Definition: A
superfluid is a special phase of matter in which, when cooled to temperatures near
absolute zero, the molecules exhibit strange quantum effects.
Some superfluids, such as helium-4 (helium with 4 nucleons - 2 protons & 2 neutrons), are bosons and therefore form a Bose-Einstein condensate when cooled into liquid form.
One effect of this is that the viscosity of superfluid helium-4 becomes zero, meaning that normal rules of surface tension, such as capillarity, are no longer obeyed. A superfluid in a glass tube will literally "crawl" up the side of the tube in a thin film because of this property.
Other superfluids, such as helium-3, are fermions, but they can also exhibit superfluid properties due to other quantum effects that are part of the BCS theory of superconducitivity.
| 