Backflipping motion of air bubbles colliding with a tilted wall

Oblique collision of solid particles with surfaces has been a topic of extensive study in Newtonian mechanics, which also explains the motion of bubbles and droplets to some extent. Here, we observe that air bubbles exhibit a backflipping behavior when they collide with a tilted surface. Our experiments reveal that bubbles with radii 0.6-0.7 mm undergo backflipping when they collide with surfaces at an angle of up to 15^o with the strongest backflipping at 3^o. Particle image velocimetry reveals that the backflipping behavior is caused by wake-induced circulation around the bubble, which applies a lift force on the bubble. We develop a theoretical model that incorporates potential flow theory to characterize the circulation caused by the interaction between the bouncing bubble and its wake. The theoretical results are in good agreement with the experiments confirming the key role of the wake-induced lift force in backflipping. Finally, we show that the backflipping behavior of air bubbles can be leveraged for sustainable cleaning of a biological surface coated with a protein solution.