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 pulse d dynamic stressing which may have significance in the context of asse ssing the safety of biomedical applications of low-frequency ultrasoun d. When incident on bubbles near a free surface (or a flexible membran e), shockwaves generated by cavitation bubble collapse are found to le ad to the production of liquid jets which are directed towards the sur face. 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. F-c, from measur ements of the velocity of these pulses suggest that F-c is far higher than has previously been reported for experiments involving the reflec tion of low-frequency ultrasound; and the estimate of F-c reported her e considerably exceeds the values of tension thought to be generated b y ultrasound in many biomedical applications.