What Is a Liquid With No Friction? Helium Climbing Out of Its Cup

Helium stays liquid all the way to absolute zero under ordinary pressure, which is already unusual, since every other substance freezes. Cooled below about two degrees above absolute zero, it undergoes a transition that is visible to the eye, because the vigorous boiling that has been going on suddenly stops and the liquid becomes perfectly still. Below that point it flows through openings too small to admit any ordinary liquid, passes around obstacles without losing energy, conducts heat enormously better than any metal, and forms a film that creeps up and over the wall of any open container until the vessel empties itself. These are not exaggerations of ordinary behaviour but a different regime.

The catalogue of behaviours is unlike anything in ordinary experience:

• •Flow through channels narrow enough to block any normal liquid entirely
• •A film about thirty nanometres thick creeping over any barrier it touches
• •A fountain effect, where gentle heating drives a jet upwards through a fine plug
• •Heat conducted as a wave rather than by diffusion
• •Vortices whose circulation comes only in fixed amounts rather than any value
• •A stirred vessel that keeps rotating indefinitely with no measurable slowing

The explanation rests on a quantum effect that normally applies only to individual particles becoming visible in a bulk liquid. Below the transition, a large fraction of the atoms occupy the same lowest energy state simultaneously and are described by a single shared wave rather than as independent particles. Losing energy to friction would require the whole population to change state together, which the available small disturbances cannot accomplish, so the flow continues undiminished. The liquid is usefully described as two interpenetrating fluids, one behaving normally and one with no viscosity at all, whose proportions change with temperature, and that two fluid picture accounts for most of the strange observations directly.

The discovery took four decades from the first liquefaction and the story is worth following. Helium was liquefied in 1908 by Heike Kamerlingh Onnes in Leiden, which was a major achievement in itself and won him a Nobel Prize, and he noticed the odd cessation of boiling below a certain temperature without pursuing what it meant. The transition point was mapped over the following years and named after a letter whose shape the plotted heat capacity resembles. Frictionless flow was demonstrated independently in 1937 by Pyotr Kapitsa in Moscow and by John Allen and Don Misener in Cambridge, whose papers appeared in the same issue of the same journal. Theoretical explanation followed through the 1940s, principally from Lev Landau, and Kapitsa received a Nobel Prize for the work in 1978.

This is not solely a curiosity, since the same underlying physics turns up in several important places. Superconductivity, where electrical resistance vanishes entirely, is the same phenomenon occurring in the electrons of a metal rather than in a liquid, and understanding one advanced understanding of the other directly. The condensates produced in ultracold atomic gases since 1995 are the same state made in a controlled and adjustable form, and they have become a laboratory for testing quantum theory. Neutron stars are thought to contain matter in this state, which is invoked to explain the sudden speed changes those stars occasionally show. Practical uses include cooling systems for the magnets in particle accelerators and extremely sensitive rotation sensors.

Cooled below about two degrees above absolute zero, liquid helium stops boiling, flows without resistance, conducts heat as a wave and creeps over container walls as a thin film. A large fraction of its atoms share one quantum state, so losing energy to friction would require them all to change together. The same physics underlies superconductivity and ultracold atomic gases.