Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G14: Electrokinetics: Nanochannels, Surface Conduction, Concentration Polarization |
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Chair: Jarrod Schiffbauer, Technion - Israel Institute of Technology Room: 202 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G14.00001: Resolving Overlimiting Current Mechanisms in Microchannel-Nanochannel Interface Devices Gilad Yossifon, Neta Leibowitz, Uri Liel, Jarrod Schiffbauer, Sinwook Park We present results demonstrating the space charge-mediated transition between classical, diffusion-limited current and surface-conduction dominant over-limiting currents in a shallow micro-nanochannel device. The extended space charge layer develops at the depleted micro-nanochannel entrance at high current and is correlated with a distinctive maximum in the dc resistance. Experimental results for a shallow surface-conduction dominated system are compared with theoretical models, allowing estimates of the effective surface charge at high voltage to be obtained. Further, we extend the study to microchannels of moderate to large depths where the role of various electro-convection mechanisms becomes dominant. In particular, electro-osmotic of the second kind and electro-osmotic instability (EOI) which competes each other at geometrically heterogeneous (e.g. undulated nanoslot interface, array of nanoslots) nanoslot devices. Also, these effects are also shown to be strongly modulated by the non-ideal permselectivity of the nanochannel. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G14.00002: Chronopotentiometric response of non-ideal ion selective microchannel-nanochannel interface device Neta Leibowitz, Jarrod Schiffbauer, Sinwook Park, Gilad Yossifon The passage of an electric current through an ion permselective medium (e.g. membranes and nanochannels) under an applied electric field is characterized by the formation of ionic concentration gradients which result in regions of depleted and enriched ionic concentration at its opposite ends, i.e. concentration polarization. As a result, it can be shown that the chronopotentiometric response of an ideal permselective interface (e.g. permselective membranes) is a monotonic function of the voltage with time regardless of the current intensity. In contrast, a microchannel-nanochannel interface device exhibits a non-monotonic chronopotentiometric response for overlimiting currents. This is shown both numerically and experimentally to result from the non-ideal ion permselectivity of the fabricated nanochannels. This is further supported using experimental visualization techniques that indicate the existence of concentration-polarization within the nanochannel itself and not only within the microchannels. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G14.00003: Electrokinetic Transport in Nanochannels Grafted with Polyelectrolyte Brushes with End-Charging Siddhartha Das, Guang Chen Electrokinetic transport in nanochannels grafted with polyelectrolyte (PE) brushes is important for applications such as ion transport, ion manipulation, flow valving, etc. We discuss here a semi-analytical mean field theory approach to study electrokinetic transport in nanochannels grafted with polyelectrolyte brushes with end-charging. The model first probes the thermodynamics and the electrostatics of the PE brushes by appropriately accounting for the entropic (elastic), excluded volume, and electrostatic effects. The resulting knowledge on the electrostatic potential and the PE configuration is next used to obtain the electroosmotic transport. Results demonstrate the role of surface charges (at the end of the PE brushes) in modifying (shrinking or swelling) the brush height. This, in turn, alters the electroosmotic body force and the PE brush layer induced drag force on the fluid flow; therefore, the flow field eventually evolves from a non-trivial interplay of the PE electrostatic, entropic, and excluded volume effects. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G14.00004: Ion Transport in 2-D Graphene Nanochannels Quan Xie, Elbert Foo, Chuanhua Duan Graphene membranes have recently attracted wide attention due to its great potential in water desalination and selective molecular sieving. Further developments of these membranes, including enhancing their mass transport rate and/or molecular selectivity, rely on the understanding of fundamental transport mechanisms through graphene membranes, which has not been studied experimentally before due to fabrication and measurement difficulties. Herein we report the fabrication of the basic constituent of graphene membranes, i.e. 2-D single graphene nanochannels (GNCs) and the study of ion transport in these channels. A modified bonding technique was developed to form GNCs with well-defined geometry and uniform channel height. Ion transport in such GNCs was studied using DC conductance measurement. Our preliminary results showed that the ion transport in GNCs is still governed by surface charge at low concentrations (10$^{\mathrm{-6}}$M to 10$^{\mathrm{-4}}$M). However, GNCs exhibits much higher ionic conductances than silica nanochannels with the same geometries in the surface-charge-governed regime. This conductance enhancement can be attributed to the pre-accumulation of charges on graphene surfaces. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G14.00005: Ion transport and rectification in a charged nanoscale cone Fan Yang, Li Zhang, Qian Mao, Howard Stone The possibility of rectification for ion transport in nanofluidic systems offers a potential route for developing a nanofluidic diode that mimics a semiconductor diode or captures some features of a biological ion channel. The rectification phenomenon, in which a solution would be enriched in one ion, results from asymmetric effects in ionic transport that can be realized by discontinuities in surface charge, concentration differences across a pore, or an asymmetric pore shape such as a cone. In this paper, we focus on the latter two effects and seek to capture the rectification effect in simple terms with a non-dimensional model representative of the many systems studied to date. Specifically, we analyze the rectification phenomenon in a charged nanoscale cone with a concentration difference and/or an electrical potential difference across the pore. Based on the Poisson-Nernst-Planck model and the assumption of one-dimensional transport, we derive a model based on two coupled ordinary differential equations to determine significant parameters such as ionic current. We identify several dimensionless parameters that have not been recognized previously and study the influence of the dimensionless parameters on the rectification. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G14.00006: Geometric effects on electrocapillarity in nanochannels Jung A. Lee, In Seok Kang Electrocapillarity phenomenon at the electrified surfaces due to an external voltage or surface charge has been regarded as an efficient tool in micro/nanofluidics. Especially in nanochannels, high surface area with small fluid volume makes the problem more attractable. However, the overlapped electric double layer (EDL) should be carefully considered. In this study, the effects of nanochannel geometry on the electrocapillarity have been studied. Poisson-Boltzmann (PB) equation is solved to get the electric potential distribution of electrolyte solution. Total stress exerted on the gas-liquid interface is expressed by the sum of electric stress from Maxwell stress tensor and the osmotic pressure due to the ionic concentration. The average value of this total stress can be regarded as the measure of electrocapillarity. In the present work, nanochannels with various cross sectional shapes are considered. Using the linearized PB equation, analytic solution for the circular cross-sectional case is obtained and this solution is compared with other cross-sectional cases with the same hydraulic diameter. Several equilateral polygon cases are also analyzed numerically and the results can be unified if they are represented in terms of hydraulic diameter. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G14.00007: Experimental study on polydisperse nanochannel system with dispersity and voltage variation Longnan Li, Daejoong Kim Ion exchange membrane (IEM) has great potential for the biological, chemical, energy and desalination applications. Generally, IEM is fabricated by the polymer material and it has non-uniform size of nanopore (nanochannel) matrix structure. We can explain this kind of nanopore non-uniformity by the dispersity of different size of nanopores. The property of IEM strongly depends on nanopore dispersity as the degree of electric double lalyer (EDL) overlap in the nanopore is depend on nanopore dimension. In this study, polydisperse nanochannel array was fabricated on the silicon wafer to model realistic IEM. To investigate ion transporting behavior through polydisperse nanochanel array, concentration polarization (CP) phenomena is examined. To quantitatively show the CP phenomena in the polydisperse nanochannel system, dispesity of nanochannels and applied voltage are examined as a variable. The experiment result shows that the high dispersity nanochannel system (even with 50{\%} dispersity) still show typical CP behavior that depletion zone at the anodic side of nanochannel. For the voltage-current characteristics of polydisperse nanochannel system, the mononanochannel (50 nm) system and lower dispersity (12.5{\%}) system show typical behavior of CP process. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G14.00008: Modelling nanofluidic field amplified sample stacking with inhomogeneous surface charge Christopher McCallum, Sumita Pennathur Nanofluidic technology has exceptional applications as a platform for biological sample preconcentration, which will allow for an effective electronic detection method of low concentration analytes. One such preconcentration method is field amplified sample stacking, a capillary electrophoresis technique that utilizes large concentration differences to generate high electric field gradients, causing the sample of interest to form a narrow, concentrated band. Field amplified sample stacking has been shown to work well at the microscale, with models and experiments confirming expected behavior [1]. However, nanofluidics allows for further concentration enhancement due to focusing of the sample ions toward the channel center by the electric double layer [2]. We have developed a two-dimensional model that can be used for both micro- and nanofluidics, fully accounting for the electric double layer. This model has been used to investigate even more complex physics such as the role of inhomogeneous surface charge. \\[4pt] [1] Bharadwaj, R., {\&} Santiago, J. G. (2005). \textit{J. of Fluid Mech.}, \textit{543}, 57--92\\[0pt] [2] Sustarich, J. M., Storey, B. D., {\&} Pennathur, S. (2010). \textit{Phys. of Fluids}, \textit{22}(11), 112003 [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G14.00009: The dominant role of surface conduction in electro-osmotic flows through periodically varying narrow channels Lotan Ludar, Ehud Yariv As surface conduction has no effect on electro-osmosis in uniform channel flows, where the tangential Debye-layer currents are longitudinally uniform, it may appear as it would only result in a small modifying correction in lubrication analyses of slowly varying channels. This misconception is refuted here by analyzing flows through periodic channels of slowly varying but otherwise arbitrary geometry. Assuming that the channel width is still large compared with the Debye thickness we employ the simplest thin-double-layer model which incorporates surface conduction. We find that surface conduction affects the leading-order flow and the consequent net volumetric flux, introducing a nonlinear dependence upon the zeta potential. Remarkably, as the channel becomes more and more narrow, the scaled flux approaches a limit which is independent of the Dukhin number yet different from that calculated for zero Dukhin number. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G14.00010: Primary, Secondary and Tertiary Vortex Formation in the Ion Concentration Polarization Francisco J. Diez, Srinivas Hanasoge The experimentally observed formation of multiple micro-vortices in the ion concentration polarization region (ICP) is presented. This is attributed to non-uniform electrokinetic phenomenon effects in the ICP such as the local increase in the electric field due to the change in the electrolyte concentration. Experimentally, the ICP is induced by a patterned nanoporous self-assembling membrane integrated inside a single microchannel. Bottom view images of the channel in the depletion region reveals the to-and-fro motion of micro particles which are a projection of a primary vortex. Side view images of the channel reveal the existence of not one, but a series of three vortices all rotating in the same direction and decreasing in size. We propose a model that predicts the formation of these vortices. It shows how the field amplification together with a 2-Dimensinally varying concentration profile is responsible for these multiple vortices. [Preview Abstract] |
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