Revisit to the mechanism of quenching: Power effects for sonochemical reactions.

In this study, the mechanism that the sonochemical reactions are quenched due to an increase in ultrasonic power was investigated through six experiments, stability analysis, and numerical simulations. The experiments involved measuring the sonochemical reaction rate, observing sono-chemiluminescence (SCL), conducting particle image velocimetry (PIV) measurement, measuring sound pressure, observing bubble motion, and measuring the degassing rate of dissolved oxygen. Through these experiments and numerical simulations, the phenomena could be classified into three regions in response to ultrasonic power. In the region of small ultrasonic power, the superposition of ultrasound is good, and the reaction rate increases with the ultrasonic power. However, at higher ultrasonic power, the superposition of ultrasound is deteriorated, suppressing the bubble nucleation and growth due to rectified diffusion. This results in a lower fluid flow velocity due to acoustic streaming, a smaller reaction rate, and smaller degassing rate. At much higher ultrasonic power, the ultrasonic standing waves are changed into traveling waves resulting in bubble cluster formation and movement, as well as a smaller chemical reaction rate. These experimental results and the proposed mechanisms were also supported by the numerical simulation and stability analysis results.