Coalescence characteristics of silica nanoparticle-laden droplets with a planar interface under direct current electric field

Abstract

Electric dehydration method is widely used in oil industry to separate water from oil. Droplets resting on the oil–water interface discharge their liquids into the bulk phase, and the process can be altered by the change of electric field strength (E) or properties of water. Phenomenon of silica (SiO2) nanoparticle-laden droplets coalescing with a planar interface under direct current electric field was observed using a high-speed digital video camera. The effects of initial droplet diameter (D), E, and weight percent of nanoparticles in deionized water (C) were studied experimentally. The results showed that increasing D or E contributed to the formation of secondary droplets. Addition of SiO2 nanoparticles to water produced two competing effects: reduction in oil–water interfacial tension and increase in water conductivity, and a shift in the dominant effect occurred with the increase of E. Furthermore, formation of a ring by the separation of vortices was visualized by movement of nanoparticles in experiments, and its downward moving velocity decreased with time, which is small compared to that reported in literature. Finally, it was confirmed that the dimensionless WO number is not suitable for describing the volume fraction of secondary droplets for droplet–interface coalescence containing nanoparticles.