We develop a theoretical model of the electrokinetic streaming potential considering the Navier-Stokes equation coupled with the Poisson-Boltzmann equation in order to elaborate the possible applicability of the microfluidic-battery from conceptualization to system validation. The ion transport in the microchannel is described on the basis of the Nernst-Planck equation. In this study, monovalent symmetric electrolytes are considered, and the profile of fluid conductivity is derived in terms of both the concentration profile and the mobilities of anions and cations. The present simulations provide that the flow-induced streaming potential increases with increasing surface potential of microchannel wall, whereas increasing the surface conductivity reduces the streaming potential. We also present the results on the change of streaming potential with variations of the electric double layer thickness normalized by the channel radius. It is of interest to find the behavior that a lower value of ion mobility leads to the enhancement of streaming potential, which tends to develop with either increasing bulk electrolyte concentration or decreasing surface conductivity. Hence, a choice of electrolyte should be considered to obtain improved performance.
This paper addresses the effects of microchannel geometry with electrically insulating posts on a particle flow driven by electrokinesis and dielectrophoresis. An in-house numerical program is developed using a numerical model proposed in literature to predict particle flows in a microchannel with a circular post array. The numerical program is validated by comparing the results of the present study to those in the literature. Results obtained from a Monte-Carlo simulation confirm the three particle flow types driven by an external DC electric field: electrokinetic flow, streaming dielectrophoretic flow, and trapping dielectrophoretic flow. In addition, we study the effects of electrokinetic and dielectrophoretic forces on particle transports by introducing a ratio of lateral to longitudinal forces exerted on a particle. As a result, we propose an improved microchannel geometry to enhance particle transports across electrokinetic streamlines for a given power dissipation. The improved microchannel has a shorter longitudinal spacing between the circular posts than a reference microchannel. We also discuss the critical values of dimensionless variables that distinguish the three particle flow types for both improved and reference microchannels.
To elaborate on the applicability of the electrokinetic micro power generation, we designed and fabricated the silicon-glass as well as the PDMS-glass microfluidic chips with the unique features of a multi-channel. Besides miniaturizing the device, the key advantage of our microfluidic chip utilization lies in the reduction in water flow rate. Both a distributor and a collector taking the tapered duct geometry are positioned aiming the uniform distribution of water flow into all individual channels of the chip, in which several hundreds of single microchannels are assembled in parallel. A proper methodology is developed accompanying the deep reactive ion etching as well as the anodic bonding, and optimum process conditions necessary for hard and soft micromachining are presented. It has been shown experimentally and theoretically that the silicon-based microchannel leads to increasing streaming potential and higher external current compared to those of the PDMS-based one. A proper comparison between experimental results and theoretical computations allows justification of the validity of our novel devices. It is useful to recognize that a material inducing a higher magnitude of zeta potential has an advantage for obtaining higher power density under the same external resistance.
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