Friction-enhanced lifetime of bundled quantum vortices

Some physical systems consist of components which interact with each other not only

directly, but also indirectly by changing the common background. A typical example

of this interplay is hydrodynamic cooperation characterising systems of active agents

such as aqueous suspensions of self-propelled bacteria, fungal spores dispersed in air, road

racing cyclists in pelotons and particle pairs trapped in optical vortices; in these systems

self-organized structures emerge from collective energy-saving mechanisms.

Here we report a similar collective effect existing in superfluid helium at finite temperatures.

By performing cutting-edge numerical simulations of superfluid helium dynamics, we show

that a toroidal bundle of quantized vortex rings generates a large-scale wake

in the normal fluid which reduces the overall friction experienced by the bundle, thus

greatly enhancing its lifetime, as observed in experiments.

This remarkable collective effect - the reduction of dissipation via

hydrodynamic cooperation - displays similar features than the ones observed in particle drafting in optical

vortices. Superfluid helium can hence be considered as a peculiar kind of active fluid,

where hydrodynamic interactions determine the characteristics of turbulence in both superfluid and normal fluid components.