Image Based Collapse Pressure Measurement of Erosive Cavitation Bubbles

Cavitation erosion of hard materials such as metals is produced under certain geometric conditions when during the collapse of a single bubble a self-focusing of shockwaves amplifies the collapse. Then, already one bubble collapse is sufficient to damage the material. In this work, we non-invasively measure the pressure of erosive single bubbles within a few micrometers of the erosion site. Conventional measurements using hydrophones are not feasible due to bandwidth and geometric constraints, possible damage to equipment, and disturbing the bubble collapse at relevant distances from it. We use high-speed shadowgraphy imaging of the shockwave front with laser illumination in bursts for multiple exposures of the same shockwave front within a camera frame, allowing for a spatially and temporally resolved measurement of 2D shockwave velocity. We account for diffraction of the initially non-spherical shockwave front using an acoustic wave equation together with the shock front images. Employing an adequate equation of state of water, the local shock pressures are found. Measurements reveal that the bubble collapse pressures crucially depend on the bubble to wall distance and show the stand-off of maximum shockwave self-focusing and collapse pressures.