Collaborative Research: Deciphering Induced-Charge Electrokinetics: Multiscale Simulations and Nanoscale Flow Characterization

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Qiao/Wang1336224/1336004Induced-charge electrokinetics (IC-EK) is a class of transport phenomena originating from the migration of induced space charges in electrolytes driven by electrical fields. Because of its distinct advantages over the classical EK in fluid/particle manipulation, IC-EK is expected to bring significant breakthroughs to microfluidic technologies. A fundamental understanding of IC-EK is, however, still lacking: although some experimental trends can be explained, theories often overpredict experimental measurement by 10-100 folds, and some observations cannot be explained even qualitatively. This suggests that some important physics are missing from the existing theories. Identifying and elucidating such physics is important for advancing the basic understanding of IC-EK and for exploring IC-EK's potential in microfluidic technologies to the fullest extent. Intellectual Merit :The objective of this collaborative effort is to investigate IC-EK using alternating current electroosmotic flow (AC-EOF), a representative member of IC-EK, as a model problem. The central hypothesis is that the discrepancy between experiments and existing theories is caused by a lack of accurate account of the Stern layer, rheology of interfacial fluids, non-equilibrium electrical double layers (EDLs), and their coupling with fluid flow in existing theories. Driven by this hypothesis, two specific objectives are planned: 1) to develop a multiscale simulation tool that accurately accounts for the Stern layer and the rheology of interfacial fluids, and explicitly resolves the ion/fluid dynamics in non-equilibrium EDLs and bulk electrolytes; 2) to elucidate the experimental anomalies of AC-EOF by integrating multiscale modeling with nanoscale flow characterization and to explore new design of AC-EOF-based device using the insights gained in this work. The planned flow measurement will resolve flow within the nanoscopic non-equilibrium EDLs and the dynamics of vortices near electrodes. Together, these studies will enable the underlying physics of the experimental anomalies of AC-EOF to be delineated with unprecedented accuracy. The research is potentially transformative. First, the insights gained here will lay foundation for the rational design of AC-EOF devices to overcome their limitations. Second, by elucidating the impact of the rheology of interfacial fluids on EOF and the role of non-equilibrium EDLs in AC-EOF, this study will significantly advance EK theory. In particular, quantitatively confirming the importance of non-equilibrium EDLs in AC-EOF can potentially lead to a paradigm shift in how the entire class of IC-EK transport is understood and controlled.Broader Impacts :A series of activities are planned to encourage and prepare undergraduate students to pursue careers in computational science and engineering. Students participating in this interdisciplinary project will be exposed to diverse fields such as computational electrohydrodynamics and interfacial sciences. Various resources, e.g., the minority recruitment/retention programs at the PIs' institutions, will be utilized to recruit students from under-represented groups to participate in this project. These activities will benefit from the PIs' experience with these programs. Research results will be developed into modules for the micro/nanofluidics courses taught by the PIs. Research results will also be developed into videos and posters for use in K-12 outreach activities and for submission to the gallery of fluid motion/images hosted by Efluids.com.