Micrometer-sized droplets from liquid helium jets at low stagnation pressures

Droplets and droplet beams produced from the breakup of micrometer-sized liquid helium jets in vacuum were studied in this work, advancing into previously unexplored regimes of low stagnation pressures. Using a 5 mu m orifice, the droplet beam shows surprisingly diverse characteristics at increasing nozzle pressures from 0.6 to 100 bar: a well-collimated beam at low stagnation pressures, a spray at some intermediate values, and a less-collimated beam at high pressures. Focusing on a nozzle stagnation of 0.6 bar and 2.7 K, we highlight the spectrum of jet disturbances, resulting in different droplet beam behaviors. On some occasions, we observed uniformly sized and equidistant droplets with diameters ranging from 11 up to more than 25 mu m and separations from 15 to 100 mu m. From simple estimates using the ratio between the droplet separations and diameters, we determined the disturbance frequencies benchmarking the production of repeatable targets for future experiments with superfluid helium droplets. Further analysis of the droplet beam behavior at farther distances from the nozzle revealed that the droplet diameter grew downstream up to 22 mu m from an initial value of 13 mu m, while their aspect ratio decreased from 1.33 to 1.16. These results indicate that droplet coagulation and superfluidity both influence the droplet beam up to several hundreds of millimeters after the nozzle exit.

Automated summary

This study examines droplets and droplet beams produced from liquid helium jets in vacuum at low stagnation pressures. It discovers diverse characteristics of the droplet beam at different nozzle pressures, and highlights the influence of disturbance frequencies on droplet beam behavior. The study also reveals the effects of droplet coagulation and superfluidity on droplet properties.