Drop-interface coalescence: Effects of interfacial properties

Stability of single oil droplets at a liquid-liquid interface in the presence of surfactants was investigated. For the systems studied, the coalescence time of oil droplets correlated favorably with interfacial viscosity of the system. Small amounts of polyvinyl alcohol (PVA) in the water phase were used to obtain finite interfacial shear viscosities in the water-ethylbenzene system. This increase in shear viscosity was accompanied by a large increase in the lifetime of ethylbenzene droplets at the oil-water interface. The increased stability was attributed to the steric hindrance of the adsorbed polymer layer. A second surface active agent composed of oleic acid, caustic soda, and sodium chloride was added to the system to obtain low interfacial tensions. It was found that addition of this surfactant further enhanced coalescence rates and decreased interfacial shear viscosities. This was explained as resulting from the dislodging of the adsorbed PVA layer by the latter surfactant. It was found that both coalescence time and interfacial viscosity were strong functions of the concentration of oleic acid.