Bubble ring entrapment during a water drop impact on viscous oil films

Air entrapment during the drop impact on a liquid surface is crucial to the ocean–atmosphere mass transfer process. Herein, we report a new mechanism of air entrapment. When a water drop impacts a highly viscous oil film, a bubble ring with a volume of approximately 2% of that of the initial drop is entrapped and disintegrated into multiple bubbles underneath the spreading lamella, which eventually float and burst to emit singular jets near the free surface. The reconstructed profile of the deformed oil film by the laser-induced fluorescence technique reveals the formation of the ridge and valley, which leads to the bubble ring entrapment between the two layers. The effect of the impact velocity on the annular ridge structure and bubble volume is discussed. The onset of the bubble ring disintegration is theoretically predicted, which agrees well with experimental data. Finally, the parameter space of bubble ring entrapment is presented in the regime maps, where three parameters including the impact Weber number, the dimensionless oil viscosity, and film thickness are considered.