Electrophoretic motion of a liquid droplet and a bubble normal to an air–water interface

Abstract

Electrophoretic motion of a charged liquid droplet or a bubble normal to an air-water interface is investigated theoretically, motivated by the rapid development of various practical applications involving micro/nano emulsions, as well as the fascinating potentials of using the air-water interface as a platform for the two-dimensional colloidal crystallization, for instance.The presence of an air-water interface reduces the droplet/bubble mobility in general when the double layer touches the interface. Special attention is given to the boundary effect upon the convection-induced polarization effect of the double layer pertinent to highly charged colloids, which tends to slow down the particle motion in general. As demonstrated by various contour plots of both electric and flow fields, the presence of an air-water interface affects the particle motion mainly due to the deformation/confinement of the double layer when it touches the interface. An axisymmetric counterclockwise vortex flow around the droplet/bubble is observed which pumps up the counterions originally in the wake to the front region. This significantly alleviates the motion-deterring polarization effect when the thickness of the double layer is comparable to the droplet/bubble radius. The release of this brake (polarization effect) refuels the electric driving force of the droplet/bubble hence affects its motion near the interface as observed.Convenient charts of correction factors of the boundary effect, expressed as a function of separation distance, are provided to facilitate the possible usage by interested researchers and engineers.