Fluid Flow and Depinning of a Metastable Vortex

We observe a vortex line pinned around a straight wire in superfluid helium, in the absence of external rotation. Unperturbed, the vortex line remains pinned indefinitely, but we can partially detach the vortex by heating the fluid in brief stints. Here we show that a key property of the heating cycle is the maximum rate of change of the temperature, which suggests that the fluid velocity generated by a temperature gradient plays an important role in the depinning. Our measurements of how depinning depends on both maximum temperature and velocity show a crossover from a velocity-dependent depinning temperature at slow heating rates to a velocity-independent temperature near 1.2 K for faster heating. We discuss how vortex loops generated by the thermal fluid flow may be responsible for the depinning.