Coalescence dynamics of a nanoparticle-laden droplet at oil-water interface under electric field: A molecular dynamics simulation

Abstract

Droplet deformation, coalescence, sedimentation and droplet-interface coalescence are common phenomena during crude oil electrodehydration. This paper investigates the coalescence dynamics of a nanoparticle-laden (NP-laden) water droplet at the oil-water interface under direct current (DC), alternating current (AC), and pulsed (PE) electric fields, using molecular dynamics (MD) method. Validation studies demonstrate strong quantitative and qualitative agreement between the experimental and numerical results. The results show that complete droplet-interface coalescence (CC), encompassing both typical and upheaval modes, as well as partial coalescence (PC), occurs under DC fields and is influenced by ion migration mechanisms. The critical cone angle transitioning from CC mode to PC mode is 47.03°, and the critical electric capillary number (Ca_E) decreases with increasing droplet-interface distance. Moreover, a robust quartic polynomial function relationship between dimensionless liquid bridge width W* and dimensionless time t* is established to describe liquid bridge evolution. The occurrence of CC mode is significantly more pronounced under AC and pulsed fields compared to DC fields. The optimal dimensionless frequencies are identified as f*=16 for AC fields and f*=25 for pulsed fields. Total interactions (TI) analysis shows that the coalescence efficiencies rank as follows: PE > DC > AC. The findings of this study offer significant potential for optimizing high-efficiency and compact electrostatic coalescence equipment.