Probing the coalescence mechanism of water droplet and Oil/Water interface in demulsification process under DC electric field

Abstract

Under electric fields, coalescence of water droplets in oil media and bulk water (droplet-interface coalescence) is a significant phenomenon during the electrostatic process of water-in-oil (W/O) emulsion. In order to reveal the microscopic mechanism of droplet-interface coalescence, MD simulations and high-speed imaging were performed to study the influence of droplet size (r0) and electric field strength (E) on the coalescence process between water layer and droplet in oil. The findings suggested that the molecular arrangement of water molecules was altered at applied electric fields, resulting in a weakened interaction force and an increased spacing between water molecules. The droplet-interface coalescence process could be classified into five temporal regimes, under the mutual effect of the associated intermolecular interactions (Coulomb interaction and van der Waals interaction) and external electric field. The interfacial charge density reached a high value of 0.15 e·nm−3 due to the charge orientation, leading to dipole–dipole interaction between the droplet and water layer. Driven by both applied electric field and polarized charge, when E = 0.44 V·nm−1, the droplet with a diameter of 6 nm had the shortest coalescence period, only 370 ps. The critical field strength (Ec) required to prevent complete coalescence between nano-droplets and oil–water interface increased approximately linearly with r0−0.5. This work has revealed the molecular interaction mechanism of electro-coalescence between water droplets and oil/water interface, and provides useful insights into the electrostatic demulsification.