Optimum Bubble Temperature for the Production of Hydroxyl Radical in Acoustic Cavitation - Frequency Dependence

Extreme temperatures and pressures associated with acoustic cavitation render each bubble as a powered microreactor within which high-energy chemical reactions (sonochemistry) and sonoluminescence occur. In this work, a series of numerical simulations of chemical reactions occurring in the interior of an oscillating argon bubble in water have been performed for two different frequencies (20 and 355 kHz). The employed model combines the dynamic of bubble oscillation in acoustic field with the chemical kinetics occurring in the bubble. In all cases, it was found that there exists an optimum bubble temperature for the production of center dot OH radical inside a bubble, which is the main precursor involved in sonochemistry and sonoluminescence. The value of the optimum bubble temperature was found to be frequency-dependent. The optimum value decreases from 5200 K at 20 kHz to 4200 K at 355 kHz. Unexpectedly, theses optimum temperatures showed an excellent agreement with the bubble temperatures determined experimentally in several reports. The existence of these optimum temperatures was attributed to the competition between the reactions of production and those of consumption of center dot OH radical at high temperatures.