The analysis of cavitation bubble dynamics by optical on-line monitoring

The formation and the dynamics of cavitation bubbles were investigated when applying pulsed mid-IR laser radiation in water. HeNe laser light reflected at the distal application fiber end was measured in order to probe refractive index changes during ablation. A Cr:Tm:YAG laser, λ = 2.01 μm, was operated in the free running and the Q-switched mode. The pulses were transmitted through a 400 μm low-OH quartz fiber into a water filled cuvette. In the Q-switched mode the reflected HeNe-laser power was found to be nearly constant during the cavitation bubble lifetime, whereas in the free running mode, transient changes of the reflected power were measured. Initial bubble wall velocities of 20 m/s in the free running mode and about 330 m/s in the Q-switched mode could be estimated using pulse energies of 110 mJ and 18 mJ, respectively. The lifetime of the cavitation bubble in both cases was measured to be about 300 μs. Relative to the reflected power when the fiber tip was in air, it was significantly lower during the lifetime of the bubble. The results indicate that condensation or sublimation of steam takes place on the fiber tip owing to low vapor temperature within the bubble. Rapid cooling due to adiabatic steam expansion and the Joule-Thompson effect at the onset of bubble formation are the most likely explanations for this unexpected result. A simple gas kinetic model predicts temperature gradients in the order of 200°C within the first microseconds. In conclusion, the optical on-line monitoring described is an excellent tool to investigate the kinetics of ablation in any medium in vitro as well as in vivo.