Nucleation and evolution of ultrasonic cavitation in a rotating exposure chamber

The nucleation and progression of ultrasonic cavitation in a rotating exposure chamber were investigated by observing hemolysis of a 0.5% suspension of erythrocytes. Bursts of 1.6 MHz ultrasound beams of 11 W/cm2 spatial peak intensity were synchronized with the rotation to aim the bursts down the axis of a cylindrical chamber. Cavitational hemolysis always occurred in fresh phosphate buffered saline (PBS) solution, but cavitation incidence declined to 38% in 4 day old PBS. The amount of hemolysis for 11 ms or 44 ms bursts with 60 rpm rotation increased in proportion to the number of bursts. Hemolysis was found above a minimum burst duration of 5.5 to 8.3 ms in the normal 2 cm chamber. This did not appear to change for 1 cm or 4 cm chamber lengths, but it did change to about 2.2 ms and about 44 ms for over- and undersaturated gas conditions, respectively. Off-times associated with rotation speeds of 15 to 30 rpm reduced the effectiveness of the cavitation, and addition of fixed cells at only 0.01% to 0.02% greatly reduced the hemolysis. For this ultrasonic exposure system, several thousand bubbles, possibly generated from as few as one cavitation nucleus, move across the chamber at up to 10 m/s, each lysing a few hundred cells, and then are cycled back to the front of the chamber by the rotation to reinitiate the cavitation on the next burst.