SuperfluidQuantum Fluid at Near Absolute Zero

Superfluid flows without friction, but only part of them. The rest is ordinary, and the temperature decides the split.

Cool liquid helium-4 below 2.17 kelvin and it changes into a phase called helium II. In 1938 Pyotr Kapitsa in Moscow, and John Allen and Don Misener in Cambridge, published side by side in Nature: helium II pours through gaps far too narrow for any normal liquid, as if it had no viscosity at all. Yet other experiments, measuring the drag on a body moving through the liquid, found a viscosity that was clearly not zero.

The resolution is the two-fluid picture, proposed by László Tisza in 1938 and developed by Lev Landau. Helium II behaves as if it were two interpenetrating liquids. The superfluid component has exactly zero viscosity and carries no entropy. The normal component is viscous and carries the heat. Narrow channels let only the superfluid through; a moving object is dragged by the normal part. As the temperature falls toward absolute zero, the superfluid fraction grows toward all of it.

Even the superfluid has limits. Landau showed it flows without dissipation only below a critical speed, the first formula on this page: below it, there is no way for the moving fluid to create an excitation and lose energy. Kapitsa received the 1978 Nobel Prize, Landau the 1962 prize.

The second formula is the strangest. Try to set the superfluid spinning and it refuses to rotate smoothly. Its circulation can only take whole-number multiples of Planck's constant divided by the mass of a helium atom, about 10⁻⁷ square metres per second. It rotates by filling with tiny quantized whirlpools, vortices, and in 1961 W. F. Vinen detected single quanta of circulation.

Superfluid never slows down. The super part, anyway, and only below the speed limit.