Coalescence, Partial Coalescence, and Noncoalescence of Two Free Droplets Suspended in Low-Viscosity Oil under a DC Electric Field.

Enhancing the electric field strength can facilitate the approach of droplets and the drainage of liquid film. However, two droplets do not coalesce but bounce off after contact under an excessively high electric field strength. To reveal the underlying mechanism, the dynamic behaviors of two free droplets suspended in low-viscosity silicone oil under a DC electric field were investigated herein. Three distinct behavior modes were successively observed by a high-speed camera with the increase in electric field strength: coalescence, partial coalescence, and noncoalescence. The mechanisms and key criteria of partial coalescence and noncoalescence were explored by studying the competition between electric force and interfacial force. The theoretical formula of critical electric field strength for droplet coalescence was derived and validated by experiments. The results indicated that the electric capillary number Ca can be used as the criterion to identify the behavior modes of two free droplets. The droplets undergo partial coalescence or noncoalescence when Ca > 0.11; otherwise, the droplets experience coalescence.