Visualization of Superfluid Helium Flow

TL;DR

This paper presents an experimental visualization technique for superfluid helium flow using micron-sized hydrogen tracers, confirming the two-fluid model and analyzing tracer-vortex interactions.

Contribution

It introduces a novel method to visualize superfluid helium flow and provides quantitative analysis of tracer behavior and normal fluid velocity dependence.

Findings

Confirmed the two-fluid model through direct visualization.

Analyzed tracer trapping mechanisms by quantized vortices.

Quantified normal fluid velocity dependence on heat flux.

Abstract

We discuss an experimental technique to visualize the motion in the bulk of superfluid $^4$He by tracking micron-sized solid hydrogen tracers. The behavior of the tracers is complex since they may be trapped by the quantized vortices while also interacting with the normal fluid via Stokes drag. We discuss the mechanism by which tracers may be trapped by quantized vortices as well as the dependencies on hydrogen volume fraction, temperature, and flow properties. We apply this technique to study the dynamics of a thermal counterflow. Our observations serve as a direct confirmation of the two-fluid model as well as a quantitative test of the normal fluid velocity dependence on the applied heat flux.