Single bubble collapse near a wall: Pressure wave and microjet impact

The wall loading mechanisms induced by the collapse of a single laser-generated cavitation bubble are investigated experimentally to resolve the long-standing debate over the relative contributions of microjet impact and shock wave emission. By synchronizing high-speed Schlieren imaging with simultaneous wall-pressure and hydrophone measurements, we isolate and quantify these mechanisms across stand-off distances γ∈[0.51,2.25]. Our measurements demonstrate that shock wave emission—not microjet impact—dominates maximum wall pressure across this range. We experimentally verify that the microjet impacts the wall before pressure wave emission for γ≤1.2, refining the previously suggested threshold of γ<1.1. Applying established topological classifications, we confirm that the acoustic regimes identified for free surfaces (torus, mixed tip-and-torus, and tip) exhibit an analogous topological progression near rigid boundaries. Critically, we identify a shock wave self-focusing mechanism within the mixed regime (1.22≤γ≤1.55) that transiently amplifies wall pressure, creating high-stress loading conditions deviating from monotonic energy decay trends predicted for non-spherical collapses. These findings provide definitive physical criteria for predicting cavitation erosion risk on marine structures, emphasizing the necessity of resolving shock-focusing phenomena in hydrodynamic impact models.