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

诱导电荷电动力学（IC-EK）是由电场驱动下电解液中诱导空间电荷迁移引起的一类输运现象。由于IC-EK在流体/颗粒操纵方面具有明显的优势，因此有望为微流体技术带来重大突破。然而，对IC-EK的基本理解仍然缺乏：虽然一些实验趋势可以解释，但理论往往高估实验测量10-100倍，一些观察结果甚至无法定性解释。这表明一些重要的物理现象在现有的理论中被遗漏了。识别和阐明这样的物理是重要的推进IC-EK的基本理解和探索IC-EK的潜力在微流控技术的最大程度。智力优势：本次合作的目的是利用IC-EK的代表成员交流电渗透流（AC-EOF）作为模型问题来研究IC-EK。中心假设是，实验与现有理论之间的差异是由于现有理论缺乏对斯特恩层、界面流体流变学、非平衡双电层及其与流体流动耦合的准确描述造成的。在这一假设的驱动下，我们计划实现两个具体目标：1)开发一种多尺度模拟工具，准确地解释斯特恩层和界面流体的流变性，并明确地解决非平衡edl和散装电解质中的离子/流体动力学；2)结合多尺度建模和纳米尺度流动表征来阐明交流eof的实验异常，并利用本工作的见解探索基于交流eof的新装置设计。计划中的流量测量将解决纳米级非平衡edl内的流动和电极附近漩涡的动力学。总之，这些研究将使AC-EOF实验异常的潜在物理以前所未有的精度被描绘出来。这项研究具有潜在的变革性。首先，本文获得的见解将为合理设计AC-EOF器件以克服其局限性奠定基础。其次，通过阐明界面流体流变对EOF的影响以及非平衡edl在AC-EOF中的作用，本研究将显著推进EK理论。特别是，定量确认非平衡edl在AC-EOF中的重要性，可能会导致如何理解和控制整个IC-EK输运的范式转变。更广泛的影响：计划开展一系列活动，鼓励和准备本科生从事计算科学和工程方面的职业。参与这个跨学科项目的学生将接触到不同的领域，如计算电流体力学和界面科学。将利用各种资源，例如pi机构的少数族裔招聘/保留计划，从代表性不足的群体中招募学生参加该项目。这些活动将受益于pi在这些项目中的经验。研究成果将被开发成由pi教授的微/纳米流体学课程的模块。研究成果还将制作成视频和海报，用于K-12外展活动，并提交给Efluids.com主办的流体运动/图像库。

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.