Relation between thresholds of free radical generation and atomization under ultrasound exposure

The relationship between thresholds of free radical generation and atomization under ultrasound exposure was investigated to elucidate the mechanisms of ultrasonic atomization. In the experiments to gradually increase the transducer driving voltage, the free radical generation, a quick transition of the water surface shape from a protuberance to a fountain, and atomization had the same threshold. The experiments using the acoustic loading conditions with the different water surface shapes also confirmed this fact. Furthermore, a focused shadowgram of the ultrasound field taken using a small container that mimics the protuberance visualized the generation of spotty-shaped high-intensity nodes inside and near the boundary of the protuberance. These results indicate that the induction of cavitation promoted by the high-intensity nodes triggers the appearance of the fountain that leads to the creation of atomization.