Oblique droplet impact on superhydrophobic surfaces: Jets and bubbles

Previous studies revealed that the perpendicular impact of low-viscosity droplets on sufficiently lyophobic surfaces would stimulate a liquid jet during droplet recoiling, and in some cases, it is accompanied with the entrapment of an air bubble. However, whether such free-surface flow phenomena occur in oblique droplet impact and how surface inclinations influence the dynamics remain open questions. Herein, we perform a comparative investigation on the perpendicular and oblique impact dynamics of water droplets on superhydrophobic surfaces. We show that the inclinations of the target surface do not influence the jet emission and the bubble entrapment in droplet impact. The jetting phenomena, which are triggered by the collapse of the air cavity, can be classified into three types of scenarios, and the jet velocity scales with its size according to two power laws as suggested by the scaling analyses in the previous studies. On the other hand, the air bubble entrapment is caused by the asymmetric cavity collapse when the recoiling speed of the droplet top is close to that of the bottom, and the bubble size can be reduced by the rising jet from the cavity bottom. We also show that some entrapped bubbles on inclined surfaces would move to the droplet surface and eventually burst, shooting out another thin jet. The correlation between the jet velocity and the bubble size is compared with different empirical scaling laws in the literature.