Spreading of Oil-in-Water Emulsions on Water Surface.

This work presents the spreading behavior of oil-in-water (o/w) emulsions on the water surface recorded using a high-speed photography method. We study a series of o/w emulsions with two different droplet sizes of 4.50 and 0.75 μm and volume fractions of the oil phase in the 20-80% range. Results show that for all the emulsions a rapid spreading occurs upon the collision with the water surface, which then forms a thin film expanding with time. Appearance of a dry spot in the center of collision is observed in the spreading of the emulsions in midvolume fraction range that induces a bursting-like spreading. For the highly concentrated emulsions, the deliberation of decompression energy from the deformed oil phase droplets inhibits the bursting, increases the equilibrium propagation radius, and reduces the dissipation time. The role of viscoplasticity (existing of the yield stress) is considered and a model describing the propagation step of the emulsion spreading is presented. The model shows that the peculiarities of the spreading are determined by the competition between yielding, plastic viscosity, and interfacial tension. By comparing the model prediction and experimental results, it is suggested that the spreading behavior of the emulsions is not only a consequence of the surface tension gradient but also the coalescence of the oil droplets during spreading.