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 A10: Microscale Flows: Electrokinetics |
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Chair: Hui Zhao, University of Nevada Las Vegas Room: 110 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A10.00001: Energy Conversion over Super-hydrophobic Surfaces Hui Zhao, Shengjie Zhai The streaming potential generated by a pressure-driven flow over a charged slip-stick surface with an arbitrary double layer thickness is both theoretically and experimentally studied. To understand the impact of the slip, the streaming potential is compared against that over a homogenously charged smooth surface. Our results indicate that the streaming potential over a super-hydrophobic surface only can be enhanced under certain conditions. In addition, the Onsager relation which directly relates the magnitude of electro-osmotic effect to that of the streaming current effect has been explicitly proved to be valid for thin and thick double layers and homogeneously charged super-hydrophobic surfaces. Comparisons between the streaming current and electro-osmotic mobility for an arbitrary electric double layer thickness under various conditions indicate that the Onsager relation seems applicable for arbitrary weakly charged super-hydrophobic surfaces though there is no general proof. Knowledge of the streaming potential over a slip-stick surface can provide guidance for designing novel and efficient microfluidic energy-conversion devices using super-hydrophobic surfaces. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A10.00002: Numerical Optimization Strategy for Determining 3D Flow Fields in Microfluidics Alex Eden, Marin Sigurdson, Igor Mezic, Carl Meinhart We present a hybrid experimental-numerical method for generating 3D flow fields from 2D PIV experimental data. An optimization algorithm is applied to a theory-based simulation of an alternating current electrothermal (ACET) micromixer in conjunction with 2D PIV data to generate an improved representation of 3D steady state flow conditions. These results can be used to investigate mixing phenomena. Experimental conditions were simulated using COMSOL Multiphysics to solve the temperature and velocity fields, as well as the quasi-static electric fields. The governing equations were based on a theoretical model for ac electrothermal flows. A Nelder-Mead optimization algorithm was used to achieve a better fit by minimizing the error between 2D PIV experimental velocity data and numerical simulation results at the measurement plane. By applying this hybrid method, the normalized RMS velocity error between the simulation and experimental results was reduced by more than an order of magnitude. The optimization algorithm altered 3D fluid circulation patterns considerably, providing a more accurate representation of the 3D experimental flow field. This method can be generalized to a wide variety of flow problems. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A10.00003: Electroconvection near the interface of ion-selective membranes and a microchannel Karen Wang, Ali Mani The transport dynamics of electroconvective flows near ion-selective membranes subject to charged sidewalls are studied using a direct numerical simulation of the coupled Poisson-Nernst-Planck and Navier-Stokes equations. Previous studies have investigated electroconvective instability near infinitely large flat membranes and have demonstrated their role in significant enhancement of transport via added advection effects. This study demonstrates how the presence of sidewalls from a connecting microchannel can affect the onset of electroconvective flows and also impact the net ion transport rate. We demonstrate that sidewalls without charge stabilize the flow and delay the onset of electroconvection while walls with properly signed charges can induce flow and lead to enhancement of transport. Impact of the sidewalls in energy and throughput efficiency will also be discussed. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A10.00004: Simultaneous Aggregation and Height Bifurcation of Colloidal Particles near Electrodes in Oscillatory Electric Fields Scott Bukosky, William Ristenpart The behavior of micron scale colloidal particles near electrodes in oscillatory electric fields is known to be sensitive to the identity of the surrounding electrolyte. For example, micron-scale particles suspended in 1 mM NaCl aggregate laterally near the electrode upon application of a low-frequency ($\sim100$ Hz) field, but the same particles suspended in NaOH are instead observed to laterally separate. Recent work has revealed that, contrary to previous reports, particles suspended in NaOH indeed aggregate under some conditions while simultaneously exhibiting a distinct bifurcation in average height above the electrode. Here, we elaborate on this observation by demonstrating the existence of a critical frequency ($\sim25$ Hz) below which particles in NaOH aggregate laterally and above which the same particles separate. At sufficiently low frequencies, particles still exhibit a distinct height bifurcation, but those particles immediately adjacent to the electrode surface also move laterally to form aggregates. These results indicate that the current demarcation of electrolytes as either “aggregating” or “separating” is misleading, and that the key role of the electrolyte instead is to set the magnitude of a critical frequency at which particles transition between behaviors. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A10.00005: 2D Flow patterning in Hele-Shaw configurations using Non-Uniform Electroosmotic Slip Evgeniy Boyko, Shimon Rubin, Amir Gat, Moran Bercovici We present an analytical study, validated by numerical simulations, of electroosmotic flow in a Hele-Shaw configuration with non-uniform zeta potential distribution. Applying the lubrication approximation and assuming thin electric double layer, we derive a pair of uncoupled Poisson equations for the pressure and the stream function, and show that the inhomogeneous parts in these equations are governed by gradients in zeta potential parallel and perpendicular to the applied electric field, respectively. We obtain a solution for the case of a disk with uniform zeta potential and show that the flow field created is an exact dipole, even in the immediate vicinity of the disk. We then illustrate the ability to generate complex flow fields using superposition of such disks. Furthermore, we study the inverse problem in which we define the desired flow pattern and solve for the zeta potential distribution required in order to establish it. We demonstrate that such inverse problem solutions can be used to create directional flows confined within narrow regions, without physical walls. We show that these solutions can be assembled to create complex microfluidic networks, composed of intersecting channels and turns, which are basic building blocks in microfluidic devices. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A10.00006: Continuous-flow Electrophoretic Separation of Particles with Dissimilar Charge-to-Mass Ratios via the Wall-induced Non-inertial Lift Cory Thomas, Andrew Todd, Xinyu Lu, Xiangchun Xuan Traditional electrophoresis separates particles with dissimilar charge-to-mass ratios along the channel length direction in a batchwise mode. We present in this talk a continuous-flow electrophoretic separation of particles in the transverse direction of a straight microchannel. This separation stems from the particle property-dependent lateral migration due to the wall-induced non-inertial electrical lift force. It is demonstrated through both a binary and a ternary separation of polymer particles based on surface charge and size. A numerical model is also developed to understand this separation and to study the parametric effects. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A10.00007: Electrothermal Flow Enhanced Sample mixing in a Ratchet Microchannel. Christian Brumme, Ryan Shaw, Yilong Zhou, Xinyu Lu, Xiangchun Xuan We present in this talk an electrokinetic method for sample mixing in a ratchet microchannel. Due to Joule heating effects in the background electrolyte, temperature gradients are created around the ratchets causing non-uniform fluid properties. The action of electric field on these thermally induced property gradients yields an electric force that can manifest itself in the flow field in the form of circulations. We demonstrate the use of electrothermal flow circulations to enhance sample mixing through both experiment and numerical modeling. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A10.00008: Control of colloid transport via solute gradients in dead-end channels Sangwoo Shin, Eujin Um, Patrick Warren, Howard Stone Transport of colloids in dead-end channels is involved in widespread applications ranging from drug delivery to geophysical flows. In such geometries, Brownian motion may be considered as the sole mechanism that enables transport of colloidal particles into or out of the channels, which is, unfortunately, an extremely inefficient transport mechanism for microscale particles. Here, we explore the possibility of diffusiophoresis as a means to control the colloid transport by introducing a solute gradient along the dead-end channels. We demonstrate that the transport of colloidal particles into the dead-end channels can be either enhanced or completely prevented via diffusiophoresis. We also observe a size-dependent focusing of the particles where, as the particle size increases, the particles tend to concentrate more, and they tend to reside deeper in the channel. Our findings have implications for all manners of controlled release processes, especially for site-specific drug delivery systems where localized targeting of drugs with minimal dispersion to the non-target is essential. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A10.00009: The Formation of Ion Concentration Polarization Layer Induced by Bifurcated Current Path Junsuk Kim, Hyomin Lee, Inhee Cho, Ho-Young Kim, Sung Jae Kim Ion Concentration Polarization (ICP) is a fundamental electrokinetic phenomenon that occurs near a perm-selective membrane and, thus, the characteristics can be significantly altered by the current path through the Nafion nanoporous membrane. In this work, a new ICP device that bifurcated the current path was fabricated using micro/\textit{micro-nano}/nano/micro hybrid channel connection, while a conventional ICP device has employed micro/nano/micro channel connection. The propagation of ICP layer was initiated from the nano-channel at high concentration regime and from \textit{micro-nano} connection at low concentration regime. Interestingly, the reverse propagation was observed at low concentration regime as well. These combined effects conveyed a competition between two distinguishable propagations at intermediate concentration regime, caused by singularity of the bifurcated current path. Experiments and an equivalent circuit analysis were conducted for this bifurcation. As a result, the conductance ratio of electrolyte to Nafion governed the bifurcation. Conclusively, the bifurcation-induced ICP layer formation was able to be characterized by analyzing current-time characteristic which have two distinct RC delay times. [Preview Abstract] |
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