Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session L3: Electrokinetics: Concentration Polarization, Nanoscale and Porous Media |
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Chair: Carlos Hidrovo, Northeastern University Room: 3004 |
Monday, November 24, 2014 3:35PM - 3:48PM |
L3.00001: Anomalous ion concentration distribution inside ion concentration polarization (ICP) layer by electrodeless measurement Inhee Cho, Wonseok Kim, Hyomin Lee, Junsuk Kim, Gun Yong Sung, Sung Jae Kim An ion concentration profile inside the cathodic side of cation-selective membrane with dc bias has been reported to be a flat with aid of numerical simulation. While rigorous experimental evidences with microelectrode array have supported the flat profile, undesirable effects such as electrode reactions have hindered an accurate measurement. In this work, a microchannel with micro-grooves inside an ion depletion zone (or ICP layer) is employed to capture a vortical electrokinetic flow in the groove. By measuring the speed of the flow, one can convert it into the local ionic concentration, since the local speed of electrokinetic flow is proportional to the local electric field which is inversely proportional to the local ionic concentration. As a result, we can indirectly measure the full ion concentration profile inside ICP layer without any undesirable disturbance and find that the profile is neither flat nor monotonic. Instead, there are peaks and, more importantly, the locations of the peaks strongly depend on the mobility of majority carrier (Li+, Na+ and K+). The samples inside the ICP layer are analyzed by mass spectrometry to confirm the dependency. [Preview Abstract] |
Monday, November 24, 2014 3:48PM - 4:01PM |
L3.00002: Capillary Ion Concentration Polarization for Power-Free Salt Purification Sungmin Park, Yeonsu Jung, Inhee Cho, Ho-Young Kim, Sung Jae Kim In this presentation, we experimentally and theoretically demonstrated the capillary based ion concentration polarization for power-free salt purification system. Traditional ion concentration polarization phenomenon has been studied for a decade for both fundamental nanoscale fluid dynamics and novel engineering applications such as desalination, preconcentration and energy harvesting devices. While the conventional system utilizes an external power source, the system based on capillary ion concentration polarization is capable of perm-selective ion transportation only by capillarity so that the same ion depletion zone can be formed without any external power sources. An ion concentration profile near the nanostructure was tracked using fluorescent probes and analyzed by solving the modified Nernst-Planck equation. As a result, the concentration in the vicinity of the nanostructure was at least 10 times lower than that of bulk electrolyte and thus, the liquid absorbed into the nanostructure had the low concentration. This mechanism can be used for the power free salt purification system which would be significantly useful in underdeveloped and remote area. [Preview Abstract] |
Monday, November 24, 2014 4:01PM - 4:14PM |
L3.00003: Numerical and analytical models of concentration polarization in a microchannel Christoffer P. Nielsen, Henrik Bruus We present a comprehensive analysis of salt transport in microchannels during concentration polarization. We have carried out full numerical simulations of the coupled Poisson--Nernst--Planck--Stokes problem governing the transport and rationalized the behaviour of the system. A surprising discovery is that bulk advection relies heavily on the surface currents, even when these surface currents do not contribute much to the overlimiting current themselves. The numerical simulations are supplemented by analytical results valid in the long channel limit as well as in the limit of negligible surface charge. Notably, by including the effects of diffusion and advection in the diffuse double layers we extend a recently published analytical model of overlimiting current due to surface conduction. [Preview Abstract] |
Monday, November 24, 2014 4:14PM - 4:27PM |
L3.00004: Concentration-Polarization, Electro-Convection and Colloid Dynamics in Microchannel-Nanochannel Interface Devices Gilad Yossifon, Neta Leibowitz, Yoav Green, Uri Liel, Jarrod Schiffbauer, Sinwook Park Understanding concentration-polarization (CP) and electroconvection processes along with colloid dynamics in microchannel-nanochannel/membrane interface devices are of particular interest in the field of micro- and nano-fluidics. Our design consists of a nano-slot/permselective membrane bounded by two micro-chambers, wherein we introduce dispersed colloids. Here we report various curios phenomena occurring in these systems. Among them: dielectrophoretic trapping of colloids at the nanoslot entrance in conjunction with the formation of electro-convective instability induced vortices; accumulation of colloids due field-focusing gradient effects within the diffusion layers; depression of the slope in the Warburg branch of the electrochemical impedance spectrum with increasing dc bias voltage as a result of nanochannel net electro-osmotic flow; suppression of the diffusion layer length via AC electrokinetics and its effect on ion transport; anomalous resistance minimum and unique chronopotentiometric signatures due to non-ideal nanochannel permselectivity. All of these stand as examples that highlight the essential differences between fabricated straight nanoslot and permselective membrane systems. [Preview Abstract] |
Monday, November 24, 2014 4:27PM - 4:40PM |
L3.00005: Concentration Polarization and Electroconvection in a Nanochannel Array System Yoav Green, Sinwook Park, Gilad Yossifon The passage of an electric current through a permselective medium (membranes/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 opposite ends of the medium, i.e. concentration polarization (CP). Here we study experimentally the effects of 3D geometric field focusing on CP in realistic three dimensional and three layers system (i.e. microchannel-permselective medium-microchannel device). Previous analytical solutions were derived under the simplifying assumptions of local-electroneutrality, ideal permselectivity and negligible convection. In particular, we studied the effect of the interchannel spacing of an array of such permselective regions/channels, on the resulting current-voltage curves and concentration profiles, wherein an increased interchannel spacing corresponds to an increased geometric heterogeneity of the permselective medium. Good qualitative agreement is obtained between these theoretical predictions and experimental data obtained for a nanoslot array system with varying interchannel spacing. These results highlight the importance of geometric field focusing, interchannel communication and electro-convection effects on the ion transport in heterogeneous permselective systems. [Preview Abstract] |
Monday, November 24, 2014 4:40PM - 4:53PM |
L3.00006: Computational modeling of electrokinetic transport in random networks of micro-pores and nano-pores Shima Alizadeh, Ali Mani A reduced order model has been developed to study the nonlinear electrokinetic behaviors emerging in the transport of ionic species through micro-scale and nano-scale porous media. In this approach a porous structure is modeled as a network of long and thin pores. By assuming transport equilibrium in the thin dimensions for each pore, a 1D transport equation is developed in the longitudinal direction covering a wide range of conditions including extreme limits of thick and thin electric double layers. This 1D model includes transport via diffusion, electromigration and wide range of advection mechanisms including pressure driven flow, electroosmosis, and diffusion osmosis. The area-averaged equations governing the axial transport from different pores are coupled at the pore intersections using the proper conservation laws. Moreover, an asymptotic treatment has been included in order to remove singularities in the limit of small concentration. The proposed method provides an efficient framework for insightful simulations of porous electrokinetic systems with applications in water desalination and energy storage. [Preview Abstract] |
Monday, November 24, 2014 4:53PM - 5:06PM |
L3.00007: Electroosmotic access resistance of a nanopore Sandip Ghosal, John D. Sherwood, Mao Mao Electroosmotic flow through a nanopore that traverses a dielectric membrane with a fixed surface charge density is considered. In the limit where the surface charge is small and the applied electric field weak, the reciprocal theorem is used to derive an expression for the electroosmotic flux through the pore up to quadratures over the fluid volume. Thus, an ``electroosmotic conductance'' (the fluid flux per unit applied voltage) may be defined in analogy to the corresponding electrical conductance of a hole in an insulating membrane immersed in a uniform conductor. In the limit when the membrane is thick compared to the pore diameter, the usual result for the electroosmotic conductance through long cylindrical channels (which varies inversely as the membrane thickness) is recovered. The electroosmotic conductance is shown to approach a finite value for an infinitely thin membrane: this residual electroosmotic resistance (inverse of conductance) is analogous to the concept of ``access resistance of a pore'' in the corresponding electrical problem. The dependence of the electroosmotic conductance on pore radius, Debye length and membrane thickness is investigated.\\[1ex] Reference: {\it JFM} (2014) {\bf 749}, 167; {\it Langmuir} (in press) [Preview Abstract] |
Monday, November 24, 2014 5:06PM - 5:19PM |
L3.00008: Electroosmotic flow through a cylindrical nanopore in a charged membrane of finite thickness Mao Mao, Sandip Ghosal, John D. Sherwood We present numerical solutions to the coupled Nernst-Planck-Poisson-Stokes equation for electroosmotic flow through a cylindrical nanopore. The pore traverses a dielectric membrane with uniform surface charge. A multi-physics solver that incorporates electrostatics, ionic transport and electroosmotic flow is developed using the OpenFOAM CFD library. In the limit of small surface charge and weak applied electric field, the numerical results of fluid flux agree with theory when the thickness of the pore $h$ is either very small or very large compared to the pore radius $a$. For intermediate $h/a$, our simulation agrees with the composite model of electroosmotic conductance [Sherwood et al. Langmuir (in press)]. When the finite permittivity of the dieletric membrane is taken into account, pairs of toroidal counter rotating eddies appear at the corner of the nanopore that expand to fill the entire pore as the pore radius is decreased. We discuss how the topology of the eddies/stagnation points varies as the aspect ratio of the pore increases. [Preview Abstract] |
Monday, November 24, 2014 5:19PM - 5:32PM |
L3.00009: Evaluating surface electrodes and hydrophobic patches for generating vortices in nanoconfined electroosmotic flows Harvey Zambrano, Marie Fuest, Nicolas Vasquez, A.T. Conlisk, Shaurya Prakash As a silica surface is exposed to an electrolyte, a net charge arise on the solid-liquid interface. In a confined electrolyte, a consequence of the net charged interface is the development of an imbalance of ions near the confining walls. The net charged region near the walls is called the Electrical Double Layer (EDL). A critical technology for the next generation of nanodevices, such as lab on a chip and electroosmotic pumps is controlling the EDL structure. Furthermore, important technical processes such as desalination using membranes could be improved by mitigating the concentration polarization, a phenomenon directly related to the EDL. Here, we study the generation of interfacial vortices in nanoconfined electroosmotic flows. We conduct molecular dynamics simulations of a multivalent electrolyte solution in a slit silica nanochannel. We apply axial electric fields and evaluate the response of the system as a counter charged patch is placed on the channel wall. Moreover, we study an alternative method for generating vorticity by employing hydrophobic surface patches. Charge, density and flow velocity profiles are computed. The profiles reveal that both types of patches are able to generate counter flow in electroosmotic devices. We compared the results against experiments. [Preview Abstract] |
Monday, November 24, 2014 5:32PM - 5:45PM |
L3.00010: ABSTRACT WITHDRAWN |
Monday, November 24, 2014 5:45PM - 5:58PM |
L3.00011: Transport and electrochemistry based characterization of porous electrodes for CDI applications and comparison with desalination performance Carlos Rios Perez, Ellen Wilkes, Luis Guitierrez, Carlos Hidrovo Development of carbon-based materials with high specific surface area at the end of last century has made researchers to look back at capacitive deionization as a potential desalination technique for brackish water. Several publications evaluate the adsorption capacity of electrode materials under different conditions. Many others present the development/characterization of new electrode materials using electrochemical analysis and other techniques. Although some work has been done to model the electro-adsorption process at the macro and micro-scale, there is still a gap to tie the characterization of the electrodes to their performance. Here a simplified one-dimensional model is used to estimate the characteristic net electro-adsorption velocities for fully-developed or developing regimes in a flow-by capacitive deionization system. This methodology is applied to three commercially available materials with very distinct structure topology to estimate electromigration velocities at a specific solution flow rate. The calculated electro-adsorption rates and other characterization parameters obtained using traditional electrochemical techniques were compared against important desalination performance parameters such as amount of salt adsorbed and desalination proficiency (amount of salt adsorbed per unit of energy). The results obtained show interesting correlations and sometimes-unexpected behavior under constant current and constant voltage operation. [Preview Abstract] |
Monday, November 24, 2014 5:58PM - 6:11PM |
L3.00012: ABSTRACT WITHDRAWN |
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