Disintegration of diminutive liquid helium jets in vacuum

Abstract

The phenomenon of liquid jets disintegrating into droplets has attracted the attention of researchers for more than 200 years. An overwhelming fraction of these studies considered classical viscous liquid jets issuing into ambient atmospheric gases, such as air. Here, we present an optical shadowgraphy study of the disintegration of a cryogenic liquid helium jet produced with a 5 µm diameter nozzle into vacuum. The physical properties of liquid helium, such as its density, surface tension, and viscosity, change dramatically as the jet flows through the nozzle and evaporatively cools in vacuum, eventually reaching the superfluid state. In this study, we demonstrate that, at different stagnation pressures and temperatures, droplet formation may involve spraying, capillary breakup, jet branching, and/or flashing and cavitation. The average droplet sizes produced in this work range from 3.4 × 1012 to 6.5 × 1012 helium atoms or 6.7–8.3 µm in diameter. This paper also reports on the distributions of sizes and shapes of the resulting droplets.

Shapes of rotating normal fluid He3 versus superfluid He4 droplets in molecular beams

“…In contrast to the temperature-independent droplet sizes reported here, previous measurements on 4 He droplets found continuous increases in sizes with decreasing temperature [10]. At T 0 < 4 K and P 0 = 20 bars, 4 He expansion leads to the formation of a jet that breaks up into micron-sized droplets due to Rayleigh instability [10,40,41]. This mechanism gives rise to an extremely collimated beam of droplets, the occurrence of which was not observed during this work with either 014504-3…”

Helium Droplet Mass Spectrometry

“…The He titration method, in which the average He droplet size is deduced from the measured attenuation of a droplet beam colliding with collisional helium atoms, shows an exponential trend of increasing average size of neutral droplets, suggesting the existence of droplets having average diameters in the micrometer range. Such micrometer sized droplets have recently been identified optically [426] by the same group. A possible source for the deviation from titration measurements lies in the charged nature of the helium droplets used in our experiment.…”

Size and Velocity Distribution of Negatively Charged Helium Nanodroplets

“…Depending on the pressure and temperature of the expanding liquid, spraying, jet branching, and flashing have been observed. 38 Particularly, at temperatures below 4 K, the divergence of the beam exceeds 5°. With an acceptance angle of 2°(0.5 mm skimmer opening at 13 mm distance to the nozzle), our skimmer is expected to exclude side jets and parts of the main droplet beam at low temperatures.…”

“…Finally, microscope images have also been used to estimate the size of micrometer-sized HNDs. 37,38 All experiments mentioned above were performed with continuous flow nozzles with diameters typically of a few micrometers. Pulsed nozzles recently became quite popular 39−46 as they provide higher peak flux and larger droplets, and they are less demanding on the pumping speed.…”