superfluid

Superfluidity is a phase of matter in which a fluid exhibits zero viscosity, allowing it to flow without dissipating kinetic energy. This behavior arises from quantum effects that become significant at very low temperatures and in certain liquids that form Bose-Einstein-like condensates.

In helium, superfluidity appears most famously in helium-4 below its lambda point at about 2.17 kelvin. Helium-3, a fermion, becomes superfluid at millikelvin temperatures through Cooper pairing to form bosonic pairs. The phenomenon was first observed in helium-4 by Kapitza, Allen, and Misener in 1937; helium-3 superfluidity was discovered in 1972 by Osheroff, Richardson, and Lee.

A widely used description is the two-fluid model, where the liquid consists of a normal component with viscosity and a superfluid component that flows without friction. The two components can move with different velocities, subject to mutual friction. Vorticity in a superfluid is quantized, so circulation occurs in discrete units, and the flow is largely irrotational except at quantized vortex lines. The Landau criterion establishes a critical velocity above which dissipation reappears.

Key phenomena include second sound (a temperature or entropy wave) in addition to ordinary density waves, persistent currents in ring geometries, and special surface effects such as the Rollin film and the fountain effect, where a superfluid can climb against gravity in a capillary.

Microscopically, helium-4 forms a Bose-Einstein condensate; helium-3 requires fermionic pairing to behave collectively as a superfluid. Beyond helium, superfluid behavior is observed in ultracold atomic gases and is studied for insights into quantum turbulence and astrophysical contexts such as neutron star interiors.