The motion of tracer particles in turbulent superfluid $^4$He down to the zero-temperature limit

TL;DR

This study visualizes and analyzes the motion of tracer particles in superfluid helium at very low temperatures, revealing complex behaviors influenced by quantum vortices and thermal excitations.

Contribution

Developed a new injection and imaging system for tracer particles in superfluid helium, enabling detailed statistical analysis of particle dynamics down to zero temperature.

Findings

Velocity distributions are Gaussian with power law tails at all temperatures.

Particle trajectories range from smooth to erratic, indicating interactions with vortices.

Particles can become trapped in vortex tangles, affecting flow behavior.

Abstract

An injection system for polymer particles, with diameters ranging from 1 to 6 $\mu$m, has been developed for visualizing flows in superfluid $^4$He at temperatures down to 0.14 K. Using an ultrasound transducer, bursts of particles were launched into a sample of superfluid and allowed to descend under gravity. The particles were imaged using their fluorescence in the presence of a sheet of laser light. We report on the statistical behavior of particles during their descent, including descriptions of a mixture of smooth and erratic trajectories, indicative of the interactions with thermal excitations and quantized vortex lines. Temperature-dependent velocity distributions were measured and analyzed, yielding Gaussian distributions with power law tails persisting into the zero temperature limit. When sampled over increasing length scales, these distributions bifurcated into exponential for the smallest particles and bimodal Gaussian for the largest. We also report observations of long-lived suspensions of small particles at temperatures near 1 K, which appear to be associated with the trapping of large numbers of particles in a turbulent vortex tangle. A method was developed for identifying and quantifying the numbers of particles bound to vortex lines, allowing for a description of the temporal dynamics of their population by an analytical model.