superfluid helium-4
/ SOO-per-FLOO-id HEE-lee-um FOR /
Helium is the gas that makes party balloons float and squeaky voices funny. Cool it down far enough and it turns into a clear, runny liquid — and then, just below about two degrees above absolute zero, something astonishing happens: the liquid stops behaving like any liquid you have ever seen and starts flowing without the slightest drag.
Superfluid helium-4 is ordinary helium-4 liquid that has crossed this threshold. Helium-4 atoms are bosons, particles happy to pile into one quantum state, and when the liquid gets cold enough a large fraction of the atoms condense into that single shared state. The condensed part flows with zero viscosity, conducts heat fantastically well, and famously creeps as a thin film up the walls of its container to siphon itself out over the top.
This was the first superfluid ever discovered, in 1938, and it remains the textbook example because helium stays liquid all the way down to absolute zero instead of freezing. A common confusion is to call the whole liquid 'the superfluid'; below the transition the liquid is best pictured as a mixture of a normal, sticky component and a frictionless superfluid component, with the superfluid fraction growing as you cool further.
Stand a small open beaker partly inside a bath of superfluid helium and the liquid will silently climb the walls as an invisibly thin film, fill the beaker, and then drip out the bottom — emptying or filling itself until the inside and outside levels match.
The creeping film of superfluid helium-4 climbs walls to seek its own level.
The temperature where helium-4 becomes superfluid is called the lambda point, named for the Greek-letter shape of the spike its heat capacity makes there; the change is gradual and continuous rather than a sudden freeze or boil.