Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session MF: Microfluidics: Electrokinetics and Transport |
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Chair: Todd Squires, University of California, Santa Barbara Room: 101F |
Tuesday, November 24, 2009 8:00AM - 8:13AM |
MF.00001: Numerical and experimental study of dispersion dynamics in isotachophoresis Giancarlo Garcia, Moran Bercovici, Juan G. Santiago Isotachophoresis (ITP) is a separation and preconcentration technique used in a variety of life science and analytical chemistry applications. Under ideal ITP conditions, sample ions focus in a narrow interface region (1-10 $\mu $m) between leading and terminating electrolytes. In practice, however, the associated electric field gradients at this interface give rise to non-uniform electroosmotic flow (EOF) and associated strong internal pressure gradients. Conductivity gradients also couple with electric fields to yield electrohydrodynamic body forces. Together, these forces disperse the ITP interface and reduce the sensitivity and resolution of ITP-based assays. Despite its importance in ITP, there has been surprisingly little research into the underlying physical mechanisms of dispersion. We performed a numerical and experimental study of dispersion dynamics in ITP using two-dimensional (axi-symmetric), time-dependent simulations of fluid flow, diffusion, and electromigration. We validated our models with controlled experiments in circular capillaries and used simulations to develop general scaling relationships. We observe localized focusing of the analyte in either near-axis or near-wall regions; and the degree to which these conditions are favored is a strong function of the axial location of the ITP interface. Our goal is to develop an area-averaged model for rapid prediction of the effects of EOF in experiments. [Preview Abstract] |
Tuesday, November 24, 2009 8:13AM - 8:26AM |
MF.00002: Concentration boundary layers in osmotic membrane transport processes Kaare Jensen, Tomas Bohr, Henrik Bruus It has long been recognized, that the osmotic transport characteristics of membranes may be strongly influenced by the presence of unstirred concentration boundary layers adjacent to the membrane [1,2]. Previous experimental as well as theoretical works have focused on the case where the solution on both sides of the membrane remain well-mixed due to an external stirring mechanism. We present a theoretical investigation the effects of concentration boundary layers on the efficiency of osmotic pumping processes in the absence of external stirring i.e. when the stirring is provided by the osmotically generated flow itself. For such systems, we show that no well defined boundary layer thickness exist and that the reduction in concentration can be estimated by a surprisingly simple mathematical relation valid across a wide range of geometries and P\'{e}clet numbers. \\[4pt] [1] T.J.Pedley, Q. Rev. Biophys., 1983, \textbf{16}, 115\\[0pt] [2] K.H.Jensen et al., Lab Chip, 2009, \textbf{9}, 2093 [Preview Abstract] |
Tuesday, November 24, 2009 8:26AM - 8:39AM |
MF.00003: Induced Charge Electrokinetics Over ``Controllably Contaminated'' Surfaces: The Effects of Dielectric Thin Films and Surface Chemistry on Slip Velocity Andrew Pascall, Todd Squires Microfluidics has renewed interest in utilizing electrokinetics (EK) for transporting fluids on small scales, and has subjected EK theories and understanding to new challenges. For example, induced-charge electro-osmosis (ICEO), a non-linear EK effect in which an externally applied AC electric field both induces and drives a layer of charged fluid near an electrically conductive surface, could provide an on-chip means to drive high pressures with low voltage [1]. Experimental data on ICEO and related phenomena have shown that the standard theory consistently overpredicts slip velocities by up to a factor of 1000[2]. Here we present experiments in which we controllably ``contaminate'' the metallic surface with a thin dielectric film or Au-thiol self assembled monolayer, and derive a theory for ICEO that incorporates both dielectric effects and surface chemistry, which both act to decrease the slip velocity relative to a `clean' metal. Data for over a thousand combinations of electric field strength and frequency, electrolyte composition, dielectric thickness and surface chemistry show essentially unprecedented quantitative agreement with our theory. [1] Squires \& Bazant. J. Fluid Mech. 2004 [2] Bazant, et al. arXiv. 0903.4790 [Preview Abstract] |
Tuesday, November 24, 2009 8:39AM - 8:52AM |
MF.00004: Concentration polarization effects in nanochannel induced-charge electro-osmosis Mathias Andersen, Henrik Bruus Concentration polarization (CP) has been observed in a variety of configurations: nanochannels, nanopores, ion-permselective membranes, and embedded conductors. It is believed, but not conclusively proven, that electric double-layer (EDL) overlap plays a significant role in CP. Consequently, further studies of fundamental electrokinetic effects related to CP are needed. We present theoretical and numerical studies of CP effects near an un-biased conductor placed in a straight nanoslit. In our model we combine the full Poisson-Nernst-Planck equations with Helmholtz-Smoluchowski slip, whereby we can construct large computational domains and at the same time resolve the EDL in the vicinity of the conductor. Based on our results we try to identify the basic physical mechanisms that lead to the CP. We show that when subjected to an external bias the axial symmetry is broken by the electro-osmotic flow. This leads to nonlinear interactions between flow, electric potential, and charged chemical species in the induced EDL on the conductor. [Preview Abstract] |
Tuesday, November 24, 2009 8:52AM - 9:05AM |
MF.00005: Transient Current and Fluid Transport in Electrolyte Displacement by Electro-osmotic Flow Hsien-Hung Wei, Szu-Wei Tang, Chien-Hsiang Chang In this work, the displacement between two different conductivity solutions by an electro-osmotic flow in a uniformly charged channel is revisited theoretically in the large Peclet number limit. A conductivity mismatch can induce an additional pressure flow due to unequal electro-osmotic slip velocities in the solutions. And yet, we argue that the notion of uniform displacement can still be valid \textit{locally} in the vicinity of the moving concentration front. We derive a coupled set of equations for the electric current and the displacement distance and obtain an analytical solution for these equations. We find that the displacement can exhibit distinct features, depending on the range of the conductivity ratio. This is demonstrated by examining three limiting scenarios: (i) when the solution conductivities are nearly matched, (ii) the displacement by a very high conductivity solution, and (iii) the use of a very low conductivity solution in advancing the displacement. Our findings provide some insights into the zeta potential measurement using the current monitoring method and sample stacking by electro-osmotic flow with conductivity gradient. Effects of dispersion are also discussed. [Preview Abstract] |
Tuesday, November 24, 2009 9:05AM - 9:18AM |
MF.00006: Charged solute transport and separation in nanochannels with surface roughness Guoqing Hu Newfound attention has been given to solute transport in nanochannels. Because the electric double layer thickness is comparable to characteristic channel dimensions, nanochannels have been used to separate ionic species with a constant charge-to-size ratio (i.e., electrophoretic mobility) that otherwise cannot be separated in electroosmotic or pressure-driven flow along microchannels. Surface roughness is usually inevitable during the fabrication of nanochannels. We develop a numerical model to investigate the transport and separation of charged solutes in nanochannels with hundreds of roughness elements. The solute transport patterns in rough channels are compared with those in smooth channels. The effects of surface roughness on the migration speed and retention (defined as the ratio of the solute speed to the fluid speed, used to characterize the efficiency of solute separation) of various solutes at different electrolyte concentrations are examined. Results indicate that solutes move slower in rough nanochannels than in smooth ones for both pressure-driven and electroosmotic flows. Solute separation can be significantly improved by surface roughness under certain circumstances. [Preview Abstract] |
Tuesday, November 24, 2009 9:18AM - 9:31AM |
MF.00007: Ion Transport in Electric Double Layer Capacitors for Water Desalination Batya Fellman, Evelyn Wang Capacitive deionization is a promising method for efficient water desalination. In this approach, salt water is passed through two polarized electrodes, whereby the salt absorbs onto the electrode surface for removal. We investigated high surface area carbon-based electrode materials, including activated carbon and activated carbon cloth, for capacitive deionization. In a 1 M NaCl electrolyte, the activated carbon cloths with a surface area of 1000-2000 m$^2$/g exhibited specific capacitance values of approximately 40 F/g, which is an order of magnitude lower than that of state-of-the-art aqueous capacitors. We speculate that the discrepancy is related to transport limitations at the electrode-electrolyte interface. Based on these studies, we fabricated new controlled electrode geometries and surface chemistries to enable detailed studies of ion transport in the electric double layer and to understand the effect on charging times and specific capacitance. Experimental techniques, including cyclic voltammetry, chronocoulometry, and impedance spectroscopy, were used. These studies help elucidate transport mechanisms and provide insight into optimal design for effective capacitive deionization. [Preview Abstract] |
Tuesday, November 24, 2009 9:31AM - 9:44AM |
MF.00008: Carbon dioxide and water multiphase flow in microchannels Ruopeng Sun, Thomas Cubaud We experimentally study the microfluidic formation and evolution of CO2 bubbles in distilled water at room temperature. Using silicon/glass microchannels, water and carbon dioxide are mixed in a cross-shaped focusing section. The decreasing length of the produced bubbles dissolving in water is measured along a square microchannel as a function of flow rates and inlet pressures. We calculate the rate of bubble dissolution from high-speed imaging and study the flow transition from elongated to spherical bubbles. We also investigate the micro-flow behavior of carbonated water and the interrelation between bubble nucleation and frictional pressure drop. [Preview Abstract] |
Tuesday, November 24, 2009 9:44AM - 9:57AM |
MF.00009: Diffusion-limited evaporation in microchannels Adam Hoffman, Kenneth Breuer We present measurements regarding the evaporation of a liquid from long microchannels. The channels are approximately 9000 microns long, 100 and 500 microns wide and range in depth from 1 to 22 microns. Both ends are open to the atmosphere. The evaporation of the liquid slug within the channel is measured by optically tracking the recession of the menisci from both ends. Different fluids are tested as well as different surface treatments, and the results are modeled in the context of a simple theory based on the diffusion of the vapor along the long channel. At these scales, the shape of the meniscus and its wetting behavior is shown to have a strong effect on the overall evaporation rates, and we hypothesize that this is due to the role of the transition and absorbed films that precede the visible meniscus. [Preview Abstract] |
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