Spatial study on a multibubble system for sonochemistry by laser-light scattering

Volumetric oscillation of multiple cavitation bubbles in an ultrasonic standing-wave field is investigated spatially through the intensity measurements of scattered light from bubbles changing the measuring position in the direction of sound propagation. When a thin light sheet finer than half of wavelength of sound is introduced into the cavitation bubbles, at an antinode of sound pressure the scattered light intensity oscillates. The peak-to-peak light intensity corresponds to the number of the bubbles which contribute to the sonochemical reaction because the radius for oscillating bubbles at pressure antinodes is restrictive in a certain range due to the shape instability and the action of Bjerknes force that expels from the antinode bubbles that are larger than the resonant size. The experimental results show that the intensity waveform of oscillating scattered light measured at the side near the sound source is similar to the waveform as seen in a single-bubble experiment. The peak-to-peak light intensity for the scattered light waveform is low at the side near the sound source where the progressive wave is dominant, while at the side near the water surface far from the sound source the intensity is relatively high and has periodic structure corresponding to the periodicity of half wavelength from the standing wave. These tendencies of high intensity near the water surface and the periodicity correspond to the periodic luminescent stripes seen in images of luminescence in an ultrasonic standing wave as reported by Hatanaka et al. [Jpn. J. Appl. Phys. 39 (2000) 2962]. The present method of light scattering is promising for evaluating spatial distribution of violently oscillating cavitation bubbles which effect sonochemical reactions.