Features of Laser-Induced Thermocavitation of Water

The features of thermocavitation of water near a fiber tip under its heating by continuous laser radiation at a wavelength of 1.94 μm have been studied. Dynamic processes have been studied using optical and acoustic methods. It has been established that the pressure pulses at the initial section of thermocavitation determined by the explosive boiling of water are significantly lower compared to the pressure pulses during the collapse of vapor-gas bubbles. The spectrum of a generated acoustic signal extends over 10 MHz, while the spectral distributions of the lowest frequency and highest frequency fluctuations are described by the 1/f law. It has been shown that the peak powers of the pressure pulses in individual instances of thermocavitation are related to their repetition rates by the dependence ~1/f^1.4. Wavelet analysis shows that in the course of thermocavitation, an alternation of “random” and “cascade” processes is observed. In a special acoustic experiment, it has been found that at the initial stage of thermocavitation, the pressure rise occurs within approximately 250 ns. The relatively long increase in pressure is explained by the fact that explosive boiling occurs at many points in the volume of a superheated liquid, and the chain reaction of the sequential appearance of critical nuclei is determined by the propagation of shock waves.