Inertial cavitation produced by pulsed ultrasound in controlled host media

The purpose of this work is to provide measurements in well-characterized media in order to better understand inertial (transient) cavitation phenomena. Focused transducers with megahertz center frequencies (2.5 MHz, 4.3 MHz) and a clinical diagnostic ultrasound system (4.0 MHz) was used to generate pulsed ultrasound to induce cavitation. An improved active cavitation detector which utilizes a highly focused transducer with much higher center frequency (30 MHz) was used to measure the threshold of inertial cavitation. In order to study the effect of the concentration of nucleation agents on cavitation thresholds, experiments were conducted in distilled water seeded with polystyrene particles. Inertial cavitation thresholds were measured for various concentrations of polystyrene particles. It was observed that the threshold decreased from 2.5 MPa at concentration of about 10(6) particles/ml to 1.6 MPa at a concentration of about 10(9) particles/ml. The effect of the concentration is not significant for smaller changes of concentration. Measurements of the cavitation thresholds were then made in specially developed phantom materials to study the effect of viscosity on the cavitation threshold when surface tension and other mechanical properties of the materials are kept relatively constant. Experimental results show that the threshold increases with increasing viscosity, consistent with theoretical predictions. Cavitation was also detected in water seeded with polystyrene particles using a clinical ultrasound system at an acoustic pressure of 3.84 MPa. Results are comparable with those obtained in the laboratory using a 4.3-MHz focused transducer.