Cavitation phenomena in water involving the reflection of ultrasound pulses from a free surface, or from flexible membranes.

We report findings from studies of water subjected to tension by pulsed dynamic stressing which may have significance in the context of assessing the safety of biomedical applications of low-frequency ultrasound. When incident on bubbles near a free surface (or a flexible membrane), shockwaves generated by cavitation bubble collapse are found to lead to the production of liquid jets which are directed towards the surface. Tension pulses generated by reflection of these shockwaves at a free surface initiate, and propagate, the growth of further cavitation bubbles. Estimates of the tensile strength of water, Fc, from measurements of the velocity of these pulses suggest that Fc is far higher than has previously been reported for experiments involving the reflection of low-frequency ultrasound; and the estimate of Fc reported here considerably exceeds the values of tension thought to be generated by ultrasound in many biomedical applications.