对浓溶液中大电压下感应电荷电动力学的理解
Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions
摘要：
古老的动电现象理论基于这样一种假设，即在弱电荷表面附近存在点状离子的稀溶液，其相对于本体的电势为热电压的量级（室温下为$kT/e \approx 25$ mV）。在非线性电动现象中，例如AC或感应电荷电渗（ACEO，ICEO）和感应电荷电泳（ICEP），将几伏的$\approx 100 kT/e$施加到微观几何形状中的可极化表面，并且所产生的电场和感应表面电荷大到足以违反经典理论的假设。在这篇文章中，我们回顾了文献，强调理论和实验之间的差异，介绍了可能的修改的理论，并分析其后果。我们认为，在一个大的外加电压，“紧凑层”和“剪切平面”有效地推进到液体中，由于拥挤的反离子。使用简单的连续模型，我们预测两个总的趋势，每个增强介电响应：（i）离子拥挤对阻塞表面扩散双层，从而降低其微分电容，和（ii）电荷诱导的粘度增加附近的表面降低了电渗迁移率。第一种效应能够预测ACEO泵中的高频流动逆转，而第二种效应可以解释ICEO流量随盐浓度增加而衰减。通过例子，如不带电的金属球在不对称电解质中的ICEP，我们表明ICEO流是离子特异性的。类似的问题出现在纳米流体（由于限制）和离子液体（由于缺乏溶剂），所以本文的结论是有限尺寸离子的修改电动方程的一般框架。
英文摘要：
The venerable theory of electrokinetic phenomena rests on the hypothesis of a dilute solution of point-like ions near a weakly charged surface, whose potential relative to the bulk is of order the thermal voltage ($kT/e \approx 25$ mV at room temperature). In nonlinear electrokinetic phenomena, such as AC or induced-charge electro-osmosis (ACEO, ICEO) and induced-charge electrophoresis (ICEP), several Volts $\approx 100 kT/e$ are applied to polarizable surfaces in microscopic geometries, and the resulting electric fields and induced surface charges are large enough to violate the assumptions of the classical theory. In this article, we review the literature, highlight discrepancies between theory and experiment, introduce possible modifications of the theory, and analyze their consequences. We argue that, in response to a large applied voltage, the "compact layer" and "shear plane" effectively advance into the liquid, due to the crowding of counter-ions. Using simple continuum models, we predict two general trends, each enhanced by dielectric response: (i) ionic crowding against a blocking surface expands the diffuse double layer and thus decreases its differential capacitance, and (ii) a charge-induced viscosity increase near the surface reduces the electro-osmotic mobility. The first effect is able to predict high-frequency flow reversal in ACEO pumps, while the second may explain the decay of ICEO flow with increasing salt concentration. Through examples, such as ICEP of an uncharged metal sphere in an asymmetric electrolyte, we show that ICEO flows are ion-specific. Similar issues arise in nanofluidics (due to confinement) and ionic liquids (due to the lack of solvent), so the paper concludes with a general framework of modified electrokinetic equations for finite-sized ions.
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