Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session J08: Microscale Flows: Electrokinetics (8:00am - 8:45am CST)Interactive On Demand
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J08.00001: Dielectrophoretic-driven deformations of free surface Israel Gabay, Antonio Ramos, Amir Gat, Moran Bercovici We present a theoretical model and experimental demonstration for the deformations of a thin liquid layer due to an electric field established by different electrode configurations. We model the spatial electric field created by the electrodes and use it to evaluate the force distribution on the interface through Maxwell's stresses. By coupling this force with the Young-Laplace equation, we obtain the deformation of the interface. These solutions serve as basic building blocks and allow us to explore the inverse problem where we seek the electrode structure and set of parameters which would yield a desired deformation. To validate our theory and demonstrate the feasibility of this mechanism, we designed an experimental setup which allows spatial dielectrophoretic actuation, while providing measurement of the microscale deformations. The system is based on microfabricated metal electrodes deposited on a glass substrate, which we cover with a thin film of oil prior to the application of the electric field. We use digital holographic microscopy to measure the induced deformations, showing highly localized deformations ranging from sub microns to tens of microns. [Preview Abstract] |
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J08.00002: Extremely-low-frequency dielectrophoretic particle focuser in an ``infinite'' microchannel. Amirreza Malekanfard, Wuzhou Zu, Apollo Wolfersberger, Xiangchun Xuan Insulator-based dielectrophoresis is an emerging technique that has been demonstrated for the passive focusing of particles under direct current electric fields. Increasing the number of insulators results in an enhanced focusing as the particle exposure to the dielectrophoretic force is extended. In this work, we explore the possibility of expanding the particle exposure to dielectrophoresis via the use of extremely low frequency (ELF) alternating current electric field. In other words, instead of increasing the number of insulators and hence the length of the microchannel under the conventional direct current electric field, we use the ELF electric field to pass the particles through the same insulator multiple times to achieve the dielectrophoretic focusing in a practically infinite microchannel. We demonstrate this focusing with both polystyrene particles and biological cells in a ratchet microchannel. [Preview Abstract] |
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J08.00003: Electroosmotic flow over superhydrophobic surfaces Baruch Rofman, Sebastian Dehe, Moran Bercovici, Steffen Hardt We present the first experimental demonstration of electroosmotic flow enhancement over superhydrophobic surfaces using gate electrodes. Utilizing a hierarchical surface composed of micropillars coated by nanoparticles, we maintain the liquid in a Cassie-Baxter state, thus entrapping air in between the microstructures. We use a gate electrode embedded in the surface to induce charge at the gas-liquid interface, and drive the flow inside a microfluidic chamber. By modifying the pitch of the pillars, we explore the dependence of the velocity on the slip length of the surfaces, confirming the theoretical predictions. For surfaces with large slip we obtain a velocity enhancement of more than an order of magnitude relative to non-slipping surfaces. Furthermore, in contrast to standard solid surfaces, over which EOF is highly sensitive to pH, we show that superhydrophobic surfaces render it essentially independent of pH, enabling a wider utility of electro-osmotic flow in microfluidic devices. [Preview Abstract] |
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J08.00004: Reversible Trapping of Colloids in Microgrooved Channels by Diffusiophoresis under steady-state solute gradients: An Experimental and Numerical Investigation Naval Singh, Goran T. Vladisavljevic, Francois Nadal, Cecile Cottin-Bizonne, Christophe Pirat, Guido Bolognesi In recent years, an increasing interest in harnessing the chemical energy has led to the exploration of colloidal particle manipulation by diffusiophoresis (DP) in microfluidic devices. In this study, we report a novel mechanism for reversible trapping of particles in dead-end micro-structures via steady-state solute gradients in a continuous flow setting. A microchannel was made of an optical adhesive glue and fitted with a transverse microgrooved wall. The charged fluorescent colloidal particles were accumulated within the microgrooves by pumping parallel electrolyte solutions into the device junction. The spatial distribution of particles within the channel was characterized via confocal microscopy and numerical investigation show that particles accumulate within the flow recirculation region beneath the groove entrance due to DP transport and hydrodynamic effects. The particles can be cyclically trapped into and released from the grooves by controlling the salt concentration via a flow switching valve. The proposed method offers great potential for microfluidic bio-analytical testing applications including signal amplification. [Preview Abstract] |
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J08.00005: Microscale membrane capacitive deionization: Concentration profiling and flow visualization with unipolar/bipolar connections Hahnsoll Rhee, Rhokyun Kwak Membrane capacitive deionization (MCDI) is a desalination method that uses electrically polarized porous electrodes as an ion adsorber and cation/anion exchange membrane to prevent co-ion desorption from the electrodes. To operate MCDI, we have two options, i.e., unipolar and bipolar connections. In unipolar connection, porous electrodes are directly connected to an electrical source, while the electrodes are floated under an electric field in bipolar connection. Here, we study the relative superiority of unipolar/bipolar connections of MCDI under various current regimes and stack numbers. To do this, we present a microscale MCDI platform that can visualize~\textit{in situ}~ion concentration and fluid flows. In both unipolar/bipolar connections, similar characteristics of ion concentration and fluid flows were observed in ohmic, limiting, and over-limiting regimes. However, the different trends of desalination performances were identified in the two connections according to the stack number. In unipolar connection with various stack numbers, salt removal ratio, energy per ion removal, and current efficiency were nearly constant under a fixed applied current, whereas the three metrics were worsened in bipolar connection as stack number increases. As a result, MCDI in bipolar connection is proper for highly efficient but low throughput applications. On the other hand, MCDI in unipolar connection is proper for high throughput but low efficient application. [Preview Abstract] |
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