**3. Flow field measurement of the collapse of cavitation bubbles**

It is found that the presence of the solid boundary has distinct influence on the flow field of a pressurized cavitation bubble and its final collapse. A distance parameter γ=d/RMAX (where RMAX is the maximum radius and *d* is the distance between the bubble center and the solid boundary) is assigned to represent the distance from the center of the bubble to the solid boundary. When γ is in the range of 1 3 , counter jet could be observed. However, no counter jet is generated under the condition of γ > 3. The results are described below:
