Electrophoresis of drops and bubbles

We have examined the electrophoresis of drops and bubbles, computing the electrophoretic mobility as a function of the ζ-potential and several other parameters. Our treatment differs from previous work in that we incorporate a more representative picture of the interface. We have found that drops and bubbles are electrophoretically distinct from particles; perhaps the most striking result obtained was that, when the diffuse layers are thin, conducting drops do not always migrate in the direction that would be anticipated from the sign of their surface charge. Thus, the ζ-potential alone is not sufficient to characterize the surface. The analysis shows the sense of the migration is dictated by the net electrochemical stress acting along the interface. For similar reasons, large inviscid spheres tend to remain stationary at modest ζ-potentials and, in contrast to rigid particles, their mobility is actually enhanced by polarization of the double layer. Further, we have uncovered conditions for which the mobility of non-conducting drops is insensitive to the interior viscosity. This 'solidification effect' stems in part from interfacial tension gradients associated with specific adsorption of the ionic solutes, as well as from polarization and, moreover, need not involve the presence of surface-active impurities.