Collapse of micrometer-sized cavitation bubbles near a rigid boundary

The interaction of cavitation bubbles with a rigid boundary and its dependence on the distance between bubble and boundary is investigated experimentally. Individual cavitation bubbles, with a maximum radius of 150 mu m, are generated by using pulsed high-intensity focused ultrasound. Observations are made with high-speed photography with framing rates of up to 200 million frames per second and exposure time of 5 ns, and the spatial resolution is in the order of a few micrometers. The significant parameter of this study is the non-dimensional stand-off parameter, gamma, defined as the distance between the ultrasound focus and the rigid boundary scaled by the maximum bubble radius. Both the velocity of the liquid jet developed during bubble collapse and the maximum pressure of the shock wave emitted during bubble rebound show a minimum for gamma approximate to 1 and a constant value for gamma > 3. The maximum jet velocity is slightly smaller than the corresponding values obtained in the case of millimeter-sized bubbles and ranges from 80 m/s (at gamma approximate to 1) to 130 m/s (for gamma > 3). No jet formation was observed for gamma > 3. The shock wave pressure, measured at a distance of 5 mm from the emission center, ranges from 0.2 MPa (at gamma approximate to 1) to 0.65 MPa (for gamma > 3). These values are an order of magnitude smaller than those obtained in the case of millimeter-sized bubbles. The shock wave duration is almost independent of gamma at a value of about 75 ns. For large gamma values (gamma > 3), a large percentage of the bubble energy (up to 60 %) is transformed into the mechanical energy of the shock wave emitted during bubble rebound but, for gamma approximate to 1, the conversion efficiency decreases to 30 %. Independent of the relative distance between bubble and rigid boundary, the shock pressure decays proportionally to r(-1) with increasing distance r from the emission center. The results are discussed with respect to cavitation damage and collateral effects in pulsed high-intensity focused ultrasound surgery.