Acoustic cavitation generated by microsecond pulses of ultrasound

Under extreme acoustic stress, liquids can be caused to rupture, with the transient generation of a vapour cavity. The subsequent collapse of these cavities is normally violent enough to be observed with the unaided eye or ear, and often generates free radicals due to the high temperatures and pressures associated with the collapse. Flynn1 has calculated that microsecond pulses of ultrasound with peak intensities in the range 10–30 W cm−2 can generate transient cavitation in water. Because some diagnostic ultrasound systems2,3 now in clinical use generate microsecond length pulses with temporal peak intensities >100 W cm−2, there is reason to believe that this mechanism could operate in diagnostic conditions in aqueous media. We show here that ultrasonic pulses as short as one cycle at a frequency of 1.0 MHz give rise to luminescence flashes characteristic of violent cavitation. This provides the first experimental confirmation of Flynn's theoretical calculations.