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Abstract The motion of quantized vortices is responsible for many intriguing phenomena in diverse quantum-fluid systems. Having a theoretical model to reliably predict the vortex motion therefore promises a broad significance. But a grand challenge in developing such a model is to evaluate the dissipative force caused by thermal quasiparticles in the quantum fluids scattering off the vortex cores. Various models have been proposed, but it remains unclear which model describes reality due to the lack of comparative experimental data. Here we report a visualization study of quantized vortex rings propagating in superfluid helium. By examining how the vortex rings spontaneously decay, we provide decisive data to identify the model that best reproduces observations. This study helps to eliminate ambiguities about the dissipative force acting on vortices, which could have implications for research in various quantum-fluid systems that also involve similar forces, such as superfluid neutron stars and gravity-mapped holographic superfluids. 

Abstract Time dependent observations of point-to-point correlations of the velocity vector field (structure functions) are necessary to model and understand fluid flow around complex objects. Using thermal gradients, we observed fluid flow by recording fluorescence of$${\text{He}}_{2}^{*}$$ ${\text{He}}_2^{\ast }$excimers produced by neutron capture throughout a ~ cm³volume. Because the photon emitted by an excited excimer is unlikely to be recorded by the camera, the techniques of particle tracking (PTV) and particle imaging (PIV) velocimetry cannot be applied to extract information from the fluorescence of individual excimers. Therefore, we applied an unsupervised machine learning algorithm to identify light from ensembles of excimers (clusters) and then tracked the centroids of the clusters using a particle displacement determination algorithm developed for PTV. 

The pioneering work of William F. Vinen (also known as Joe Vinen) on thermal counterflow turbulence in superfluid helium-4 largely inaugurated the research on quantum turbulence. Despite decades of research on this topic, there are still open questions remaining to be solved. One such question is related to the anomalous increase in the vortex-line density L(t) during the decay of counterflow turbulence, which is often termed as the “bump” on the L(t) curve. In 2016, Vinen and colleagues developed a theoretical model to explain this puzzling phenomenon (JETP Letters, 103, 648-652 (2016)). However, he realized in the last a few years of his life that this theory must be at least inadequate. In remembrance of Joe, we discuss in this paper his latest thoughts on counterflow turbulence and its decay. We also briefly outline our recent experimental and numerical work on this topic.