Jetting and shear stress enhancement from cavitation bubbles collapsing in a narrow gap

The dynamics of bubbles near infinite boundaries has been studied in great detail. Once viscosity is accounted for, large wall shear stresses are generated upon jet impact and spreading. Although earlier works covered bubble dynamics in thin gaps and revealed rich fluid dynamics, viscosity and the resulting mechanical action on the surface have not been addressed. Here, we report experimental and numerical studies of cavitation bubbles expanding and collapsing inside a narrow gap. High-speed recordings and numerical simulations demonstrate an unexpected enhancement of the jetting velocity, a centre of mass translation and a dramatic increase of the wall shear stress. For the latter, we use computational simulations and present the results as spatio-temporal shear stress maps, while the bubble is recorded with high-speed photography. To test the implications of the high wall shear stress combined with the bubble translation, we conducted two experimental demonstrations. The first shows particulate removal on the distant wall, and the second cell detachment and molecule delivery through the cell membrane.