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Electromigration dispersion in a capillary in the presence of electro-osmotic flow

Published online by Cambridge University Press:  09 March 2012

S. Ghosal  and
Z. Chen
S. Ghosal*
Affiliation:
Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208, USA
Z. Chen
Affiliation:
Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208, USA
*
†Email address for correspondence: [email protected]
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Abstract

The differential migration of ions in an applied electric field is the basis for the separation of chemical species by capillary electrophoresis. Axial diffusion of the concentration peak limits the separation efficiency. Electromigration dispersion is observed when the concentration of sample ions is comparable to that of the background ions. Under such conditions, the local electrical conductivity is significantly altered in the sample zone making the electric field, and, therefore, the ion migration velocity, concentration dependent. The resulting nonlinear wave exhibits shock-like features and, under certain simplifying assumptions, is described by Burgers’ equation (Ghosal & Chen Bull. Math. Biol., vol. 72, 2010, p. 2047). In this paper, we consider the more general situation where the walls of the separation channel may have a non-zero zeta potential and are therefore able to sustain an electro-osmotic bulk flow. The main result is a one-dimensional nonlinear advection diffusion equation for the area averaged concentration. This homogenized equation accounts for the Taylor–Aris dispersion resulting from the variation in the electro-osmotic slip velocity along the wall. It is shown that in a certain range of parameters, the electro-osmotic flow can actually reduce the total dispersion by delaying the formation of a concentration shock. However, if the electro-osmotic flow is sufficiently high, the total dispersion is increased because of the Taylor–Aris contribution.

JFM classification

Low-Reynolds-number flows: Lubrication theory Micro-/Nano-fluid dynamics: Microfluidics
Type
Papers
Information
Journal of Fluid Mechanics , Volume 697 , 25 April 2012 , pp. 436 - 454
Copyright
Copyright © Cambridge University Press 2012

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