Surface acoustic cavitation understood via nanosecond electrochemistry

The application of high intensity ultrasound to liquids leads to cavitation. In contrast to the homogeneous situation this is poorly understood close to a surface despite the implications for many biological, chemical, and physical applications. By using ultrafast electrochemical equipment and arrays of electrodes, we prove that the acoustic bubbles in the range of power ultrasound are hemispherical, not spherical as usually supposed, possessing a large range of possible diameters and oscillating at harmonics and sub-harmonics of the driving frequency (20 kHz). Most importantly, contrary to inferences made previously at much lower frequencies, no liquid microjet inside the bubble is observed.