Editing Superfluid helium-4 (section)

== Properties ==
{{see also|Helium#Helium II|superfluidity}}
Superfluids, such as helium-4 below the lambda point (known, for simplicity, as '''helium II'''), exhibit many unusual properties. A superfluid acts as if it were a mixture of a normal component, with all the properties of a normal fluid, and a superfluid component. The superfluid component has zero viscosity and zero entropy. Application of heat to a spot in superfluid helium results in a flow of the normal component which takes care of the heat transport at relatively high velocity (up to 20&nbsp;cm/s) which leads to a very high effective thermal conductivity.

=== Film flow ===

Many ordinary liquids, like alcohol or petroleum, creep up solid walls, driven by their surface tension. Liquid helium also has this property, but, in the case of He-II, the flow of the liquid in the layer is not restricted by its viscosity but by a critical velocity which is about 20&nbsp;cm/s. This is a fairly high velocity so superfluid helium can flow relatively easily up the wall of containers, over the top, and down to the same level as the surface of the liquid inside the container, in a siphon effect. It was, however, observed, that the flow through nanoporous membrane becomes restricted if the pore diameter is less than 0.7&nbsp;nm (i.e. roughly three times the classical diameter of helium atom), suggesting the unusual hydrodynamic properties of He arise at larger scale than in the classical liquid helium.<ref>{{Cite journal | doi=10.1038/srep28992| pmid=27363671| pmc=4929499| title=Limited Quantum Helium Transportation through Nano-channels by Quantum Fluctuation| journal=Scientific Reports| volume=6| pages=28992| year=2016| last1=Ohba| first1=Tomonori| bibcode=2016NatSR...628992O}}</ref>

=== Rotation ===
Another fundamental property becomes visible if a superfluid is placed in a rotating container. Instead of rotating uniformly with the container, the rotating state consists of quantized vortices. That is, when the container is rotated at speeds below the first critical angular velocity, the liquid remains perfectly stationary. Once the first critical angular velocity is reached, the superfluid will form a vortex. The vortex strength is quantized, that is, a superfluid can only spin at certain "allowed" values. Rotation in a normal fluid, like water, is not quantized. If the rotation speed is increased more and more quantized vortices will be formed which arrange in nice patterns similar to the [[Abrikosov lattice]] in a superconductor.

=== Comparison with helium-3 ===
Although the phenomenologies of the superfluid states of helium-4 and helium-3 are very similar, the microscopic details of the transitions are very different. Helium-4 atoms are bosons, and their superfluidity can be understood in terms of the [[Bose–Einstein statistics]] that they obey. Specifically, the superfluidity of helium-4 can be regarded as a consequence of Bose–Einstein condensation in an interacting system. On the other hand, helium-3 atoms are fermions, and the superfluid transition in this system is described by a generalization of the [[BCS theory]] of superconductivity. In it, [[Cooper pair]]ing takes place between atoms rather than [[electron]]s, and the attractive interaction between them is mediated by [[Spin (physics)|spin]] fluctuations rather than [[phonon]]s. (See [[fermion condensate]].) A unified description of superconductivity and superfluidity is possible in terms of [[spontaneous symmetry breaking|gauge symmetry breaking]].