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16 - AC Electrokinetics and the Dynamics of Diffuse Charge

Published online by Cambridge University Press:  05 June 2012

Brian J. Kirby
Affiliation:
Cornell University, New York
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Summary

Equilibrium models of the EDL (Chapter 9) assume that the ion distribution is in equilibrium and use a Boltzmann statistical description to predict ion distributions. The equilibrium assumption is appropriate for the EDL at an electrically insulating surface such as glass or most polymers, because the ion distribution processes are typically fast relative to the phenomena that change the boundary condition ϕ0 (e.g., surface adsorption or changes in electrolyte concentration or pH).

In this chapter, we address the dynamics of diffuse charge. We focus primarily on the formation of thin double layers at electrodes with attention to the dynamics of doublelayer formation and equilibration. Unlike for the double layer formed at the surface of an insulator, the double-layer equilibration at an electrode (owing to the potential applied at that electrode) is not necessarily fast compared with the variation of the voltage at the electrode – high-frequency voltage sources can vary rapidly compared with double-layer equilibration. Thus the dynamic aspects of double-layer equilibration are critically pertinent.

A full continuum description of these phenomena comes from the Poisson, Nernst–Planck, and Navier–Stokes equations combined with boundary conditions describing electrode kinetics. Because such analysis is daunting, we approximate the problem as that of predicting surface electroosmosis with time-dependent electrokinetic potentials, and use 1D models of the EDL to form equivalent circuits that can be used to model the temporal response of ϕ0.

Type
Chapter
Information
Micro- and Nanoscale Fluid Mechanics
Transport in Microfluidic Devices
, pp. 355 - 372
Publisher: Cambridge University Press
Print publication year: 2010

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