Friction-enhanced lifetime of bundled quantum vortices

TL;DR

This paper explains the unexpectedly long lifetime of bundled quantum vortices in superfluid helium by showing that their collective motion creates a wake in the normal fluid, reducing friction and enhancing their persistence.

Contribution

It introduces a fully coupled dynamics model demonstrating how vortex bundles generate wakes that decrease friction, explaining their extended lifetime.

Findings

Vortex bundles generate large-scale wakes in the normal fluid.

Wakes reduce friction, prolonging vortex bundle lifetime.

Collective effects are analogous to drag reduction in active matter systems.

Abstract

Experiments in the early 1980s have shown that a compact bundle of quantum vortex rings in superfluid helium remains coherent and travels a significant distance compared to its size. This is surprising because a single vortex ring, under the effect of friction with the background of thermal excitations (the so-called normal fluid), would quickly lose energy and decay. The observation of these long-lived vortex structures has remained an unsettled question since their experimental detection. In this work, by taking into account the fully coupled dynamics of superfluid vortices and the normal fluid, we show that the motion of the superfluid bundle generates a large-scale wake in the normal fluid which reduces the overall friction experienced by the bundle, enhancing its lifetime. This collective effect is similar to the drag reduction observed in systems of active, hydrodynamically cooperative agents such as bacteria in aqueous suspensions, fungal spores in the atmosphere and cyclists in pelotons.