Vortex pinning by surface geometry in superfluid helium

TL;DR

This study investigates how vortex lines in superfluid helium interact with surface bumps, revealing multiple pinning sites and less frequent pinning than predicted, which impacts understanding of vortex-surface interactions.

Contribution

It provides experimental insights into vortex pinning behavior on macroscopic surface features, challenging previous computational predictions and highlighting complex pinning dynamics.

Findings

Multiple pinning sites observed along the bump surface.

Pinning is less frequent than computational models suggest.

Vortices can pass near or through the bump without pinning.

Abstract

We present measurements of how a single vortex line in superfluid helium interacts with a macroscopic bump on the chamber wall. At a general level our measurements confirm computational work on vortex pinning by a hemispherical bump, but not all the details agree. Rather than observing a unique pin location, we find that a given applied velocity field can support pinning at multiple sites along the bump, both near its apex and near its edge. We also find that pinning is less favorable than expected. A vortex can pass near or even traverse the bump itself with or without pinning, depending on its path of approach to the bump.