Acoustic bubble cavitation at low frequencies

Electricite de France is involved in understanding the complex physical mechanism induced by power ultrasounds in the bulk of a liquid. The propagation of a ultrasound wave (from 20 to 800 kHz) through a liquid initiates a little-known phenomenon called acoustic cavitation. A film shot at 500 frames s(-1) shows the acoustic stable and transient cavitation activity in a stationary field at 28 kHz frequency. Stable cavities are simply bubbles that oscillate, often non-linearly, around some equilibrium size during many cycles of acoustic pressure. They are collected at pressure nodes and give rise to large bubble clouds. Transient cavities exist for less than one cycle. Cavities expand rapidly, often to many times their original size. They then collapse violently, generally breaking up into many smaller bubbles, These bubbles are gathered into a small cloud which grows to up to 10 times the emergence size during 15 ms. They move from arising locations at pressure antinodes to pressure nodes at velocity around 50 cm s(-1). Such high-speed photographs can help to better understand acoustic cavitation phenomena. (C) 1998 Elsevier Science B.V.