Abstract
Enhancement of multibubble sonoluminescence was observed for pulsed ultrasound at 103 kHz. High-speed photography was also used to obtain the images of cavitation bubbles at a frame rate of 150,000 fps in continuous wave mode and pulsed wave mode. Most bubbles oscillated spherically and showed frequent coalescence in continuous wave mode at an applied voltage of 28 Vpp. The maximum bubble radius was measured to be 20–30 µm. Only in pulsed wave mode the following complex structure of bubble oscillation was captured. The radii of bubbles that survived during the pulse-off time were in the range from 5 to 10 µm, and these act as bubble nuclei for the successive pulse. These bubble nuclei grow to large bubbles, which oscillate non-spherically and emit many tiny bubbles. The daughter bubbles oscillate spherically and probably contribute to the enhancement of sonoluminescence. Subharmonic components and the increase in broadband noise obtained in the acoustic spectrum result from bubbles oscillating non-spherically and chaotically.