Vorticity in Liquid He II Flow through Orifices

Recent experiments in wide channels have shown a marked difference between the critical velocity observed in pure superfluid flow and thermal counterflow; because of the lack of a satisfactory hydrodynamical theory, the reason for this difference remains obscure. In the present work we have investigated the onset of superfluid vorticity for these two types of flow through small orifices of some tenths of a millimeter i.d., by means of the technique of trapping of negative ions. Our results show that the same difference is also observed in our much simpler geometry; namely, we find the same paradox already found in channels, that is the law $v_{s,c}d=\mathrm{const}$, expected to hold in pure superfluid flow, is instead followed only in the thermal counterflow, and the effect is geometry-independent.