The Coalescence of Charged and Uncharged Drops Under Electric Field

Electrocoalescence is an application of drop electrokinetics. It can be used for the separation of an aqueous phase from an oil phase. In the electrocoalescence process, small water drops, which are dispersed in an oil phase, are promoted to aggregate into a bulk aqueous phase by the electric field. Existing studies focused on the electrocoalescence of either charged or uncharged drops. However, the coalescence behavior has not been clarified comparatively for charged and uncharged water drops in the same oil phase. In this work, we have performed an experiment to observe the coalescence of charged and uncharged drops with an oil-water interface under a dc pulsed electric field. A de-ionized water drop was injected into an oil phase for coalescence with a water plane below to observe the drop behavior and determine the effects of charge and electric field on the coalescence. In addition, mineral oil and silicone oil were used as the oil phase to investigate the effect of the medium properties, the interfacial tension coefficient and the permittivity, which have opposite effects on the coalescence. The results demonstrated that an increase in the electric field accelerated the drop movement, but the deformation of the drop also increased, leading to partial coalescence. The stability of the drop and the coalescence in silicone oil were better compared to those of mineral oil due to the predominant interfacial tension property. Due to the effect of charge, the electrical force was increased, enhancing the coalescence time. However, non-coalescence or disintegration could occur at high electric field when using charged drop in the oil with low interfacial tension.