Coalescence characteristics of droplets at oil–water interface subjected to different electric waveforms

Abstract

Droplet–interface electrocoalescence may be partial, which is undesirable for separating water from oil. This paper focused on the coalescence characteristics of water droplets at oil–water interface, and the influences of droplet diameter, electric field strength (E), waveform and frequency were experimentally evaluated. The results showed that the normalized volume of secondary droplets (Vnor) increased as E rose, and it was larger for large initial droplet. Increasing the frequency of pulsatile electric fields (PEF) reduced Vnor. The droplet was periodically stretched under PEF according to the waveform’s periodic characteristic. It was proved that the increase of frequency suppressed the deformation of the droplet in vertical direction, which was conducive to droplet’s vertical collapse, and therefore inhibited the formation of secondary droplets. Critical electric field strength (Ecrit) for the occurrence of partial coalescence also increased with the elevated frequency, and the increase became relatively slow at high frequencies. Ecrit of square waveform was the largest in experiments, indicating it was most effective in suppressing partial coalescence; moreover, Ecrit of PUAC could exceed that of sine AC and DC waveforms at high frequencies. As the initial droplet radius (R) reduced, Ecrit increased and showed a linear correlation with R−0.5.