superfluid film
/ SOO-per-FLOO-id FILM /
Spill a little water and it stays in its puddle; it has no urge to climb the walls and escape. Superfluid helium is not so well behaved. Leave a beaker of it sitting in a cold chamber and a ghostly thin layer creeps silently up the inside wall, over the rim, and down the outside, until the beaker has quietly drained itself dry.
A superfluid film is that incredibly thin sheet of superfluid — only a few dozen atoms thick — that coats every surface it can reach. Ordinary liquids cling to walls too, but their own stickiness pins them in place. A superfluid has no internal friction, so the gentle attraction between the liquid and the wall is enough to make the film flow freely along the surface, carrying liquid uphill and over any barrier in its path until levels everywhere equalize.
It matters as a striking everyday proof of zero viscosity, and it is also a real nuisance: the creeping film lets liquid helium escape from any open container and is a notorious source of heat leaks in cold experiments. The common misconception is that the film is pulled up by some special force unique to superfluids; the lifting force is just ordinary surface attraction, and it is the total absence of friction that lets such a feeble pull move the liquid so dramatically.
Lift a small cup brimming with superfluid helium out of its bath and the level slowly sinks all by itself — the liquid climbs the walls as an invisible film and drips off the bottom, refusing to stay put inside any open vessel.
A lifted cup of superfluid empties itself: the film climbs out over the rim.
Because the film is so thin, only a limited amount of superfluid can flow through it per second, which sets a maximum creeping rate — so the cup empties at a steady, measurable pace rather than instantly.