Experimental study on the intensity variation of scattered light from ultrasonic cavitation bubbles

Variation in intensity of scattered light from cavitation bubbles in an ultrasonic standing wave field at different contents of dissolved air is studied experimentally. The number of cavitation bubbles that repeat expansion and contraction correlates with the sonochemical reaction yield and the evaluation of the number of bubbles is important for perspective on the yield. Peak-to-peak of intensity waveform of scattered light from cavitation bubbles corresponds to the number of the active bubbles which can contribute to the sonochemical reaction, because the waveform measured is the superposition of the waveform by independent bubble and a range of the active bubble radius is constrained due to its shape instability. In this paper, time evolution of the peak-topeak intensity under the irradiation of 142 kHz ultrasound is measured at various contents of dissolved air. It is shown that, as time goes by, the light intensity varies intermittently with period of several second at most. The intensity variation is marked at high content of dissolved air and the tendency reduces at the lower content. From the results it is interpreted that, in the ultrasonic cavitation field, the following sequential process is repeated intermittently; 1.Generation of tiny cavitation bubbles, 2.Generation of large-sized cavitation bubbles due to coalescence of tiny bubbles, 3.Movement of the large cavitation bubbles towards the pressure node by Bjerknes force with quasi-acoustic streaming, and these are responsible for the intermittent variation of the number of cavitation bubbles.