Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session R22: Microscale Flows: Electrokinetics 
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Chair: Prashanta Dutta, Washington State University Room: E141/142 
Tuesday, November 22, 2016 1:30PM  1:43PM 
R22.00001: Stable Rotation of Microparticles using a Combination of Dielectrophoresis and Electroosmosis Prashanta Dutta, Walid Rezanoor Electric field induced microparticle rotation has become a powerful technique to evaluate cell membrane dielectric properties and cell morphology. In this study, stable rotations of microparticles are demonstrated in a stationary AC electric field created from a set of coplanar interdigitated microelectrodes. The medium, particle size, and material are carefully chosen so that particle can be controlled by dielectrophoretic force, while a sufficiently high AC electroosmotic flow is produced for continuous particle rotation. Stable rotation up to 218 rpm is observed at 30 V$_{\mathrm{pp}}$ applied sinusoidal potential in the frequency range of 80  1000 Hz. The particle spin rate observed from the experimental study is then validated with a numerical model. The model is formulated around complex charge conservation equation to determine the electric potential distribution in the domain. Stokes equation is employed to solve for AC electroosmotic fluid flow in the domain. Complexity arising from nonlinear potential drop across the electric double layer due to the application of a very large electric potential is also addressed by introducing modified capacitance equation which considers steric effect. [Preview Abstract] 
Tuesday, November 22, 2016 1:43PM  1:56PM 
R22.00002: Influence of Electrolyte Concentration on the Aggregation Of Colloidal Particles Near Electrodes in Oscillatory Fields Scott Bukosky, Sukhleen Saini, William Ristenpart Micronscale particles suspended in various aqueous electrolytes have been widely observed to aggregate near electrodes in response to oscillatory electric fields, a phenomenon believed to result from electrically induced flows around the particles. Most work has focused on a narrow range of ionic strengths. Here we demonstrate that an applied field causes micronscale particles in aqueous NaCl to rapidly aggregate over a wide range of ionic strengths, but with significant differences in aggregation morphology. Optical microscopy observations reveal that at higher ionic strengths ($\sim$1 mM) particles arrange as hexagonally closedpacked (HCP) crystals, but at lower ionic strengths ($\sim$0.05 mM) the particles arrange in randomly closedpacked (RCP) structures. We interpret this behavior in terms of two complementary effects: an increased particle diffusivity at lower ionic strengths due to increased particle height over the electrode and the existence of a deep secondary minimum in the particle pair interaction potential at higher ionic strength that traps particles in close proximity to one another. The results suggest that electrically induced crystallization will readily occur only over a narrow range of ionic strengths. [Preview Abstract] 

R22.00003: ABSTRACT WITHDRAWN 
Tuesday, November 22, 2016 2:09PM  2:22PM 
R22.00004: Mathematical analysis of electromigration dispersion fronts Ivan C. Christov It is of interest to understand traveling electromigration wave phenomena, such as \emph{isotachophoretic boundaries}, because of their applications in electrophoretic separation methods. To this end, we construct exact solutions to an unusual nonlinear advectiondiffusion equation arising in the study of TaylorAris (also known as shear) dispersion due to electroosmotic flow during electromigration in a capillary. An exact reduction to a Darboux equation is found under a travelingwave anzats. The equilibria of this ordinary differential equation are analyzed, showing that their stability is determined solely by the (dimensionless) wave speed without regard to any (dimensionless) physical parameters. Integral curves, connecting the appropriate equilibria of the Darboux equation that governs traveling waves, are constructed, which in turn are shown to be asymmetric kink solutions ({\it i.e.}, nonTaylor shocks). Furthermore, it is shown that the governing Darboux equation exhibits \emph{bistability}, which leads to two coexisting nonnegative kink solutions for (dimensionless) wave speeds greater than unity. [Preview Abstract] 
Tuesday, November 22, 2016 2:22PM  2:35PM 
R22.00005: Reconfigurable microfluidic nanoparticle trapping using dielectrophoresis for chemical detection Reza Salemmilani, Brian Piorek, Martin Moskovits, Carl Meinhart We report a microfluidic particle manipulation platform based on dielectrophoresis (DEP) to capture and release nanoscale particles cyclically via reconfigurable traps. DEP is routinely used in microfluidic devices for capturing and trapping cells and particles of various sizes, however the trapping of small nanoparticles by DEP is challenging due to the inverse relationship of the DEP force with particle size. The architecture we describe uses electrically insulating silica beads of micron scale in conjunction with DEP electrodes configured to manipulate nanoscale particles for microfluidic applications such as filtration and chemical detection. [Preview Abstract] 
Tuesday, November 22, 2016 2:35PM  2:48PM 
R22.00006: Fluctuating Hydrodynamics of Electrolytes Solutions JeanPhilippe Peraud, Andy Nonaka, Anuj Chaudhri, John B. Bell, Aleksandar Donev, Alejandro L. Garcia In this work, we develop a numerical method for multicomponent solutions featuring electrolytes, in the context of fluctuating hydrodynamics as modeled by the LandauLifshitz Navier Stokes equations. Starting from a previously developed numerical scheme for multicomponent low Mach number fluctuating hydrodynamics [1], we study the effect of the additional forcing terms induced by charged species. We validate our numerical approach with additional theoretical considerations and with examples involving sodiumchloride solutions, with length scales close to Debye length. In particular, we show how charged species modify the structure factors of the fluctuations, both in equilibrium and nonequilibrium (giant fluctuations) systems, and show that the former is consistent with DebyeHuckel theory. We also discuss the consistency of this approach with the electroneutral approximation in regimes where characteristic length scales are significantly larger than the Debye length. Finally, we use this method to explore a type of electrokinetic instability. [1] A. Donev {\&} al.,"Low Mach Number Fluctuating Hydrodynamics of Multispecies Liquid Mixtures", \textit{Phys. Fluids}, 27, 3, 2015 [Preview Abstract] 
Tuesday, November 22, 2016 2:48PM  3:01PM 
R22.00007: Timedependent electrokinetic flows of nonNewtonian fluids in microchannelarray for energy conversion MyungSuk Chun, Byoungjin Chun, JiYoung Lee We investigate the externally timedependent pulsatile electrokinetic viscous flows by extending the previous simulations concerning the electrokinetic microfluidics for different geometries. The external body force originated from between the nonlinear PoissonBoltzmann field and the flowinduced electric field is employed in the Cauchy momentum equation, and then the NernstPlanck equation in connection with the net current conservation is coupled. Our explicit model allows one to quantify the effects of the oscillating frequency and conductance of the Stern layer, considering the shear thinning effect and the strong electric double layer interaction. This presentation reports the new results regarding the implication of optimum frequency pressure pulsations toward realizing mechanical to electrical energy transfer with high conversion efficiencies. These combined factors for different channel dimension are examined in depth to obtain possible enhancements of streaming current, with taking advantage of pulsating pressure field. From experimental verifications by using electrokinetic power chip, it is concluded that our theoretical framework can serve as a useful basis for micro/nanofluidics design and potential applications to the enhanced energy conversion. [Preview Abstract] 
Tuesday, November 22, 2016 3:01PM  3:14PM 
R22.00008: Deformations of a prestretched elastic membrane driven by nonuniform electroosmotic flow Moran Bercovici, Evgeniy Boyko, Amir Gat We study viscouselastic dynamics of fluid confined between a rigid plate and a prestretched elastic membrane subjected to nonuniform electroosmotic flow, and focus on the case of a finitesize membrane clamped at its boundaries. Considering small deformations of a strongly prestretched membrane, and applying the lubrication approximation for the flow, we derive a linearized leadingorder nonhomogenous 4th order diffusion equation governing the deformation and pressure fields. We derive a timedependent Green's function for a rectangular domain, and use it to obtain several basic solutions for the cases of constant and time varying electric fields. In addition, defining an asymptotic expansion where the small parameter is the ratio of the induced to prescribed tension, we obtain a set of four oneway coupled equations providing a first order correction for the deformation field. [Preview Abstract] 
Tuesday, November 22, 2016 3:14PM  3:27PM 
R22.00009: Flow of PowerLaw Liquids in a HeleShaw Cell Driven by NonUniform Electroosmotic Slip in the Case of Strong Depletion Evgeniy Boyko, Moran Bercovici, Amir Gat We analyze flow of a nonNewtonian fluid in a HeleShaw cell, subjected to spatially nonuniform electroosmotic flow. We specifically focus on powerlaw fluids with wall depletion properties and derive a pPoisson equation governing the pressure field, as well as a set of linearized equations representing its asymptotic approximation for weakly nonNewtonian behavior. To investigate the effect of nonNewtonian properties on the resulting fluidic pressure and velocity, we consider several configurations in one and two dimensions, and calculate both exact and approximate solutions. We show that the asymptotic approximation is in good agreement with exact solutions even for fluids with significant nonNewtonian behavior. The asymptotic model thus enables prediction of the flow and pressure fields for nonNewtonian fluids, and may be particularly useful for the analysis and design of microfluidic systems involving electrokinetic transport of such fluids. [Preview Abstract] 
Tuesday, November 22, 2016 3:27PM  3:40PM 
R22.00010: Novel propulsion of active colloids by selfinduced field gradients with potential for cargo transport Alicia Boymelgreen, Gilad Yossifon, Touvia Miloh Localized electric field gradients, induced by the dual symmetrybreaking of an asymmetric particle adjacent to a wall are shown to potentially drive particle motion, even in a uniform field. Since the driving gradient is induced by the particle itself, we have termed this propulsion mechanism ``self\textbf{}dielectrophoresis'' (sDEP), to distinguish from traditional DEP where the driving nonuniform field is externally fixed and particle direction is restricted. It is also shown that sDEP driven particles are natural cargo carriers, since the localized gradients can also trap and release targets selectively and on demand. This phenomenon is specifically characterized for GoldPolystyrene Janus spheres, including the establishment of a nondimensional parameter marking the critical frequency at which sDEP dominates lowfrequency ICEP evidenced by a reversal in particle direction. Additionally we demonstrate that localized gradients can transform the translating Janus particles into an externally controlled, mobile floating electrode with the ability to collect, transport and release a target sample a target 1/50 of its size. It is also shown that calculated control of the frequency enables selective sorting and transport  if the driving frequency is aligned with the positiveDEP (pDEP) response of a specific ``target'' and negativeDEP (nDEP) of any other contaminants, only the former will be transported with the Janus sphere. [Preview Abstract] 
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