Formation of water in oil in water particles by drop impact on an oil layer

We present an experimental study of water drop impact on an oil layer floating on a water bath. With the fixed oil layer thickness delta = 1 mm and viscosity mu = 10 mPa s, different phenomena are presented depending on the impact velocity. A regime map is built to classify these phenomena into bouncing, trapped, trapped + breakup, water in oil in water (W-O-W) particles, and multi-layered W-O-W particles. Focus is on the formation of the W-O-W particle which has a combined structure: a water globule enclosed by an oil film in the water bath. Generally, one intermediate-speed impact produces two W-O-W particles named as the primary and satellite particles, respectively. The diameter of the satellite particle significantly increases with the impact velocity, while the primary particle remains almost constant. There exists a maximal diameter of the satellite particle which is equivalent to that of the primary particle. After that, multi-layered W-O-W particles form and the satellite particle diameter decreases with the impact velocity. In the limit of an extremely thin oil film, a theoretical model is built to predict the threshold for generating W-O-W particles and the variation of the particle diameter with the impact velocity. Moreover, the threshold for generating W-O-W particles is found to depend on the oil layer thickness and viscosity. A qualitative analysis is presented to explain this phenomenon. Published under license by AIP Publishing.