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 AM: Microfluids: General I: Electrokinetic |
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Chair: Minami Yoda, Georgia Institute of Technology Room: Long Beach Convention Center 202B |
Sunday, November 21, 2010 8:00AM - 8:13AM |
AM.00001: Colloidal particle motion in micro galvanic reactors through tailored electrokinetic fluid flow Linda Jan, Christian Punckt, Boris Khusid, Ilhan A. Aksay Using an array of galvanic micro electrodes (e.g., anodic copper and cathodic gold) in contact with an acidic colloidal suspension, we have previously demonstrated autonomous control of particle trajectory and the location of particle deposition which affected the crystallinity of 2D colloidal crystals on the anodes. Particle velocities and the locations of initial particle deposition are affected by the electrode geometry and reaction time. We now present data on the effects of geometry and time on the copper dissolution rate and the associated electrokinetic phenomena. Particle velocities increase with the copper dissolution rate and the steepness of its lateral variation. Experiments and theoretical results reveal that the different location of deposition is related to the difference in the lateral gradient of the dissolution rate. [Preview Abstract] |
Sunday, November 21, 2010 8:13AM - 8:26AM |
AM.00002: Suppression of Electrokinetic Flows by Surface Roughness Robert Messinger, Todd Squires In microfluidic systems, electro-osmotic flows are a promising alternative to mechanical pressure-driven flows, since electrokinetic flow rates are independent of microchannel dimensions and may enable the design of portable (e.g., battery-operated) devices. We show that nanoscale surface roughness, which commonly occurs on microfabricated metal electrodes, can significantly suppress electro-osmotic flows when excess surface conductivity is appreciable. We demonstrate the physical mechanism of electro-osmotic flow suppression due to surface curvature, compute the effects of varying surface conductivity and roughness amplitude on the slip velocities of a model system, and identify scalings for flow suppression in different regimes of surface conduction. We suggest that surface roughness may be one factor that accounts for large discrepancies between classical electrokinetic theory and modern microfluidic experiments. [Preview Abstract] |
Sunday, November 21, 2010 8:26AM - 8:39AM |
AM.00003: Note on the Nonlinear Electrokinetic Effects in Mircochannel Flow: Exact Analytical Solutions for Sinh-Poisson Equation Alan Cheng Hou Tsang, Kwok Wing Chow Electrokinectic effects are important phenomena for fluid flow in microchannels, especially in mechanical systems involving movable micromechanical devices. Electrokinectic effects arise from electric double layer, which is a layer of charges attached to the dielectric surfaces as a result of the interaction of charges between ionized solution and dielectric surfaces. Electric potential inside the flow field is governed by the nonlinear Poisson-Boltzmann equation. Owing to the difficulty in solving the nonlinear equation, Debye - H\"{u}ckel approximation, having an assumption of small electric potential, is a common approach to solve the linearized problem. In the present work, exact analytical expressions are obtained for the fully nonlinear sinh - Poisson equation without invoking the linear approximation. These solutions give insight on treating flow problems when Debye - H\"{u}ckel approximation does not hold. Selected examples of solutions for a rectangular cell with zero homogenous boundary conditions applied on three wall surfaces are used for comparisons between the fully nonlinear and the linearized cases. Significant discrepancies are observed if the potential is not small, hence the present nonlinear theory is essential to better describe the physics involved. [Preview Abstract] |
Sunday, November 21, 2010 8:39AM - 8:52AM |
AM.00004: The electrokinetics of near-wall colloidal particles measured by evanescent-wave particle velocimetry Yutaka Kazoe, Minami Yoda Understanding the near-wall dynamics of suspended colloidal particles subject to electric fields is of interest in microfluidics. Most previous colloid science studies using total internal reflection microscopy to study these dynamics have considered a single particle in a quiescent fluid. We instead analyze the dynamics of an ensemble of fluorescent particles illuminated by evanescent waves using multilayer nano-particle tracking velocimetry (MnPTV). The technique exploits the decay of the evanescent-wave intensity with wall-normal distance $z$ to extract near-wall particle $z$-distributions and flow velocities at different distances from the wall. Here electrokinetically driven flows through $\sim $40 $\mu $m deep fused-silica channels are studied using MnPTV. The results for tracers of radii $a=100\mbox{ nm}$ to 500 nm show that the particle distributions near the wall are highly nonuniform due to electrostatic and van der Waals effects, and that the distributions vary with both the applied electric field $E$ and $a$ due to forces that scale as $E^2$and $a^2$. Despite this variation in the near-wall particle distributions, the MnPTV results give Brownian diffusion coefficients that agree with theoretical predictions and the uniform velocity profiles typical of electroosmotic flow. [Preview Abstract] |
Sunday, November 21, 2010 8:52AM - 9:05AM |
AM.00005: Secondary flow effect on electrokinetic transport in curved channels and microfluidic mixing Myung-Suk Chun, Jin-Myeong Lim This presentation reports the numerical framework and important new results regarding the velocity pattern, vorticity, and mixing property, with variations of channel geometry and heterogeneity of surface properties. Extending our previous studies, secondary Dean flow in curved rectangular microchannels is examined by applying the finite volume/SIMPLE algorithm for the pressure-driven electrokinetic transport coupled with the Poisson-Boltzmann/Navier-Stokes/Nernst-Planck equations. Hydrophilic glass and hydrophobic polymers with fluid slip are combined to create different channel configurations with ranging complementary aspect ratios. Simulation results show that, contrary to the case of narrow-bore channels, the streamwise axial velocity tends to shift toward the inner wall caused by a stronger effect of the spanwise pressure gradient. We observe the presence of pairs of counter-rotating vortices perpendicular to the flow direction and evaluate the circulation magnitude. The increasing rate of inner shift with increasing curvature ratio is more significant in the shallow channel, and the patterns of axial velocity and vorticity alter by the heterogeneity effect of surfaces occupying a large area. In addition, the inertial force should be considered for precise control of the micro- or nanoflows. [Preview Abstract] |
Sunday, November 21, 2010 9:05AM - 9:18AM |
AM.00006: Electrokinetic locomotion due to Reaction Induced Charge Auto-Electrophoresis Jeffrey Moran, Jonathan Posner Synthetic nanomotors, like their biological counterparts, propel themselves through aqueous solutions by harvesting chemical energy from their local environment and converting it to mechanical energy. We study bimetallic rod-shaped particles which move autonomously by catalytically decomposing hydrogen peroxide to oxygen and water. We present a scaling analysis and computational simulations that describe the locomotion of bimetallic rod-shaped motors in hydrogen peroxide solutions due to reaction-induced charge auto-electrophoresis. The model shows that the locomotion results from electrical body forces in the surrounding fluid, which are generated by a coupling of an asymmetric dipolar charge density distribution and the electric field it generates. The simulations make the predictions, in agreement with experiment, that the rods' velocity depends linearly on both the surface charge and reaction rate. [Preview Abstract] |
Sunday, November 21, 2010 9:18AM - 9:31AM |
AM.00007: Electrophoretic mobility of deformable elastic particles in confined geometries Tong Gao, Howard Hu Electrophoretic motion of a dielectric neo-Hookean elastic particle in a confined microchannel is simulated by an Arbitrary Lagrangian-Eulerian moving mesh technique. The particle with a fixed zeta potential is initially elliptical and aligned perpendicular to the direction of the applied electric field. The size of the electrical double layer is assumed to be negligible compared with the particle size and the classical Helmholtz-Smoluchowski slip boundary conditions are applied on the particle surface. When the Reynolds number is low, the elastic deformation is purely induced by the viscous shear force distribution along the body. In the unbounded domain, it is known that the particle will move with a constant Helmholtz-Smoluchowski velocity which is independent of the particle deformation. However, in the confined channel, the rigid walls not only alter the particle-electrical field interaction but also tend to slow the particle motion. To explore the wall effect on the electrophoretic mobility of the particle, the migration velocity is examined by systematically changing both the channel size and the material properties. Also the particle motion in non- Newtonian fluids are simulated and compared with Newtonian cases. [Preview Abstract] |
Sunday, November 21, 2010 9:31AM - 9:44AM |
AM.00008: Lateral Migration and Three-dimensional Focusing of Particles in Microchannel Electrophoresis Litao Liang, Shizhi Qian, Xiangchun Xuan The fundamental study of particle electrophoresis in microchannels is relevant to many applications. It has long been accepted that particles move parallel to the applied electric field in a straight uniform microchannel. In this talk we present the first experimental demonstration of lateral particle migration in electrophoresis through a rectangular microchannel. This phenomenon is due to the electrical force induced by the asymmetric electric field around the particle near a channel wall. We demonstrate that such cross-stream particle motion in electrophoresis can focus neutrally buoyant particles to the centerline of a rectangular microchannel. This three-dimensional electrokinetic particle focusing may potentially be used in micro flow cytometers. [Preview Abstract] |
Sunday, November 21, 2010 9:44AM - 9:57AM |
AM.00009: Highly non-linear induced-charge electroosmosis for flat electrodes Gaurav Soni, Carl Meinhart, Mathias B. Andersen, Henrik Bruus We have simulated induced-charge electroosmotic flow over a flat surface in the highly nonlinear regime, where the notion of the double layer as a linear capacitor is invalid. We have developed two completely independent solution methods: one resolving the double layer under continuum assumption and the other treating the double layer as an effective boundary condition balancing the normal and tangential flux of ions. We show that tangential transport within the double layer leads to gradients in bulk scalar fields. By comparing the effective-boundary model with continuum model, we are able to quantify the accuracy of the effective boundary model. There are certain simplifications of the effective boundary model. One simplification ignores the bulk concentration gradients but incorporates surface conduction. Another simplification is based on linearization and ignores both bulk concentration gradients and surface conduction. We quantify the accuracy of these simplifications. [Preview Abstract] |
Sunday, November 21, 2010 9:57AM - 10:10AM |
AM.00010: Nonlinear waves in electromigration dispersion Sandip Ghosal, Zhen Chen Electromigration dispersion occurs in CE when sample concentrations are sufficiently high. The signal is known to exhibit features such as sharp concentration ``shocks'' that are reminiscent of nonlinear waves. We consider a simplified 3 ion model consisting only of strong electrolytes that are equi-diffusive. The sample concentration is then shown to obey a one dimensional advection diffusion equation with a concentration dependent advection velocity which reduces to Burgers' equation if the sample loading is not too high. Thus, the time dependent problem is exactly solvable with arbitrary initial conditions and in the case of small diffusivity concentration shocks are formed. Analytical formulas are derived for the shape, width, and migration velocity of the sample peak and it is shown that axial dispersion at long times may be characterized by an effective diffusivity that is exactly calculated. [Preview Abstract] |
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