Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W8: Focus Session: Nonlinear Electrokinetics |
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Sponsoring Units: GSNP DFD Chair: Martin Bazant, Massachusetts Institute of Technology Room: Baltimore Convention Center 314 |
Thursday, March 16, 2006 2:30PM - 3:06PM |
W8.00001: Electro-Convective and Non-Equilibrium Electro-Osmotic Instability of Electric Conduction from an Electrolyte Solution into a Charge Selective Solid Invited Speaker: Electro-convection is reviewed as a mechanism of mixing in the diffusion layer of a strong electrolyte adjacent to a charge-selective solid, such as an ion exchange (electrodialysis) membrane or an electrode. Two types of electro-convection in strong electrolytes may be distinguished: bulk electro-convection , due to the action of the electric field upon the residual space charge of a quasi-electro-neutral bulk solution, and convection induced by electro-osmotic slip, due to electric forces acting in the thin electric double layer of either quasi-equilibrium or non-equilibrium type near the solid/liquid interface. According to recent studies, the latter appears to be the likely source of mixing in the diffusion layer, leading to `over-limiting' conductance in electrodialysis. Electro-convection near a uniform charge selective solid/liquid interface sets on as a result of hydrodynamic instability of one-dimensional steady state electric conduction through such an interface. We discuss instabilities of this kind appearing in the full electro-convective and limiting non-equilibrium electro-osmotic formulations. The short- and long-wave aspects of these instabilities are discussed along with the wave-number selection principles and possible sources of low frequency excess electric noise experimentally observed in these systems. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W8.00002: Nonlinear Surface Transport in the Thin Double-Layer Limit Kevin Chu, Martin Bazant At high applied electric fields, ionic transport within the double layer plays a significant role in the overall response of electrokinetic systems. It is well-known that surface transport processes, including surface electromigration, surface diffusion and surface advection, may impact the strength of electrokinetic phenomena by affecting both the zeta-potential and the magnitude of the tangential electric field. Therefore, it is important to include these effects when formulating the effective boundary conditions for the equations that govern electrokinetic flow outside of the double layer. In this talk, we discuss the application of a general formulation of ``surface conservation laws'' for diffuse boundary layers to derive effective boundary conditions that capture the physics of electrokinetic surface transport. Previous analyses (\textit{e.g.} Deryagin \& Dukhin 1969) are only valid for weak applied fields and are based on a linearization of the concentration and potential about a reference solution, but our results are fully nonlinear and hold at large applied fields as long as the double layer is sufficiently thin. We compare our nonlinear surface transport theory with existing linear analogues and apply it to the canonical problem of induced-charge electro-osmosis around a metal sphere or cylinder in a strong DC field. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W8.00003: Experiments on Nonlinear Electrokinetic Pumps in Microfluidics John Paul Urbanski, Todd Thorsen, Jeremy A. Levitan, Martin Z. Bazant Nonlinear electrokinetic pumps are attractive in the development of portable and flexible microfluidic analysis systems, since they operate without moving parts using low (battery powered) alternating potentials. Since the discovery of AC electro-osmosis (ACEO) in the late 1990s, there has been much work in designing and building two-dimensional, periodic micro-electrode geometries, which exploit broken symmetry to rectify AC forcing and produce steaming flow over a surface. Building on this work, we exploit more general principles of induced-charge electro-osmosis (ICEO) in three-dimensional electrode geometries to enhance pumping in microfluidic devices. Our fabrication efforts are guided by theoretical analysis and simulations using the standard low-voltage theory, which, in some cases, predict flow rates faster than existing planar ACEO pumps by an order of magnitude (for the same voltage and feature size). We test various microfabricated pump geometries in a microfluidic loop following the methodology of Studer et al (2004). We are also measuring the strong effect of solution chemistry (e.g. ion valence and concentration) on ICEO flow to guide further developments in the theory of nonlinear electrokinetics. [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W8.00004: Microfluidic pumps based on Induced Charge Electroosmosis and flow Field Effect Transistor phenomena Gaurav Soni, Todd Squires, Carl Meinhart We are developing AC electrokinetic micropumps to drive electrically conductive biological fluids in microchannels. These pumps work on the principles of Induced Charge Electroosmosis (ICEO) and flow Field Effect Transistor (flowFET) phenomena. AC (as well as DC) electric fields can induce electrical double layer on a polarizable surface and in turn, cause fluid motion by moving this layer. However, when the polarizable surface is electrically floating, symmetric vortices are observed on the surface. Symmetry of this flow leads to zero net pumping. In order to achieve pumping, we apply a second AC signal to the polarizable surface. When the magnitude of this second AC signal is different from the floating potential of the surface, unidirectional flow is observed i.e., pumping is achieved by modulating the induced zeta potential of the surface. Pumping caused by modulation of zeta potential is also known as flowFET phenomenon and has been shown to work with DC electric fields by other community members. We show a detailed study of flowFET with AC electric fields. [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W8.00005: Nonlinear studies of AC electrokinetic micropumps Henrik Bruus, Laurits H. Olesen, Armand Ajdari Recent experiments have demonstrated that AC electrokinetic micropumps permit integrable, local, and fast pumping (velocities $\sim$~mm/s) with low driving voltage of a few volts only. However, they also displayed many quantitative and qualitative discrepancies with existing theories. We therefore extend the latter theories to account for three experimentally relevant effects: $(i)$ vertical confinement of the pumping channel, $(ii)$ Faradaic currents from electrochemical reactions at the electrodes, and $(iii)$ nonlinear surface capacitance of the Debye layer. We report here that these effects indeed affect the pump performance in a way that we can rationalize by physical arguments. [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W8.00006: Electric field driven motion of flexible polyelectrolytes Tak Shing Lo, Boris Khusid, Andreas Acrivos, Joel Koplik Our work aims to study dielectrophoresis of biomolecules in micro/nano-fluidics using molecular dynamics (MD) simulations. Our model combines electrohydrodynamics with molecular theories for the macromolecule polarization caused by the distortion of the counterion cloud. Unlike most available MD studies of polyelectrolytes, solvent atoms are explicitly represented in our model, so that hydrodynamic interactions are included naturally with no ad hoc assumption. The polyelectrolyte is modeled as a negatively charged bead-spring chain. The charges interact through the Coulomb potential and other molecular interactions are included via Lennard-Jones potentials. We study the transport properties of flexible polyelectrolytes suspended in a solvent, with or without added salt, under the action of DC or AC electric fields. MD data provide the information needed to compute the dipole moments of the molecule and the surrounding double layer, which are required for understanding the dielectrophoretic behavior of these molecules in nanoscopic channels. [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W8.00007: Anomalous ionic transport at nanometer-scale electrodes Diego Krapf, Bernadette Quinn , Meng-Yue Wu, Henny Zandbergen, Cees Dekker, Serge Lemay We probe the transport of charged species in high ionic strength solutions on the scale of a few nanometers by monitoring electrochemical reactions at correspondingly sized electrodes. The electrical current through the nanoelectrode provides a direct measure of the flux of ions across the diffuse double layer. Because both the concentration gradient and the electric field are localized in the immediate vicinity of the nanoelectrode, this provides very local information. Furthermore, when the electrode dimensions are in the order of a few nanometers, very steep concentration gradients are achieved. To carry out these experiments, we have developed a method for fabricating nanoelectrodes with a well-defined size and geometry. A pore is first drilled in an insulating membrane with a focused electron beam and it is then filled with a noble metal yielding conically shaped, convex electrodes with radii as small as 2 nm. We observe pronounced non-linearities of ion flux versus concentration when transport is localized within a region smaller than 10 nm in size. We numerically calculate the predicted ion flux using the Stern-Poisson-Nernst-Planck formalism. We show that our observations cannot be explained using this widely applied mean-field description of ionic transport. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W8.00008: Nonlinear Electroosmosis and Biomolecule Electrokinetic Trapping Induced by Ion Selective Nanofluidic Channels Ying-Chih Wang, Jongyoon Han This paper describes a nanofluidic device that can concentrate dilute biomolecule by electrokinetic trapping mechanism. This device has nanofluidic channels with a depth down to 40 nm, therefore, having significant Debye layer overlap. Depending on the strength of the applied potential across the nanochannel, one can observe phenomena such as concentration polarization; charge depletion and nonlinear electrokinetic flow in the adjacent microfluidic channel using fluorescent microscopy. By manipulating the electric field, the device can generate an extended space charge region, maintained for several hours, within a microchannel as a mean to collect and trap biomolecules. Our studies demonstrate such device can achieve up to 10 million fold sample preconcentration within 30 minutes. Besides, if applied a higher potential, a much faster chaotic flow can be seen in the microchannel adjacent to nanochannels. This kind of nonlinear electrokinetic flow is often called the electroosmosis of the second kind or induced-charge electroosmosis in electrode and ion exchange membrane studies. The presented device can be used as either a preconcentrator or an injector to other separation and detection systems preferred its performance and integrabilty. Also, it is an ideal experimental platform for studying such nonlinear electrokinetic effects, by directly tracking molecules \textit{in situ}. [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W8.00009: Modeling of Selectively Permeale Vesicle Membrane in Electrolytes: An Energetic Variational Formulation Chun Liu, Rolf Ryham We introduce a self consistent coupled system to model the deformation of selectively permeable vesicle membranes in electrolytes via an energetic variational formulation. The equations governing a diffuse charge system and the evolution of the vesicle membrane are coupled with the momentum equations of the electrolyte through the Lorentz force along with the induced elastic forces due to the membrane bending energy. The force coupling and charge flux selectivity are consequences of the energetic balance and competition of kinetic, electric and membrane elastic bending energy. We will also present some numerical simulation results of the system. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W8.00010: Reversible Transitions on Electrically-Tunable Nanostructured Superhydrophobic Surfaces Tom Krupenkin, J. Ashley Taylor, Paul Kolodner, Marc Hodes, Joanna Aizenberg Recently demonstrated electrically tunable nanostructured superhydrophobic surfaces provide a promising new way of manipulating liquids at both micro and macro scales. Dynamic control over the interaction of liquids with the solid substrate is of great interest to many research areas ranging from biology and chemistry to physics and nanotechnology. In this work the reversibility of the electrically induced superhydrophobic -- hydrophilic transition on nanostructured surfaces is addressed. Recently demonstrated approach based on momentarily induction of film boiling in a very thin layer of liquid adjacent to the solid-liquid interface is discussed. The dependence of the hydrophilic -- superhydrophobic transition on the topography of the nanostructured layer, as well as on the energy and duration of the electrical pulse is investigated. Several emerging applications of these surfaces, including lab-on-a-chip, chemical microreactor, and on-chip power sources are discussed. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W8.00011: Nonlinear alternating current susceptibilities of rotating microparticles in electrorheological fluids Kin Wah Yu, J.P. Huang, W.J. Tian A perturbation approach [1] has been employed to investigate the nonlinear alternating current (AC) responses of the rotating microparticles in electrorheological (ER) fluids under AC or direct current electric fields. The shear flow of ER fluids exerts a torque on the particles and leads to the rotational motion of the particles about their centers [2]. We show that the dynamic effects can play a significant role in the AC responses. Our results can be conveniently interpreted in the dielectric dispersion spectral representation [3], thus offering a convenient method to determine the relaxation time and the rotation velocity of the ER particles by measuring the nonlinear AC responses. \newline \newline [1] G. Q. Gu and K. W. Yu, Phys. Rev. B 46, 4502 (1992); K. W. Yu, P. M. Hui, and D. Stroud, Phys. Rev. B 47, 14150 (1993). \newline [2] Jones T. K. Wan, K. W. Yu, and G. Q. Gu, Phys. Rev. E 62, 6846 (2000). \newline [3] Jun Lei, Jones T. K. Wan, K. W. Yu, and Hong Sun, Phys. Rev. E 64, 012903 (2001). [Preview Abstract] |
Thursday, March 16, 2006 5:06PM - 5:18PM |
W8.00012: Electrophoresis in bounded doamins Ehud Yariv Electrophoretic motion is usually described by a linear model, based upon the smallness of the applied field relative to the equilibrium field in the screening Debye layer surrounding the particle. This model, in turn, leads to the Smoluchowski's slip condition and eventually results in mobility relations. The mobility concept is valid provided the external electric force is neglected --- a procedure superficially supported by the net electric neutrality of the combined particle-layer system. In this talk, however, I will show that this force does not vanish in general. Accordingly, a consistent scheme is formulated for analyzing the nonlinear motion of a particle in an applied field. This motion is illustrated in two contexts: rotation of non-spherical particles, and drift of a spherical particle away from a planar wall. I will also describe an analogy to incompressible and inviscid fluid motion. This analogy enables for experimental verification of three-dimensional potential flow solutions. [Preview Abstract] |
Thursday, March 16, 2006 5:18PM - 5:30PM |
W8.00013: How accurate is the Poisson-Boltzmann theory for monovalent ions near highly charged interfaces? Wei Bu, Alex Travesset, David Vaknin Monovalent ion distributions next to highly charged interfaces were determined by synchrotron surface X-ray sensitive techniques. A lipid phosphate (dihexadecyl hydrogen-phosphate) was spread as a monolayer at the air-water interface, containing CsI at various concentrations. Using anomalous reflectivity off and at the $L_3$ Cs$^+$ resonance, we provide, for the first time, spatial counterion distributions (Cs$^+$) next to the negatively charged interface over a wide range of ionic concentrations. We argue that at low salt concentrations and for pure water the enhanced concentration of hydroniums H$_3 $O$^+$ at the interface leads to proton-transfer back to the phosphate group by a high contact-potential, whereas high salt concentrations lower the contact-potential resulting in proton- release and increased surface charge-density. The experimental ionic distributions are in excellent agreement with a renormalized-surface-charge Poisson-Boltzmann theory without fitting parameters or additional assumptions. [Preview Abstract] |
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