Cavitation Bubbles and Their Interactions with Granular Boundaries*

We study the dynamics of a laser-induced cavitation bubble interacting with granular boundaries of different sand grain sizes. High-speed visualizations of the bubble and sand motion are performed for stand-off distances (dimensionless bubble-boundary distance) ranging between γ ≈ 5.3 and γ ≈ 0.3. Overall, we find that the presence of a nearby granular boundary leads to a bubble with a shorter lifetime and reduced centroid migration compared to a similar bubble that develops near a flat, rigid boundary. Above γ ≈ 1.3, the behavior of the bubble is nearly independent of the sand grain size. Between γ ≈ 1.3 and γ ≈ 0.6, a mound of sand forms under the bubble, forcing the latter to assume a conical shape as it collapses. The extent of the sand mound and the bubble deformation both depend on the grain size. Below γ ≈ 0.6, the bubble develops a bell-shaped form leading to the formation of thin and very fast micro-jets (v_jet > 1000 m/s). Moreover, between γ ≈ 1.3 and γ ≈ 0.3, we observe granular jets erupting from the sand surface following the bubble collapse.