Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session M6: Microfluids: Electrokinetics |
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Chair: Ali Mani, Stanford University Room: 328 |
Tuesday, November 26, 2013 8:00AM - 8:13AM |
M6.00001: High Order WENO Simulation of Electrokinetic Instability in a Cross-Shaped Microchannel Qian Li, Yann Delorme, Steven Frankel Electroosmotic flow with electrokinetic effects is the primary method of fluid handling in micro-total analysis systems. Knowledge of electrokinetic instabilities (EKI) is required to trigger instabilities in applications like low Reynolds number micromixing or to suppress them in applications such as sample injection, separation and controlled diffusion-limited reaction processes where the minimum sample dispersion is needed. A novel multiblock high order in-house solver based on WENO scheme is applied to simulate the EKI for multiple electrolyte solutions with different electric conductivities in a cross-shaped microchannel. 3D simulations are performed to explore the effects of variations of applied electric field, electric field ratio, and conductivity ratios on the EKI phenomena, and to determine the critical value of electric field required for instabilities. The validity of the numerical study is assessed by comparing the numerical results with the experimental data. [Preview Abstract] |
Tuesday, November 26, 2013 8:13AM - 8:26AM |
M6.00002: Electro-osmotic Flow over a Charged Super-hydrophobic Surface Hui Zhao A super-hydrophobic surface has a large effective hydrodynamic slip length compared to a smooth hydrophobic surface and holds the promise of enhancing electrokinetic flows that find many applications in microfluidics. However, recent theoretical studies suggested that electro-osmotic flows over a weakly charged, super-hydrophobic surface can only be enhanced when liquid-gas interfaces are charged. So far there is little work reported when the zeta potential of the surface is comparable or even larger than the thermal potential. Here we numerically investigate electro-osmotic flows over a periodically striped slip-stick surface by solving the standard Poisson-Nernst-Planck equations. Our results indicate that at large zeta potentials, even if liquid-gas interfaces are charged, the non-uniform surface conduction due to the mismatch between surface conductions over no-shear and no-slip regions leads to electric field lines penetrating the double layer and thus the non-uniform surface conduction weakens the tangential component of the electric field which primarily drives electro-osmotic flows. Our results imply that in the presence of strong non-uniform surface conduction, enhanced electro-osmotic flows over a super-hydrophobic surface are possible only in certain conditions. In particular, the enhancement due to the slip can potentially be lost at large zeta potentials. [Preview Abstract] |
Tuesday, November 26, 2013 8:26AM - 8:39AM |
M6.00003: Modification of the local electric field around a sharp corner due to surface conductance Hsien-Hung Wei, David Halpern It is well known that the electric field at the tip of an insulated wedge is singular when solving the two-dimensional Laplace equation for the electric potential. But if a wedge is highly charged to possess strong electric currents along the wedge surface, an imbalance of these currents can produce the so-called surface conductance effects that can either draw the electric field lines into or out of the wedge surface, and hence modify the local electric field behavior around the tip. We find that how an external field is applied is crucial to how surface conductance impacts the corner field, depending on if the applied field cuts around the wedge (cutting mode) or acts symmetrically over the wedge (impinging mode). For each mode, we not only examine how the field around the wedge behaves as the strength of surface conductance varies, but also address whether the singularity at the tip is enhanced/relieved by identifying how the field grows/decreases with distance from the tip. [Preview Abstract] |
Tuesday, November 26, 2013 8:39AM - 8:52AM |
M6.00004: Electrokinetic instability of isotachophoresis shocks Giancarlo Garcia, Juan Santiago, Ali Mani Isotachophoresis (ITP) is an electrokinetic focusing technique used in a variety of life science and analytical chemistry applications. In ITP, an electrokinetic shock wave forms at the interface between leading and trailing electrolytes with relatively high and low conductivities. The ITP interface is self-sharpening, as restoring electromigration fluxes counteract molecular diffusion. However, the large electric field gradient at the shock interface also gives rise to free charge and strong electrostatic body forces. At large applied currents, electrostatic forces cause recirculating flows which destabilize the ITP interface. We performed stability analysis and direct simulation of ITP shocks through numerical solutions to the coupled Nernst-Planck and Navier-Stokes equations using a quasi-electroneutral approximation. In both experiments and numerical simulations, we observe two modes of instability: 1) a distorted ITP interface which is steady in time, and 2) an oscillating perturbation which persists. In addition, at the highest simulated electric fields, we observe transition towards more chaotic oscillatory modes. We use our stability analysis and numerical simulations to characterize instability of ITP shocks using two dimensionless parameters. [Preview Abstract] |
Tuesday, November 26, 2013 8:52AM - 9:05AM |
M6.00005: Electrokinetic instability and hydrodynamic chaos near electrodes Scott M. Davidson, Mathias B. Andersen, Ali Mani It is known that ion-concentration-polarization (ICP) near ion-selective membranes can lead to electrokinetic instability of an aqueous solution. Consistent with experimental observations, recent DNS studies demonstrate these instabilities and even predict hydrodynamic chaos when ICP is subject to high voltage. Through direct numerical simulation (DNS) of the coupled Poisson-Nernst-Planck and Navier-Stokes equations in two dimensions, we demonstrate that this phenomena is not limited to membranes, but is much more general. Our DNS results predict sustained chaotic behavior between blocking parallel electrodes under applied AC forcing and at an ideally polarizable cylinder in a DC electric field. Comparison with asymptotic predictions in the linear, nonlinear, and chaotic regimes is performed as well as analysis of transport effects. [Preview Abstract] |
Tuesday, November 26, 2013 9:05AM - 9:18AM |
M6.00006: Streaming potential and conductivity measurements reveal electrokinetic properties of porous and charged layers Alexander Barbati, Brian Kirby We perform streaming potential, conductivity, and supporting physical and chemical measurements on thin Nafion polymer films spun on rigid glass slides. Our data reveals a phenomenological zeta potential that scales inversely with the negative logarithm of ionic strength (electrolytes NaCl and HCl) and displays a weak, but unexpected, dependence on pH. Using derived coupling coefficients for streaming current and conductivity, we analyze the phenomenological zeta potential to extract porous layer resistance, fixed charge density, and the Donnan potential within the porous layer. We supplement these electrokinetic studies with physical and chemical measurements of the sample, using profilometry, XPS, and ellipsometry measurements to further inform the state of the system. [Preview Abstract] |
Tuesday, November 26, 2013 9:18AM - 9:31AM |
M6.00007: Shear Flow induced Electrical Current Generation Claus-Dieter Ohl, Silvestre Roberto Gonzalez Avila, Chaolong Song, Luong Trung Dung Electro-osmotic flows are driven by an electric potential difference along a channel where the driving force is acting very close to the boundary at the electric double layer (EDL). The charge separation within the EDL gives rise to an electric current. Conversely, one may expect that a strong shear flow can induce an electric current that could be picked up with electrodes and a closed circuit. Previous experiments relied on a steady free jet at a nozzle exit driven by a strong pressure gradient [1]. Here we utilize a laser induced cavitation bubble near an electrode equipped surface to generate strong shear from the impinging jet. Correlation of high-speed recordings of the spreading jet with current measurements reveals that the shear stress is causing the electric current. We make an attempt to calibrate this sensor in a better defined shear flow within a microfluidic channel. \\[4pt] [1] A.M. Duffin and R.J. Saykally, ``Electrokinetic Power Generation from Liquid Water Microjets,'' J. Phys. Chem. C 112, 17018-17022 (2008). [Preview Abstract] |
Tuesday, November 26, 2013 9:31AM - 9:44AM |
M6.00008: Probing electrokinetics in microchannels and nanochannels with electrochemical measurements Jarrod Schiffbauer, Sinwook Park, Gilad Yossifon We present a brief review of recent experimental and theoretical results concerning the use of electrochemical impedance spectroscopy (EIS,) in conjunction with other electrochemical measurements (chronoamperometry, linear sweep voltammetry,) to characterize the response of micro- and nanofluidic systems. Using these techniques, the interplay between conduction, diffusion, and convection are probed across a range of time- and length scales. The resulting information permits characterization of the respective roles of processes in both micro- and nanchannel regions of a fluidic device. Such techniques provide a useful probe of transient behavior at the micro-nanochannel interface, have great potential in biomolecular sensing applications, and may be useful in the study of surface properties at the fluid-solid interface. [Preview Abstract] |
Tuesday, November 26, 2013 9:44AM - 9:57AM |
M6.00009: Geometric Modulation of Electro-Osmosis of the Second Kind in Micro-Nanochannel Interface Devices Gilad Yossifon, Neta Leibowitz, Yoav Green, Jarrod Schiffbauer, Sinwook Park The charge-selective ionic transport through the nanochannel induces a concentration polarization effect. At sufficiently high currents, the depleted region develops an extended space charge layer adjacent to the micro-nanochannel interface. As the applied voltage exceeds a critical threshold, the loss of mechanical stability in this space-charge region leads to the formation of fast fluid vortices which undergo a complex wavelength-selection process. Both microchannel dimensions and interfacial geometry have been shown to affect the onset and subsequent development of the vortex flow field. Here we present results concerning suppression and control of the onset of instability as well as demonstrating competition between different vortex mechanisms. These effects modulate the interfacial mass transport and, hence, ionic current, through the interface and produce observable patterns. These results are of both fundamental and practical interest, with implications regarding early transitions from limiting to over-limiting currents and colloid-hydrodynamic interactions. The practical applications of such effects range from bio-molecular concentration, separation, and detection to micro-purification and on-chip electro-dialysis. [Preview Abstract] |
Tuesday, November 26, 2013 9:57AM - 10:10AM |
M6.00010: Characterization of electrochemical response of a hybrid micro-nanochannel system using computational impedance spectroscopy (CIS) Vishal Nandigana, Narayan Aluru Single molecule/particle sensing using micro/nanochannel integrated systems has attracted tremendous interest in recent years. The molecule in an aqueous ionic solution is translocated from the source microchannel towards the drain microchannel across a nanochannel under the influence of an external electric field. The translocated molecules are characterized from the electrical response of the system. In order to develop an efficient design for accurate characterization of single molecules, it is important to first understand the ion-transport dynamics in these integrated systems. To this end, we develop a computationally efficient area-averaged multi-ion transport model (AAM), considering an ion-selective nanochannel integrated with a microchannel on either side. Further, we study the ion transport dynamics both under equilibrium and non-equilibrium regimes. In each regime, the base state is perturbed with an external harmonic electrical disturbance over a wide range of frequency spectrum and the electrochemical impedance response is computed. We correlate each characteristic frequency present in the system to its corresponding physical phenomena and also characterize the microscopic diffusion boundary layer lengths (DBL) observed in the microchannel. [Preview Abstract] |
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