Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D6: Electrokinetics II |
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Chair: David Salac, SUNY Buffalo Room: 24B |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D6.00001: Multiscale simulation of electroosmotic flows Lin Guo, Mark Robbins, Shiyi Chen, Jin Liu We develop an efficient hybrid multiscale method for simulating nano-scale electroosmotic flow based on spatial ``domain decomposition'' [1]. Molecular dynamics (MD) is used in the near wall region where atomistic details are important. AmultigridParticle-Particle Particle-Mesh (PPPM) method [2] is used to calculate the long-range Coulombic interaction between charged ions. Continuum (incompressible Navier-Stokes) equations for the solvent are solved in the bulk region, reducing the computational cost substantially. A discrete description of ions is retained in the continuum region because of the low density of ions and the long-range of electrostatic interactions. Langevin dynamics is used to model the Brownian motion of these ions in the implicit solvent. The fully atomistic and continuum descriptions are coupled through ``constrained dynamics'' [1] in an overlap region. Flux of charged and solvent particles between continuum and MD regions is included. Simulation results for different channel sizes are provided. To benchmark this multiscale scheme, we compare results with pure MD simulations. \\[4pt] [1] X. B. Nie, S. Y. Chen, W. N. E, and M. O. Robbins, J. Fluid Mech., 500:55--64, 2004.\\[0pt] [2] J. Liu, M. R. Wang, S. Y. Chen, and M. O. Robbins, J. Comput. Phys., 229:7834-7847, 2010. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D6.00002: Colloidal Trapping within a Second Kind Electroosmotic Vortex Pair at a Microchamber-Nanoslot Interface Yoav Green, Gilad Yossifon It has been shown in previous works that due to strong tangential electric fields, electroosmotic vortices of the second kind appear at the interface of a microchamber-narrow nanoslot. Continuity of fluxes causes strong field focusing effects which change the characteristic behavior of the current-voltage curve and traps particles, larger than the nanoslot height, at the nanoslot interface. We solve the steady state Poisson-Nernst-Planck-Stokes equations for a 2D model of a microchamber-narrow nanoslot interface. Afterwards, the particle's dynamic equations of motion including a non-divergence free dielectrophoresis (DEP) force are solved to obtain the particle trajectories. It is demonstrated that due to the short range DEP force particles in the vicinity of the interface are quickly trapped which stands in qualitative agreement with the experimental findings. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D6.00003: Effect of Electro-Osmotic Flow on Energy Conversion on Superhydrophobic Surfaces Seshadri Gowrishankar, Tobias Baier It has been suggested that a superhydrophobic surface, by virtue of the presence of no-shear zones, can greatly enhance the transport of surface charges. This would lead to a considerable increase in the streaming potential, a feature which could find possible applications in micro-energy harvesting devices. Such devices are of promise in micro-fluidic studies in view of their ability to act as effective energy conversion devices on the micro-scale. In our paper, we use a theoretical approach to show that the generation of a streaming potential in such superhydrophobic geometris is significantly limited from that otherwise expected because of the current generated from a reverse electro-osmotic flow. We also show that, for large values of free surface charge densities, the electro-osmotic flow current would engender a saturation in both the power extracted and efficiency of energy conversion that is achievable in such systems. Our analysis therefore indicates that fluids with very low conductivity should be preferred in energy conversion devices. Finally, we extrapolate our results to show that a saturation to both the energy conversion and the efficiency would be obtained in all flow geometries, although the charge density of the free surface at which this happens can vary. [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D6.00004: Electroosmotic Flow of Power-Law Fluids in a Cylindrical Microcapillary M.H. Saidi, Ashkan Babaie, Arman Sadeghi In biological applications where most fluids are considered to be non-Newtonian, Newtonian law of viscosity looks insufficient for describing the flow characteristics. In the present work, the electroosmotic flow of power-law fluids in a circular micro tube is investigated. The Poisson-Boltzmann equation for electrical potential is solved numerically in the complete form without using the Debye-H\"{u}ckel approximation. The physical model includes the Joule heating and viscous dissipation effects. Once the momentum and energy equations are solved numerically, a parametric study is done to investigate the effects of different parameters such as flow behavior index, wall zeta potential and the Debye-H\"{u}ckel parameter on thermal and hydrodynamic characteristics of the flow. Results show that based on the value of viscous dissipation and the Debye-H\"{u}ckel parameter the non-Newtonian characteristics of the flow can lead to significant changes regarding to Newtonian behaviors. The provided results in this study would lead to accurate prediction of temperature of biofluids in Lab-on-a-chip devices which is vital for retaining samples in a healthy condition. [Preview Abstract] |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D6.00005: Measurements of Induced-Charge Electroosmotic Flow Around a Metallic Rod Ali Beskok, Cetin Canpolat A cylindrical gold-coated stainless steel rod was positioned at the center of a straight microchannel connecting two fluid reservoirs on either end. The microchannel was filled with 1 mM KCl containing 0.5 micron diameter carboxylate-modified spherical particles. Induced-charge electro-osmotic (ICEO) flow occurred around the metallic rod under a sinusoidal AC electric field applied using two platinum electrodes. The ICEO flows around the metallic rod were measured using micro particle image velocimetry (micro-PIV) technique as functions of the AC electric field strength and frequency. The present study provides experimental data about ICEO flow in the weakly nonlinear limit of thin double layers, in which, the charging dynamics of the double layer cannot be presented analytically. Flow around the rod is quadrupolar, driving liquid towards the rod along the electric field and forcing it away from the rod in the direction perpendicular to the imposed electric field. The measured ICEO flow velocity is proportional to the square of the electric field strength, and depends on the applied AC frequency. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D6.00006: Nonlinear electrokinetic repulsion effects in combined electroosmotic and Poiseuille flow through microchannels Necmettin Cevheri, Minami Yoda Recent evanescent-wave particle velocimetry studies in electrokinetically driven flow where aqueous solutions are driven by an electric field of magnitude $E$, have shown that the radius $a=O$(0.1-1 $\mu $m) particle tracers suspended in the solution are subject to a wall-normal force that drives particles away from the wall [Kazoe {\&} Yoda, \textit{Langmuir} \textbf{27}:11481]. The magnitude of this force appears to scale as $E^{2}$ and $a^{2}$, albeit over a limited range of $E$ and $a$, suggesting that particles of different sizes will have different average wall-normal positions, and hence sample different velocity distributions in a shear flow. To verify this hypothesis, evanescent-wave particle velocimetry was used to measure near-wall particle distributions and velocities of $a$ = 0.2 $\mu $m and 0.5 $\mu $m particles in the combined electroosmotic and Poiseuille flow of a bidisperse dilute aqueous solution through fused-silica channels about 30 $\mu $m deep for $E$~$<$~45 V/cm and pressure gradients $\Delta p$/$L$~$\le $~1.3Bar/m. To evaluate the whether this nonlinear electrokinetic force can be used separate particles based on their size, near-wall particle distributions for both particle sizes were measured at different streamwise locations in the combined flow. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D6.00007: Electrokinetic investigations of uniformly dissociated polymer films Alexander Barbati, Brian Kirby We execute electrokinetic investigations of a Nafion polymer film attached to a rigid glass substrate. These measurements reveal a film charging mechanism that follows Donnan potential scalings over several decades of hydronium and salt concentration, showing invariance of the film dissociation with respect to both pH and solution ionic strength. Electrokinetic measurements are additionally supplemented by observations characterizing the physical (ellipsometry) and chemical (XPS) film state. Our experimental results are analyzed using analytical and numerical modeling of the volumetrically--charged soft interface to interpret measured fluxes with more familiar quantities such as conductivities, surface potentials, and apparent slip lengths. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D6.00008: Streaming Potential of an Electrolyte in a Microchannel with an Axial Temperature Gradient Mathias Dietzel, Steffen Hardt The effect of a temperature gradient parallel or antiparallel to the main flow direction of a pressure-driven symmetric electrolyte in a slit-microchannel is investigated. Based on the non-isothermal Nernst-Planck equations as well as on the Poisson equation, and under the assumption that the intrinsic Soret coefficient $S^*$ is the same for each ion species, an analytical expression of the electric double layer (EDL) potential is derived. Since the local EDL thickness is found to increase exponentially with temperature, a temperature difference $\delta T$ applied along a channel exhibiting a constant surface zeta potential leads to a corresponding gradient in the EDL thickness. For large pressure differences $\delta p_0$, the non-isothermal streaming potential can be adequately described by the well known isothermal expression if the local modification of the Debye length due to the thermal effect is taken into account. For small channel heights at small driving pressure differences, the streaming potential is seen to be over-predicted (under-predicted) by the (Debye-length corrected) isothermal expression for positive (negative) values of $S^* \delta T/\delta p_0$. With vanishing pressure difference, the steady-state thermoelectric potential of confined electrolytes is derived. [Preview Abstract] |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D6.00009: Ion Altered Fluorescence Imaging (IAFI): A Non-invasive, Visualization Method Which Simultaneously Images Scalar Fields and Quantifies Local Ion Concentration Viktor Shkolnikov, Juan G. Santiago Electrokinetic flows are leveraged for a wide range of microfluidic and lab-on-a-chip systems, and are often used to mix, preconcentrate, and/or separate analytes. Traditionally, temperature, conductivity, electrochemical, and UV absorbance detectors have been used to indirectly estimate analyte concentration profiles in these flows. However, these typically are point detectors and thus do not permit dynamic, full-field visualization of unsteady scalar fields. To address this, we propose a novel visualization and quantitation method we term ion altered fluorescence imaging (IAFI). IAFI leverages florescence quenching or enhancement of electrically neutral dyes by ions. IAFI therefore provides a non-intrusive quantitation of full-field concentration of non-fluorescent ions endogenous to the flow and its application. We demonstrate this method in visualization of two non-linear electrokinetic flows: isotachophoresis (ITP) and electrokinetic instability (EKI) in an electrokinetic focusing flow. We have quantified shock propagation and ion concentrations upstream and downstream of shocks in cationic and anionic ITP. We quantified and visualized chaotic EKI flow, including complex secondary flows and local ion densities as the flow develops downstream. [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D6.00010: Diffusion of molecules along incompressible interfaces due to electric fields Ebrahim Kolahdouz, David Salac The diffusion of insoluble molecules, such as surfactants or lipids, on incompressible interfaces due to electric fields is important in understanding the behavior of vesicles. Here a three-dimensional model is presented to investigate the motion of molecules on an arbitrary curved and incompressible interface in the presence of electric fields and an analytic fluid flow field. The interface is described using the gradient-augmented level set method while the electric field is solved with the immersed interface method. The motion of molecules on the surface are modeled using a set of coupled convection-diffusion equations. These equations are solved using the implicit closest point method. The model and sample results are presented. [Preview Abstract] |
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