Experimental study on the falling and coalescence characteristics of droplets under alternating electric fields

Abstract

Droplet–interface collision coalescence is an important pattern of phase separation, especially for removing water from water-in-oil emulsions in electrostatic dehydrators. Deformation, migration to the interface, and coalescence of droplets driven by electric fields are fundamental processes but have not been completely understood. In this work, we studied the influences of three typical alternating current or pulsed electric fields on these dynamic processes of a single water droplet falling to the planar oil-water interface. The results depicted that the falling droplet oscillated in both shape and trajectory in an electric field. Specifically, the deformation of droplets should be able to follow the change of electric fields, while the migration may lag behind that. The pattern of droplet deformation matched the electric field waveform, and the frequency had different multiple relationship with the electrical frequency in the electric field of different waveforms. Moreover, the increment of electrical frequency could reduce the degree of droplet deformation and extend the falling time required for droplets to reach the interface. When the droplet coalesced with the interface, secondary droplets were observed if the applied electric field strength exceeded a critical value (Ecrit), detrimental to dehydration efficiency. Ecrit varied from different electric field waveforms and increased with elevated electrical frequencies. Compared to sinusoidal AC and square electric fields, the values of Ecrit in a pulsed DC electric field were highest in the experiments which could be more effective in suppressing the formation of secondary droplets. These findings will be useful for designing high efficient electro-dehydration plants.