Droplet Coalescence of W/O Emulsions under an Alternating Current Electric Field

Electrocoalescence is a common method for water droplet separation in oils. By using a high-viscosity oil as the continuous phase and cavity substrate, the visualization research of droplet coalescence of water-in-oil emulsions (W/O, 10%(v/v)) under a power-frequency alternating current (AC) electric field is realized. The combination of a microscope and a high-speed camera is used to make the visualization at different planes come true. The radial distribution and motion of the smaller water droplets around the ends of a large droplet were observed at different planes by changing the focal length. In addition, it is found that droplet chains formed are not stable, and besides, droplet chains directly join together to extend and form Y-shaped droplet chains, especially in the case of low voltage. The effects of application time, voltage, and surfactant concentration on droplet coalescence are studied through the microscopic pictures and particle size distribution of emulsions. The coalescence process of W/O emulsions under an AC electric field is observed and summarized as follows: (1) the adjacent droplets coalesce rapidly, and a few droplets coalesce into chains; (2) a large number of droplet chains form and crossly combine; and (3) large droplets appear, attracting small droplets to approach radially and coalesce. The enhancement of voltage can promote water droplets to coalesce. But there exists a critical voltage of 2.54 kV, and after it, the coalescence effect gently decreases. Furthermore, as the surfactant concentration increases, the coalescence effect first increases and then decreases. Both the deformation extent and liquid film mechanical strength influence the droplet coalescence.