Visualization study of counterflow in superfluid helium-4 using metastable helium molecules

TL;DR

This study visualizes the normal fluid flow in superfluid helium-4 during counterflow, providing evidence of turbulence at high velocities using metastable helium molecules and laser-induced fluorescence.

Contribution

It introduces a novel visualization technique for the normal fluid in superfluid helium-4 and confirms turbulence in the normal component at high velocities.

Findings

Normal fluid flow becomes turbulent at high velocities.

Visualization technique successfully images metastable helium molecules.

Evidence supports turbulence in the normal component during counterflow.

Abstract

Heat is carried in superfluid He-4 by the motion of the normal fluid$^{1}$, a counterflowing superfluid component serving to eliminate any net mass flow. It has been known for many years that above a critical heat current the superfluid component in this counterflow becomes turbulent. This turbulence takes the form of a disorganized tangle of quantized vortex lines and is maintained by the relative motion of the two fluids$^{2-3}$. It has been suspected that the normal fluid may also become turbulent$^{4}$, but experimental verification is difficult without a technique for visualizing the flow. Here we report a series of visualization studies on the normal-fluid component in a thermal counterflow performed by imaging the motion of seeded metastable helium molecules using a laser-induced-fluorescence technique$^{5-8}$. We present evidence that the flow of the normal fluid is indeed turbulent, at least at relatively large velocities. Thermal counterflow in which both components are turbulent presents us with a new and theoretically challenging type of turbulent behaviour.