Conservation of energy in the oscillations of laser-induced cavitation bubbles

Abstract

The oscillations of Nd:YAG laser-induced cavitation bubbles close to a free liquid surface are studied using a Mach–Zehnder interferometer and high-speed photography. For an initial bubble depth of 2.6 mm and a laser energy of 5.1 mJ, three complete expansion cycles are observed. During the second cycle, a high-speed liquid jet is directed away from the free surface and the whole bubble is displaced downwards. Particular attention is paid to the production of acoustic transients during the initial and subsequent expansions of the cavitation bubbles. Optical measurements of the changes in refractive index of the liquid yield the pressure profiles of these acoustic waves. The respective energies of the four transients are found to be 101, 59, 55, and 2 μJ; the energies of the three cavity expansions are 640, 350, and 70 μJ. By conservation of energy, it follows that the thermal energies given out during the initial supersonic expansions of the second and third acoustic waves are 230 and 220 μJ, respectively. These are large proportions of the initial shock-wave energies.