Chemical Effects of Ultrasonics—``Hot Spot'' Chemistry

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Abstract
The present work was undertaken with the purpose of considering why and how reaction takes place in an ultrasonic field, rather than studying the chemical kinetics of such reactions. Experimental evidence is presented which supports the conclusions that chemical processes brought about by ultrasonics require cavitation. Furthermore, the experiments were planned so as to show whether the reactions take place in the bubble or at the interface by studying the effect of the thermal conductivity and of the ratio of the heat capacities of various dissolved gases and their mixtures on the yields. From the results it was concluded that primary reactions seem to be gas phase reactions, probably of a thermal nature, taking place inside the gas bubbles which serve as ``hot spots'' in the liquid. The calculations show that, under the experimental conditions reported here, temperatures of several hundreds or thousands degrees can be easily reached inside the cavitating bubbles in resonance with the sound field; it is believed that at these temperatures the reaction which gives rise to H2O2 takes place. At this time it is not possible, however, to rule out completely the concomitant effects of gaseous discharge as suggested by others. It was also found that in conducting experiments like those in the present work, the cavitation threshold should be determined, effective intensities used, and systems maintained as simple as possible.

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