A fluid, such as a liquid form of helium, exhibiting a frictionless flow at temperatures close to absolute zero.
'Superfluid' is a state of matter in which the matter behaves like a fluid without viscosity and with extremely high thermal conductivity. The substance, which appears to be a normal liquid, will flow without friction past any surface, which allows it to continue to circulate over obstructions and through pores in containers which hold it, subject only to its own inertia. Since even gasses have viscosity, superfluids have less resistance to shear than a gas does.
Despite its lack of viscosity, the liquid still has surface tension, which allows it to rise up the sides of its containers without any normal frictional restrictions to flow. This allows the liquid to flow up the sides of containers, over the top, and down to the same level as the surface of the liquid inside the container, in a siphon effect.
These unusual effects are observed when liquids, typically helium-4, overcome friction in surface interaction at a stage known as the "lambda point". This is the temperature and pressure at which the liquid's viscosity becomes zero. Although liquid helium forms at 4.2 kelvin at 1 atmosphere, it does not become a superfluid until it is cooled below its lambda point at 2.18 kelvin.
Known as a major facet in the study of quantum hydrodynamics, the superfluidity effect was discovered by Pyotr Kapitsa, John F. Allen, and Don Misener in 1937. It has since been described through phenomenological and microscopic theories. The formation of the superfluid is known to be related to the formation of a Bose-Einstein condensate. This is made obvious by the fact that superfluidity occurs in liquid helium-4 at far higher temperatures than it does in helium-3. Each molecule of helium-4 is a boson particle, by virtue of its zero spin. Helium-3, however, is a fermion particle, which can form bosons only by pairing with itself at much lower temperatures, in a process similar to the electron pairing in superconductivity.
In the 1950s, Hall and Vinen performed experiments establishing the existence of quantized vortex lines in superfluid helium. In the 1960s, Rayfield and Reif established the existence of quantized vortex rings. Packard has observed the intersection of vortex lines with the free surface of the fluid, and Avenel and Varoquaux have studied the Josephson effect in superfluid helium-4. Wikipedia