Cascade partial coalescence phenomena at electrolyte–oil interfaces and determination of bounds for the surface potential

Abstract

Small oil droplets (diameter <500 μm) in slow, steady, buoyancy-driven rise through pure or salty water at neutral pH exhibit a cascade partial-coalescence phenomenon upon soft collision with quasi-planar aqueous electrolyte solution–oil interfaces. For droplets of pure toluene, n-heptane, and their mixtures (heptol), and for moderate-strength electrolytes, the cascade partial-coalescence process continues until a critical droplet size is reached. We infer that this outcome is due to the oil–water interface being electrically charged and use the last partial-coalescence event in the sequence to estimate lower- and upper-bounds for the absolute value of the electrical potential of the interface.