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Electrophoresis of a colloidal sphere parallel to a dielectric plane

Published online by Cambridge University Press:  21 April 2006

Huan J. Keh  and
Shing B. Chen
Huan J. Keh
Affiliation:
Department of Chemical Engineering, National Taiwan University. Taipei 10764 Taiwan, R.O.C.
Shing B. Chen
Affiliation:
Department of Chemical Engineering, National Taiwan University. Taipei 10764 Taiwan, R.O.C.
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Abstract

An exact analytical study is presented for the electrophoretic motion of a dielectric sphere in the proximity of a large non-conducting plane. The applied electric field is parallel to the plane and uniform over distances comparable with the particle radius. The particle and plane surfaces are assumed uniformly charged and the thin-double-layer assumption is employed. The presence of the wall causes three basic effects on the electrophoretic velocity: first, an electro-osmotic flow of the suspending fluid exists owing to the interaction between the electric field and the charged wall; secondly, the electrical field lines around the particle are squeezed by the wall, thereby speeding up the particle; and thirdly, the wall enhances viscous retardation of the moving particle. In the analysis, corrections to Smoluchowski's classic equation for the electrophoretic velocity in an unbounded fluid are presented for various separation distances between the particle and the wall. Of particular interest is the electrophoresis for small gap widths, in which case the net effect of the plane wall is to enhance the particle velocity. The particle mobility can be increased by as much as 23% when the surface-to-surface spacing is about 0.5% of the sphere radius. For the case of moderate to large separations, the electrophoretic velocity of the particle is reduced by the wall, but this effect is much weaker than for sedimentation. In addition to the translational migration, the electrophoretic sphere rotates at the same time in the direction opposite to that which would occur if the sphere sedimented parallel to a plane wall. The ratio of rotational-to-translational speeds of the sphere is in general larger for electrophoresis than for sedimentation.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 194 , September 1988 , pp. 377 - 390
Copyright
© 1988 Cambridge University Press

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