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Quenching Mechanism of Multibubble Sonoluminescence at Excessive Sound Pressure

Decrease in luminous intensity of multibubble sonoluminescence (MBSL) at excessive ultrasonic intensity is studied in connection with the behavior of cavitation bubbles. The intensity of MBSL from aerated distilled water in a rectangular vessel with six transducers, in which a standing wave field was established, was measured while the voltage applied to the transducers was increased. The corresponding change in the distribution of cavitation bubbles in the field was observed. The distributions of bubbles were compared with those of MBSL using an intesified charge-coupled device (ICCD) camera, and the bubble motion was observed with a high-speed camera. It is clarified that the expulsion of bubbles from the pressure antinode in the field is responsible for the decrease in MBSL intensity at high ultrasonic intensity. The expulsion is caused by the primary Bjerknes force which changes from an attractive force to a repulsive one depending on pressure amplitude and bubble radius, assisted by coalescence of bubbles due to the secondary Bjerknes force.