A hemispherical bubble induced by ultrasonic vibration observed by high-speed X-ray imaging

Acoustic cavitation, also called ultrasonic cavitation, can produce aggressive intensity leading to cavitation erosion due to high pressure wave amplitudes, and impact forces on surfaces. However, its intensity changes drastically with the standoff distance between the vibratory horn and workpiece surface. The type of cavitation bubble collapse at the proximity on the surface has been found in two-fold, implosion of bubbles causing erosion of the surface and causation of microjets on the surface, with hemispherical collapse being more aggressive than microjet-type collapse. It has also been confirmed that the frequency of intense bubble collapse is less than one-tenth of the vibration frequency. This study aims to elucidate the mechanism underlying the generation of intense acoustic cavitation bubble collapse. The acoustic cavitation phenomenon was observed using high-speed X-ray imaging with synchrotron radiation at SPring-8 while varying with the standoff distance. Cavitation noise was simultaneously recorded using a hydrophone over 1,000 vibration cycles. It was found that hemispherical bubble collapse produced intense cavitation noise and that hemispherical bubbles gradually grew larger over several vibration cycles. The maximum size of the hemispherical bubble varied with the standoff distance, reaching a peak at a certain standoff distance where cavitation noise amplitude was also maximum.