A 1 kHz resolution frequency study of a variety of sonochemical processes

Abstract

A variety of reactions, which are known to be enhanced or driven by sonochemical effects, have been studied and their absolute rate measured as a function of the ultrasonic frequency employed within a cylindrical reactor. The rate is shown to be highly dependent on the ultrasonic frequency employed in the range of 20 kHz to 160 kHz. The frequency dependence of the system is the net result of the frequency dependencies of the transducer, the reverberant sound field, the cavitation dynamics and the chemistry. Rate variation of the reactions studied is correlated to light emission (sonoluminescence) as a function of the acoustic driving frequency with a resolution down to 1 kHz. The results are discussed with reference to the acoustic characteristics (particularly the modal nature) of the cell employed. The results are compared to the spatial peak acoustic pressure amplitude within the cell and broadband audio emission. Chemical activity could be predicted by sonoluminescence activity, which correlated with the more spatially complex sound field produced at higher frequencies. The most important finding is that characterisation of the sound field is vital in sonochemical experiments: a <3% change in the driving frequency was found to change the chemical activity by 3 orders of magnitude, because of the tuning effect of the modal sound field.