Inhomogeneous nucleation of superfluid vorticity at a sharp edge

We report an experiment in which superfluid flow through a single 10-µm diameter orifice is examined at pressure heads as low as 0.03 dyne/cm². Accurate measurements of low pressure head are made possible by a recirculating flow cryostat, capable of generating a calibrated, continuous flow of superfluid helium. Current vs. potential data for temperatures between 1.46 and 2.14 K are analyzed according to the Iordanskii-Langer-Fisher thermal nucleation theory, modified to apply to a model in which vortex half-rings are inhomogeneously nucleated at the sharp-edged mouth of the orifice. We offer two possible interpretations of the results.Parts of this work were carried out with the support of National Science Foundation grants DMR-72-02971 and DMR-78-10778.