Sub-micron solid air tracers for quantum vortices and liquid helium flows

Abstract

The dynamics of quantized vortices in superfluids has received increased attention recently because of novel techniques developed to visualize them directly. One of these techniques [G. P. Bewley et al., Nature 441, 588 (2006)] visualized quantized vortices and their reconnections in superfluid flows of (4)He by using solid hydrogen tracers of micron-size or larger. The present work improves upon the previous technique by using substantially smaller particles created by injecting atmospheric air diluted in helium gas. These smaller particles are detectable thanks to the higher index of refraction of nitrogen compared to hydrogen and thanks to an improved visualization setup. The optical counting estimate, which agrees with terminal velocity estimates, suggests that the tracer diameter is typically 400 ± 200 nm and could be as small as 200 nm; being smaller, but not so small as to be influenced by thermal motion, the particles get trapped on the vortices faster, perturb the vortices less, possess smaller Stokes drag, and stay trapped on fast-moving vortices, as also on vortices generated closer to the superfluid transition temperature. Unlike the past, the ability to create particles in the superfluid state directly (instead of creating them above the λ-point and cooling the fluid subsequently), ensures greater temperature stability for longer periods, and enables the tracking of long and isolated vortices. These advantages have also led to the direct visualization of Kelvin waves. The use of other seed gases could lead to the visualization of even smaller tracers for quantized vortices. We discuss the visualization setup and provide suggestions for further improvement.

Reconnection scaling in quantum fluids

“…34 and is described in detail in ref. 33. It consists of an optical cryostat filled with liquid helium in which we inject a dilute mixture of atmospheric air in helium gas.…”

“…Compared with these previous experimental studies we use a visualization technique that uses frozen air tracers which are much finer, described briefly in Materials and Methods and in detail in ref. 33. The technique, previously used to visualize kelvin waves (34), uses submicrometer particles injected directly in the superfluid phase, permitting the visualization of longer and straighter vortices.…”

Non-isothermal physical and chemical processes in superfluid helium

“…Indeed the dense packing of atoms in the nanowires "large" (in comparison with vortex core's thickness) diameter, and the presence of metallic spheres with the perfect shape and atomically smooth surface in the products were observed [16]. Our model of nanowire formation in He II based on the assumption of adiabatic regime of the coagulation process that leads to the cluster melting [16] has been recognized in several independent studies [21,22]. Nevertheless, we decided primarily to perform the systematic comparison of diameters of the nanowires grown in a low temperature experiments from various metals with those predicted for them by the model [16].…”

An apparatus for generation and quantitative measurement of homogeneous isotropic turbulence in He ii

“…Previous work has shown that a combination of low condensible gas partial pressure and low flow rate produces solidified tracer particles with size less than one micron in He ii. 20 Therefore, a small quantity of deuterium is added to the evacuated mixture cylinder, which is then charged to 10 psig with helium gas. The mixture is supplied at a regulated pressure of 600 mbar and enters the delivery tube through a metering valve (flow coefficient C v < 0.003).…”

“…Visualization of grid turbulence has been successfully applied to He i, 18,19 the classical liquid phase, but applying the same to He ii requires a number of non-trivial considerations. For example, new methods for introducing passive tracer particles into He ii reduce unwanted heating and turbulence 20 and incorporating second sound attenuation would provide a more comprehensive measurement of the fluid behavior. Additionally, the magnetic coupling between the grid and drive system used for He i grid turbulence 18 is unlikely to provide careful control of the grid motion.…”