Laser-induced cavitation in normal, critical and supercritical Helium*

Laser-induced cavitation dynamics have been thoroughly studied in many liquids under saturation conditions, while in some applications, such as supercritical fluid, cavitation can occur at supercritical conditions.  Supercritical fluids offer two significant features in studying cavitation, surface tension vanishes and the buoyancy force becomes negligible due to the convergence of liquid and gas densities.  Herein, we experimentally investigate laser-induced cavitation dynamics in Helium-4 at temperatures below and higher than critical temperature, these temperatures are sufficiently low to ensure liquid purity and absence of dissolved air.  Our optical-access cryogenic device is described in Speirs et al. [1].  The device uses four windows, with two used for illumination and ultra-high-speed image capturing, with frame rates up to 7 million frames per second.  A different window is used to transmit a pulsed Nd:YAG laser beam, focused by a parabolic mirror, to generate the cavitation bubble.   We measure shock speeds and show cavitation generated by reflected shocks.   The collapse dynamics are compared to the prevailing models, showing significant deviations.   We also show myriad of microbubbles generated in the spinodal regime.