This is taken from the excellent PBS/BBC documentary Absolute Zero, which you can view in its entirety on YouTube. This particular segment concerns the strange properties of helium at temperatures just above absolute zero.
Helium becomes liquid at around 4 K. Below 2.18 Kelvin, or -270.97 degrees Celsius, it turns into a superfluid. Now, ordinary fluids always have some measure of viscosity, or resistance to flow: if you drop a heavy object into water, it’ll make a big splash and sink relatively quickly; in thicker fluids like honey, it will make a smaller splash and sink more slowly, since honey has higher viscosity. A superfluid has zero viscosity. In the video, you see an “eternal fountain” of helium that keeps flowing until the temperature goes above 2.18 Kelvin, and the way a thin film of liquid will escape unsealed containers, seemingly defying gravity.
Superfluids have another interesting property: something called second sound. In superfluid helium, heat transfer doesn’t occur in the usual way, by diffusion. Rather, it occurs in a wave-like fashion: where pressure waves make up sound, waves of entropy make up second sound. This allows extremely high thermal conductivity: heat flows through superfluid helium a thousand times faster than through copper. There is no known material that can conduct heat faster than superfluid helium.
The entire program is well worth a watch. It starts four hundred years ago, with the first scientific theories of just what heat is, and moves up to present day experiments at tiny fractions of a degree above absolute zero. It runs 1h 40 min, but if you don’t have that much time, the above clip is one of the highlights.
I LOVE this video, I saw it my freshman year and now I’m working towards making this subject my life’s work.