Laser-induced spherical shocks generated in water near a free surface (liquid-air) break through the boundary layer and spread out as an elliptical or a cylindrical shock wave in air. After break through, the shock wave is accelerated and its velocity in air is higher than the shock velocity in liquid during the first 100 ns.
After some μs both shocks propagate with sound velocities in liquid and in air.
The initial laser-induced spherical shock propagates in liquid and breaks through the phase boundary as mentioned before. But also a rarefaction wave is reflected from the boundary layer back into the liquid and the decreasing pressure may generate heterogeneous vapor bubbles.
It is well known that a single laser-induced breakdown in liquid does not only produce a strong spherical shock (typically ns time scale) but also a cavitation bubble (typically μs time scale) following the shock. As a consequence, both the initial shock and the cavitation bubble propagate through the boundary layer.
Both phenomena are studied by observing the boundary layer simultaneously from below and above the water surface with two synchronized CCD cameras. Two different cases with a boundary layer free in three dimensions and a boundary layer in a small glass tube (nearly 1-dimensional free) are analyzed. The multiple illuminations are provided by a double-cavity Nd:YAG laser with an exposure time of 6 ns.
