Electrocoalescence dynamics of two unequal-sized nanodroplets under a direct-current electric field

Electrocoalescence effectively enables the high-performance separation of finely dispersed water droplets from oils. Molecular dynamics (MD) simulations were used to investigate the dynamic process of unequal-sized pure water droplets under a direct current (DC) electric field. Droplet electrocoalescence is accelerated with increasing volume ratio and proceeds faster when the electric field is directed toward the larger droplet compared to the smaller one. The results show that the droplet electrocoalescence process comprises two phases: the coalescence phase (two droplets gradually move toward each other and make contact) and the diffusion phase (two droplets undergo free diffusion). To quantify the rate of the coalescence process, a prefactor α of the square root of the shrinkage function was introduced, which increases with the volume ratio and leads to faster droplet coalescence. Moreover, by altering the electric field strength, this study discovered that two threshold values for the electric field strength exist in droplet coalescence. When the electric field strength exceeds the upper threshold E_c,high, the coalesced droplet deforms, and spindle-shaped droplets readily form; while below the lower threshold E_c,low, droplets coalesce due to the random motion. Additionally, both these thresholds decrease as the ratio of the droplet volume increases. These findings can be applied to crude oil dehydration, oily wastewater treatment, electrostatic demulsification, microfluidic droplet manipulation, and other oil-water separation processes.