopy technique to track the dynamics of nanoparticle aggregation \emph{in situ}. Fluorescent 20-nm polystyrene nanoparticles are observed to form optically visible aggregates in response to an applied AC field. Although single particle resolution is lost, the existence of aggregates on the electrode surface is marked by growing clusters of increasingly bright intensity. We present a systematic investigation of the effects of applied potential and frequency on the aggregation rate, and we interpret the behavior in terms of a mechanism based on electrically induced convective flow. [Preview Abstract] |
Tuesday, November 23, 2010 2:08PM - 2:21PM |
QV.00007: Numerical Simulations of Electrostatically Induced Aggregation and Coalescence in Polydisperse Emulsions G.R. Magill, W.D. Ristenpart Although electrostatic coalescers have long been used to destabilize emulsions of polarizable droplets, the dynamics of droplet aggregation and coalescence remain poorly understood. The aggregation is believed to be primarily driven by dipolar interactions between droplets, suggesting that increasing the electric field strength should increase the rates of aggregation and coalescence. However, recent evidence suggests that coalescence is inhibited above a critical field strength. Here we numerically investigate the dynamics of aggregation and coalescence of polydisperse emulsions. The simulations are based on the point dipole approximation coupled with pseudo hard sphere repulsion at small separations. Two limiting cases are examined in detail: immediate coalescence upon contact, and perfect stability against coalescence. We compare the numerical results to previous experimental work, and we discuss the implications for optimizations of electrostatic coalescers. [Preview Abstract] |
Tuesday, November 23, 2010 2:21PM - 2:34PM |
QV.00008: Magnetically Guided Propulsion of Osmotic Motors Glenn Vidal, Carlos Rinaldi, Ubaldo C\'{o}rdova-Figueroa Propulsion of artificial nano- and micro-scale objects induced by chemical reactions is one of the most exciting challenges in colloidal physics. Recent experiments have shown that directed motion of catalytic motors is hindered by their rotary Brownian motion, preventing its potential to be fully realized. The present work investigates the magnetically guided propulsion of a colloidal particle--the osmotic motor-- immersed in a dispersion of colloidal $`$bath$'$ particles subject to an unidirectional magnetic field using Brownian dynamics simulation. The osmotic motor is propelled by a chemical reaction that consumes bath particles over a portion of its surface. The non-equilibrium microstructure of bath particles induced by the surface reaction creates an $`$osmotic pressure$'$ imbalance on the motor$'$s surface causing it to move to regions of lower bath particle concentration. The strength of the magnetic field is controlled by the Langevin parameter, which physically measures the relative importance of magnetic to Brownian torques, and dictates the directionality of the osmotic motor. The translational self-diffusivity is measured for different reaction speeds, particle sizes, bath particle concentrations, and magnetic dipole orientations. Finally, a theory to determine the long-time self-diffusivity and time-averaged particle velocity is developed and compared to the simulation results. [Preview Abstract] |
Tuesday, November 23, 2010 2:34PM - 2:47PM |
QV.00009: The Rheology and Microstructure of Dense Suspensions of Elastic Capsules Jonathan Clausen, Daniel Reasor, Cyrus Aidun We use a recently developed hybrid numerical technique [MacMeccan et al. (2009)] that combines a lattice-Boltzmann (LB) fluid solver with a finite element (FE) solid-phase solver to study suspensions of elastic capsules. The LB method recovers the Navier-Stokes hydrodynamics, while the linear FE method models the deformation of fluid-filled elastic capsules for moderate levels of deformation. The simulation results focus on accurately describing the suspension rheology, including the particle pressure, and relating these changes to changes in the microstructure. Simulations are performed with hundreds of particles in unbounded shear allowing an accurate description of the bulk suspension rheology and microstructure. In contrast to rigid spherical particles, elastic capsules are capable of producing normal stresses in the dilute limit. For dense suspensions, the first normal stress difference is of particular interest. The first normal stress difference, which is negative for dense rigid spherical suspensions, undergoes a sign change at moderate levels of deformation of the suspended capsules. [Preview Abstract] |
Tuesday, November 23, 2010 2:47PM - 3:00PM |
QV.00010: Solid Particle Erosion in Slug Flow Netaji Ravikiran Kesana, Jon Throneberry, Brenton McLaury, Siamack Shirazi Erosion is a common problem faced by oil and gas industries, and the repair of pipeline fittings damaged by erosion is extremely costly. Therefore measuring erosion under different flow conditions and in different flow geometries is important to help better understand the effect of various parameters on erosion and to provide information to develop protective guidelines for the oil and gas producers. Specifically, this work examines solid particle erosion in multiphase slug flow and the transition to annular flow regime in a 3-inch pipe with solid (sand) particles of different average sizes (20, 150 and 300 microns) and with different liquid viscosities (1cP, 10 cP). The metal loss is measured using intrusive Electrical Resistance (ER) probes which work on the principle of resistivity of the sample and reference elements. Erosion is measured at three different locations in the pipe, two in a bend and one in a straight section. Results demonstrate that metal loss increases by increasing the superficial gas velocity, superficial liquid velocity, or particle diameter; however, metal loss decreases by increasing the liquid viscosity. [Preview Abstract] |
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