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 EK: Suspensions I |
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Chair: Grae Worster, University of Cambridge Room: Hilton Chicago Joliet |
Sunday, November 20, 2005 4:10PM - 4:23PM |
EK.00001: Effects of DC and AC Electric Fields on Emulsions of Leaky Dielectric Drops Immersed in Simple Shear Flow. Arturo Fernandez DNS is used to examine the effects of DC and AC electric fields on the rheological properties of emulsions. Drops are immersed in a bath of fluid between two plates, and a simple shear flow is imposed. Assuming quasi-static conditions, the electric field is described by using the leaky dielectric model proposed by Taylor. The electric stresses have two effects: (1) a dielectrophoretic attraction in the direction parallel with the electric field, (2) at the interface between the fluids, a viscous fluid motion generated by the tangential component of the electric stresses. The electrorheological response of the emulsion is governed by the competition between hydrodynamical and electrical forces. The former include viscous, inertial and capillary forces, and the latter ones depend on the electrical properties of the fluids, the strength of the electric field and the frequency, if the electric field is alternating. The emulsions exhibit a yield stress, which depends on the strength of the electric field, and behave as either shear-thinning or shear-thickening material depending on the electrical properties. [Preview Abstract] |
Sunday, November 20, 2005 4:23PM - 4:36PM |
EK.00002: Breaking symmetries in induced-charge electro-osmosis and electrophoresis Todd Squires, Martin Bazant In induced-charge electro-osmosis (ICEO), an applied field induces an electric double-layer around a polarizable surface, and then forces that same induced double-layer to drive a nonlinear electrokinetic flow. This allows steady flows to be driven even with AC fields, unlike in standard electro-osmosis. In this talk, we discuss ICEO in systems in which a symmetry has been broken in any of a number of ways: conductors with inhomogeneous surface properties, asymmetrically-shaped bodies, and nonuniform electric fields. We highlight several paradigmatic examples that are interesting for both colloidal science and microfluidics. We describe the induced-charge electrophoretic motion of asymmetrically-shaped colloids, and provide principles for the design of metallic colloids that rotate to orient themselves in a desired direction, then translate in a desired direction relative to an applied electric field. In the microfluidic context, asymmetric conductors allow strong, steady microfluidic flows to be driven along channels with relatively small potentials applied across channels, suggesting methods for portable, self-powered microfluidic devices. [Preview Abstract] |
Sunday, November 20, 2005 4:36PM - 4:49PM |
EK.00003: Effect of the Double Layer on the Dielectrophoretic Motion of Particles T.N. Swaminathan, Howard Hu Most suspensions involve the formation of ionic double layers next to the surface of particles. The double layer formed due to the induced-charge on the particle affects its motion even under sinusoidal electric fields through a phenomenon termed as induced-charge electro-osmosis. A method to numerically evaluate the effect of the double layer on the dielectrophoretic motion of particles has been developed. The technique, developed herein, involves a matched asymptotic expansion of the electric field near the particle surface, where the double layer is formed, and is written as a jump-boundary-condition for the electric potential when the thickness of the double layer is small compared to the size of the particle. The developed jump-boundary-condition is amenable to numerical evaluation and has been implemented in an Arbitrary Lagrangian Eulerian based finite element scheme using a discontinuous Galerkin method which naturally permits for such discontinuous boundary conditions in its formulation. The effect of the induced-charge electro-osmosis on the dielectrophoretic motion of particles has been observed using this technique. [Preview Abstract] |
Sunday, November 20, 2005 4:49PM - 5:02PM |
EK.00004: New Electric Field-Driven Mesoscale Phase Transitions in Polarized Suspensions Anil Kumar, Andreas Acrivos, Zhiyong Qiu, Boris Khusid We report the discovery of a new electric-field induced mesoscale phase transition in a confined suspension under the action of a spatially uniform AC electric field ($\sim $kV/mm, 0.1-3kHz). The experiments were conducted on suspensions of neutrally buoyant, negatively polarized, non-Brownian particles confined between two parallel electrodes. Within few seconds following the field application, as expected, the particles align themselves along the field direction forming chains and columns. But, surprisingly, after $\sim $10-20 minutes, several nucleation sites suddenly appeared throughout the whole experimental cavity. At these nucleation sites, the particle columns, formed initially, began moving radially outward until they interfered with one another at which point they created a stationary cellular pattern. We characterized the morphology of this pattern and found that it depended only on the initial suspension concentration and the interelectrode gap. The growth kinetics of the structure formation was governed only by the initial particle concentration and the magnitude of the applied field strength. Strangely, both the morphology as well as the kinetics, are insensitive to the particle size and the field frequency. Lastly, we wish to note a surprising similarity of the observed cellular structure formation and a hotly debated paradoxical scenario of multi-scale phase transitions proposed by Lebowitz and Penrose nearly 40 years ago. [Preview Abstract] |
Sunday, November 20, 2005 5:02PM - 5:15PM |
EK.00005: Mechanisms of ice lens formation in freezing soils. Stephen Peppin, M. Grae Worster Frost heave is a process in which the ground swells upon freezing. This phenomenon plays a central role in the formation of unique geological features in permafrost areas. Frost heave also causes annual damage to roads and buildings in northern climates. During the freezing process regions of nearly pure ice (ice lenses) form in the soil. Similar phenomena occur during the freezing of tissue, food products, and many other materials. In the last several years, the first experiments capable of viewing the formation of ice lenses in a well-characterized model soil consisting of glass beads in water have been performed. These experiments have yielded insight into the basic mechanisms behind frost heave. Recently we have developed mathematical models of these experiments. Our models indicate that ice lenses can form via three distinct mechanisms: nucleation of ice beyond a compacted layer of particles, periodic rejection and engulfment of particles by the ice front, and (in highly colloidal soils) morphological instability of the solidification front. The interplay between these mechanisms could explain some of the great variation in ice lens structure and orientation seen in real soils. [Preview Abstract] |
Sunday, November 20, 2005 5:15PM - 5:28PM |
EK.00006: Segregation and Pattern Formation in a Rotating Suspension. Tony Ladd, Jonghoon Lee A rotating suspension of non-neutrally buoyant particles can be unstable to axial perturbations in concentration over a range of angular frequencies. Numerical simulations show that a highly regular pattern of particle density and fluid flow coexist in a non-equilibrium stationary state. Similar patterns were observed in laboratory experiments under equivalent conditions. We have discovered that the mean angular velocity of the particles is an order parameter, which distinguishes between a low-frequency segregated phase and a high-frequency dispersed phase, where the particles fill the whole volume uniformly. The order parameter is a function of a single dimensionless frequency, with a characteristic length that is the mean interparticle separation. As the rotational frequency increases, the particle distribution becomes more homogeneous, and the band structure disappears. Hydrodynamic diffusion stabilizes the suspension against centrifugal forces, allowing for a uniformly dispersed phase that can be used to grow three-dimensional cell cultures in an artificial microgravity environment. [Preview Abstract] |
Sunday, November 20, 2005 5:28PM - 5:41PM |
EK.00007: Near wall dynamics of a large particle in a highly bidisperse colloidal solution Sukalyan Bhattacharya, Jerzy Blawzdziewicz, Eligiusz Wajnryb Small particles (or macromolecules) added to a colloidal solution of much larger species produce the effective structural force acting between the large particles and the walls. When the large particles are moving, the macromolecules can also produce non-equilibrium effects, which include the hydrodynamic and Brownian resistance forces. Our talk will focus on these non-equilibrium phenomena for a large sphere near a planar wall in a dilute solution of much smaller spherical particles. The gap between the wall and large particle is of the order of the small-particle diameter. It is thus much larger than the size of the lubrication region, and our analysis relies on this length-scale separation. To find the hydrodynamic contribution to the resistance force we evaluate the induced-force distribution on a sphere in a gap between (locally) planar, parallel walls. The pressure field in the original system, where the gap width is slowly varying, is then obtained by solving lubrication equations with the source term corresponding to this induced force. The Brownian contribution is obtained in a similar manner. [Preview Abstract] |
Sunday, November 20, 2005 5:41PM - 5:54PM |
EK.00008: On the droplet deformation by internal colloidal particles Takuya Tsuji, Takashi Sako, Masahiro Sakai, Toshitsugu Tanaka Spray drying of colloidal droplet is a promising method for the fabrication of nano-structured particles. It is know that thermal / hydrodynamic conditions of the process control the morphology of final particles. Addition to the surface tension and shear stress induced by the velocity difference between suspended droplet and carrier gas, internal colloidal particles are expected to have influence on the droplet shape. However, this is still not known well. In this study, we investigated the effect of colloidal particle loading, colloidal particle size and droplet size on the droplet deformation. Free-falling silica colloidal water droplet was observed by using the high speed camera. Colloidal particles, depending on the conditions, tend to enhance the instability and vice versa. [Preview Abstract] |
Sunday, November 20, 2005 5:54PM - 6:07PM |
EK.00009: Detailed Simulations of Rigid Fiber Suspensions Anna-Karin Tornberg, Katarina Gustavsson In this talk, we present a numerical method designed to simulate the challenging problem of the dynamics of slender fibers immersed in an incompressible fluid. Specifically, we consider microscopic rigid fibers, that sediment due to gravity. Such fibers make up the micro-structure of many suspensions for which the macroscopic dynamics are not well understood. Our numerical algorithm is based on a non-local slender body approximation that yields a system of coupled integral equations, relating the forces exerted on the fibers to their velocities, which takes into account the hydrodynamic interactions of the fluid and the fibers. The system is closed by imposing the constraints of rigid body motions. The fact that the fibers are straight have also been further exploited in the design of the numerical method. In difference to spheres, isolated fibers can have motion perpendicular to gravity and the velocity depends strongly on the fiber orientation. Also, the forming of clusters, sometimes referred to as flocculation, enhances the settling velocity to a larger value than the maximum speed of an single and vertical aligned isolated fiber. We present results from simulations including a larger number of fibers, and discuss phenomena on the microscopic scale as well as macroscopic properties such as average sedimentation speed and fiber orientation. [Preview Abstract] |
Sunday, November 20, 2005 6:07PM - 6:20PM |
EK.00010: Modeling of motion of flexible fibers in nonzero Reynolds number flows Dewei Qi Based on a lattice Boltzmann equation, methods for direct simulations of flexible filament suspensions in a nonzero Reynolds number flow are developed. A flexible fiber is discretised into a chain of either consecutive spherical or cylindrical segments. For spherical model, a constraint force algorithm is proposed to warrant constant bonding distance and non-slip contacting velocity between two neighboring segments so that the filament moves and rotates as a whole body. For cylindrical model, cylindrical segments move and rotate with additional constraint forces that keep the joint point between two neighboring segment have the same displacement. A constraint algorithm for rotations of cylindrical segment is developed. The present flexible fiber methods are tested by using a rigid particle method at large fiber stiffness and by comparing the present results with theoretical and experimental results. It is demonstrated that the present results have a reasonable accuracy and that the computational results are consistent with the existed experimental results at nonzero Reynolds number flows. The behavior of flexible fibers in shearing and sedimentation flows is studied and reported. [Preview Abstract] |
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