Ultrasound combined with microwave irradiation: Cavitation regimes and acoustic emissions.

The irradiation of water by intense ultrasound (US) without and with microwave (MW) heating is investigated by analysis of acoustic emission spectra and high-speed imaging of cavitation bubbles. To this end, pure airborne sound detection proves sufficient for a rough assessment of cavitation quality generated by a 20.5 kHz glass horn inside a MW oven. Results show essentially two cavitation states: soft (or gassy) and hard (inertial) cavitation. Application of US alone leads to soft cavitation in strongly pre-heated water, and to hard cavitation otherwise. The addition of MW irradiation to hard cavitation triggers after a certain delay time the transition to soft cavitation, but a return to hard cavitation is observed after switching MW off. The findings are discussed in the context of water temperature and relative air saturation of the liquid. It is conjectured that rapid MW heating during US irradiation can drive the water into stronger oversaturation, while US alone does not. Further experiments for exploration of the observed effects are suggested, and potential optimization strategies for US/MW applications are proposed.