Ultrasonic cavitation under microwave irradiation: bubbles and acoustic emissions

Ultrasound (US) irradiation and microwave (MW) heating of aqueous solutions have shown synergetic effects and can foster advanced processing when applied simultaneously or sequentially [1]. Here we try to characterize in more detail the acoustic cavitation phenomena taking place in such setups. Our aims comprise a better understanding of the physical effects, but also a suitable way to monitor cavitation activity of the system. To this end, we conduct several types of experiments where acoustic emissions of sonicated and heated water are recorded and analyzed. Partly the bubbles are observed in parallel with high-speed imaging (see Fig. 1). Results show that essentially two different states of cavitation can appear, depending on state and history of the system parameters: (i) a "gassy" cavitation state where large and weakly collapsing bubbles occur with acoustic emissions remaining on a lower level; and (ii) a "hard" cavitation regime with smaller and stronger collapsing bubbles where acoustic emissions are on a high level. The regimes are similar to the non-degassed and degassed state of cavitating liquid that are encountered, for instance, during pre-treatment of water by heating and ultrasonic degassing in the context of cleaning applications [2]. Here, however, the microwave heating plays a special role that is not yet fully understood. In particular, turning MW on and off during sonication can alternately switch between gassy (MW on) and hard (MW off) cavitation regimes, even without significant temperature (i.e. air saturation) changes of the liquid. Our results indicate (a) that acoustic cavitation emissions into air can be used for cavitation assessment, (b) that simultaneous application of US/MW can compromise sonochemical activity by switching to a soft, gassy cavitation regime, (iii) that alternating or pulsed US/MW application might be a favorable strategy for improvement of synergy of both energy sources, and (iv) that the microscopic degassing and nucleating effects of MW in connection with US still need more exploration.