[论文解读] Towards an understanding of induced-charge electrokinetics at large applied voltages in concentrated solutions

Martin Z. Bazant, Mustafa Sabri Kilic+2 more|ArXiv.org|Mar 26, 2009

Electrostatics and Colloid Interactions被引用 5

一句话总结

本文提出了一种改进的连续介质理论，用于描述在高电压下浓电解质中的诱导电荷电动力学，考虑了离子聚集、介电饱和以及表面附近的粘度变化。结果表明，这些效应导致紧密层和剪切面在溶液中有效前移，从而降低双电层电容和电渗迁移率——解释了在ACEO中高频流动反转以及在ICEO中随盐浓度增加而出现的速度衰减现象。

ABSTRACT

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.

研究动机与目标

• 解决经典电动力学理论与微流体器件在大电压下实验结果之间的差异。
• 解释高频率ACEO中流动反转以及盐浓度增加时ICEO速度衰减等未解现象。
• 重新评估在极端表面电势下，紧密层和剪切面在非线性电动力学中的作用。
• 建立一个考虑有限离子尺寸、介电饱和以及高带电表面附近粘度变化的连续介质框架。
• 为浓溶液、纳米流体以及离子液体中的非线性电动力学现象提供统一的理论基础。

提出的方法

• 采用考虑空间位阻效应的改进泊松-玻尔兹曼理论（如硬球液体、格气模型），以描述有限离子尺寸和离子聚集效应。
• 在高带电表面附近引入介电常数的介电饱和效应，以模拟非线性介电响应。
• 引入粘压电效应模型，描述电压诱导的表面附近粘度增加，从而降低电渗迁移率。
• 应用连续介质模型，预测在高表面电荷下紧密层和剪切面在溶液中的有效前移。
• 结合非平衡热力学与连续介质静电学，描述大电压下的动态弛豫和电动力响应。
• 通过ACEO中的流动反转和ICEO中盐浓度依赖的流动衰减等实验观测结果，验证理论预测。

实验结果

研究问题

• RQ1 为何交流电渗流（ACEO）在高频下出现与经典理论预测相反的流动反转？
• RQ2 为何诱导电荷电渗流（ICEO）的速度随盐浓度增加而衰减，与线性理论预测相反？
• RQ3 在大电压下，有限离子尺寸与离子聚集如何改变电双电层的结构？
• RQ4 表面附近的介电饱和与粘度变化在多大程度上改变电动力迁移率与电容？
• RQ5 是否能建立一个统一的连续介质模型，解释包括微流体、纳米流体和离子液体在内的多种系统中的非线性电动力学现象？

主要发现

• 在阻塞表面附近，离子聚集使扩散双电层扩展，导致大电压下其微分电容降低。
• 表面附近局部粘度因电压升高而增加，从而降低电渗迁移率，解释了盐浓度增加时ICEO流速衰减的现象。
• 介电饱和同时增强电容降低与粘度增加效应，放大其非线性电动力学后果。
• 由于离子聚集与介电饱和，紧密层与剪切面的有效位置在高表面电荷下向溶液中前移。
• 改进的连续介质模型成功预测了ACEO泵在高频下的流动反转，这一现象未被经典理论捕捉。
• 该理论揭示了非线性电动力学中的离子特异性行为，例如在不对称电解质中未带电金属球的诱导电荷电渗（ICEP），其差异源于离子尺寸与溶剂化能的不同。