I've been reading up on Superfluids, and they've fascinated me. I understand why their superfluid component has zero viscosity, but there's one aspect that's bothering me, and that's the formation of so called "vortices". Why is their existence necessary?
The superfluid can be in a state with zero vortices. But if one imparts angular momentum to the superfluid, then the angular momentum is physically expressed either by the superfluid rotating as a whole, or by the presence of vortices.
In an ordinary fluid vortices can also appear of course, but they get damped away by viscosity. This is not so for the superfluid: the vortices persist and they are quantized in angular momentum (owing to the fact that the phase increment around a loop has to be a multiple of $2\pi$, and with non-zero angular momentum there is a term $\exp(i m \phi)$ in the wavefunction).