Cavitation activation by dual-frequency ultrasound and shock waves.

High-speed photographic observations of cavitation occurring under a low-frequency (21 kHz) sonotrode tip in the presence of an additional, high-frequency (355 kHz) ultrasound source have been made in water and in dilute aqueous solute solutions. Acoustic emission spectra were measured to support the visual observations. It was seen that a nucleating effect of the high-frequency action on cavitation at the low-frequency sonotrode was highly power dependent, with cavitation being homogenous at low acoustic power and highly localised at high acoustic power. The presence of solutes was found to significantly affect the cavitation structures and the bubble fragmentation process. Both the fundamental high-frequency acoustic emission peak and the higher order low-frequency harmonics were significantly intensified in the dual-frequency mode in the presence of these solutes. Additionally, the application of a high-voltage induced acoustic shock-wave to two different ultrasound fields was investigated in water and surfactant solutions.