Adam M. Calabrese
Natl. Ctr. for Phys. Acoust., Univ. of Mississippi, University, MS 38677
Ronald A. Roy
Univ. of Washington, Seattle, WA 98105
The threshold pressure for the onset of transient acoustic cavitation due to high-frequency, pulsed ultrasound has been shown to be dependent on the frequency and initial nucleus size. Therefore, assuming that a range of optimally sized nuclei are present in a fluid, the amount of cavitation one gets depends on the applied acoustic pressure and the frequency. This idea has been incorporated into the concept of the ``cavitation index'' [Apfel and Holland, Ultrasound Med. Biol. 17, 179--185 (1991)], which specifies the conditions below which cavitation should not occur. However, the influence that time-averaged effects, such as rectified diffusion, have on the cavitation threshold has not been extensively studied yet. A set of laboratory experiments designed to study these effects has been performed. Clean water is seeded with polystyrene spheres (to act as nucleation sites), and the cavitation threshold is measured using a modified version of a passive detection technique [Atchley et al., Ultrasonics 26, 280--285 (1988)]. The dependence of the threshold pressure on duty cycle (0.1%--10%) and pulse length (about 3--100 cycles) is presented at frequencies of 1, 2.25, and 5 MHz. Results are interpreted in light of existing theories for bubble dynamics. [Work supported by the NIH.]