Ejection of Metal Particles into Superfluid ⁴He by Laser Ablation

The dynamics of laser ablation of a metal target immersed in superfluid 4He is studied through time-resolved shadowgraph photography. Delayed ejection of hot micrometer-sized particles from the target surface into the liquid was indirectly observed by monitoring the formation and growth of gaseous bubbles around the particles. The experimentally determined particle average velocity distribution appears to be similar to that previously measured in vacuum but exhibits a sharp cutoff at the speed of sound in the liquid. The propagation of the subsonic particles terminates in slightly elongated nonspherical gas bubbles residing near the target, whereas faster particles reveal an unusual hydrodynamic response in the liquid. On the basis of the previously established semiempirical model developed for macroscopic objects, the ejected transonic particles exhibit a supercavitating flow to reduce their hydrodynamic drag. Supersonic particles appear to follow a completely different propagation mechanism as they leave discrete and semicontinuous bubble trails in the liquid. The relatively low number density of the observed nonspherical gas bubbles indicates that only large micron-sized particles are visualized in the experiments. Although the unique properties of superfluid helium allow detailed characterization of these processes, the developed technique can be used to study the hydrodynamic response of any liquid to fast-propagating objects on the micrometer scale.