Mapping shear stress in inclined bubble impact*

Bubbles are used in a range of applications related to cleaning, from ultrasonic baths to wastewater treatment. The cleaning is typically attributed to shear stresses in the fluid or scavenging of particles. The case of injected bubbles rising along a surface has gained interest recently for its potential in preventing biofouling and cleaning produce. We conduct experiments and numerical simulations of a bubble rising underneath an inclined surface held at various angles. From simulations, we calculate snapshots of shear stress induced at the wall, showing that the largest shear stresses occur where the wall-bubble film thickness is minimum. We combine these snapshots to obtain maximum shear stress maps, allowing us to determine the area of the wall subject to a specified stress level during a single bounce of a bubble. We find that the shear stress maps predict the cleaning patterns from experiments remarkably well except for the steepest case, where bubble trajectories in experiments vary considerably due to the presence of algae in the water. Additionally, we compare this to the similar case of a drop impacting an inclined surface.