Visualization of acoustic streaming produced by lithotripsy field using a PIN method

We visualized the acoustic streaming produced in water by an experimental lithotripter using a particle image velocimetry (PIV) method. Streaming generated around the beam focus has been optically visualized using light scattering particles and was easily noticeable even with naked eye for all electrical settings of the lithotripter. Spatial distributions of velocity vectors are complicated and several local peaks and vortices are observed. Measured streaming velocities are found to be in ranges of 1.5 - 3 cm/s. It should be noted that the measured velocity was averaged over 1/30 sec, the time resolution limited by video frame rate, and the true velocity is expected to be at least 10 times higher. Despite such an underestimation, it was shown that the streaming velocity increased with voltage settings and, as predicted by theory, is proportional to intensity and closely related to the shock-wave pressures generated. In particular, the velocity has almost a linear correlation with peak-negative pressures (r=0.98683, p=0.0018). This suggests that the streaming velocity measured using the PIV technique can be used to estimate the generated peak-pressures without disturbing the field.