Electrophoresis of a spheroid in a spherical cavity

The boundary effect of electrophoresis is investigated by considering the case of a spheroid in a spherical cavity; both are maintained at a low constant surface potential, and the axis of the former is aligned with the direction of the applied field. We show that the qualitative behavior of the particle depends largely on its position in the cavity and its aspect ratio. In general, the closer the particle to cavity, the smaller its electrophoretic mobility; the reverse is true, however, if it is too close to the cavity. The latter arises from the presence of an attractive force between the particle and the cavity. For a fixed particle volume, the attractive force for a prolate is greater than that for an oblate. Also, charge reversal may occur to the cavity surface when a particle is sufficiently close to it. In this case, it is possible that a particle moves in the opposite direction of that of the applied electric field. For a fixed particle volume, if the wall effect is important, the relative magnitudes of the electrophoretic mobilities for particles of various shapes follow the order prolate < sphere < oblate, and the reverse is true if the wall effect is unimportant.