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.
In studies on the bioeffects of ultrasound exposure on cells cultured in a Petri dish, an understanding of the pressure field generated inside the dish is essential to achieve better dosimetry of ultrasound exposure. In this study, ultrasound fields generated inside four types of small containers designed to mimic a Petri dish were visualized using the focused shadowgraphy technique. The results revealed that wall propagation and water surface reflection of ultrasound make a complex standing wave field in the entire space of the container, which is very different from a standing wave field predicted by a simple model with reflection at the flat water surface without wall propagation.
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