Editing Helium (section)

====Helium II====
{{main|Superfluid helium-4}}
Liquid helium below its lambda point (called ''helium&nbsp;II'') exhibits very unusual characteristics. Due to its high [[thermal conductivity]], when it boils, it does not bubble but rather evaporates directly from its surface. [[Helium-3]] also has a [[superfluid]] phase, but only at much lower temperatures; as a result, less is known about the properties of the isotope.<ref name="enc" />
[[File:helium-II-creep.svg|thumb|upright|Unlike ordinary liquids, helium&nbsp;II will creep along surfaces in order to reach an equal level; after a short while, the levels in the two containers will equalize. The [[Rollin film]] also covers the interior of the larger container; if it were not sealed, the helium&nbsp;II would creep out and escape.<ref name="enc" />|alt=A cross-sectional drawing showing one vessel inside another. There is a liquid in the outer vessel, and it tends to flow into the inner vessel over its walls.]]

Helium&nbsp;II is a superfluid, a [[Macroscopic quantum phenomena|quantum mechanical state]] of matter with strange properties. For example, when it flows through capillaries as thin as 10 to 100 [[Nanometre|nm]] it has no measurable [[viscosity]].<ref name="nbb" /> However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Existing theory explains this using the ''two-fluid model'' for helium II. In this model, liquid helium below the lambda point is viewed as containing a proportion of helium atoms in a [[ground state]], which are superfluid and flow with exactly zero viscosity, and a proportion of helium atoms in an excited state, which behave more like an ordinary fluid.<ref>{{Cite journal|doi = 10.1006/aphy.2000.6019 |title = Microscopic Theory of Superfluid Helium |journal = Annals of Physics |volume = 281 |issue = 1–2 |date = 2000|pages = 636–705 12091211 |author = Hohenberg, P. C. |author2 = Martin, P. C.|bibcode = 2000AnPhy.281..636H }}</ref>

In the ''fountain effect'', a chamber is constructed which is connected to a reservoir of helium&nbsp;II by a [[sintering|sintered]] disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. In order to maintain the equilibrium fraction of superfluid helium, superfluid helium leaks through and increases the pressure, causing liquid to fountain out of the container.<ref>{{cite web|last=Warner|first=Brent|url=http://cryowwwebber.gsfc.nasa.gov/introduction/liquid_helium.html |title=Introduction to Liquid Helium |publisher=NASA|access-date=2007-01-05 |url-status=dead|archive-url=https://web.archive.org/web/20050901062951/http://cryowwwebber.gsfc.nasa.gov/introduction/liquid_helium.html |archive-date=2005-09-01}}</ref>

The thermal conductivity of helium&nbsp;II is greater than that of any other known substance, a million times that of helium&nbsp;I and several hundred times that of [[copper]].<ref name="enc" /> This is because heat conduction occurs by an exceptional quantum mechanism. Most materials that conduct heat well have a [[valence band]] of free electrons which serve to transfer the heat. Helium&nbsp;II has no such valence band but nevertheless conducts heat well. The [[heat transfer|flow of heat]] is governed by equations that are similar to the [[wave equation]] used to characterize sound propagation in air. When heat is introduced, it moves at 20 meters per second at 1.8&nbsp;K through helium&nbsp;II as waves in a phenomenon known as ''[[second sound]]''.<ref name="enc" />

Helium&nbsp;II also exhibits a creeping effect. When a surface extends past the level of helium&nbsp;II, the helium&nbsp;II moves along the surface, against the force of [[gravity]]. Helium&nbsp;II will escape from a vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30&nbsp;nm-thick film regardless of surface material. This film is called a [[Rollin film]] and is named after the man who first characterized this trait, [[Bernard V. Rollin]].<ref name="enc" /><ref>{{Cite journal|doi = 10.1103/PhysRev.76.1209 |title = Rollin Film Rates in Liquid Helium |journal = Physical Review |volume = 76 |issue = 8 |pages = 1209–1211|date = 1949 |author = Fairbank, H. A. |author2 = Lane, C. T. |bibcode=1949PhRv...76.1209F}}</ref><ref>{{Cite journal|doi = 10.1016/S0031-8914(39)80013-1 |title = On the 'film' phenomenon of liquid helium II |journal = Physica |volume = 6 |issue = 2 |date = 1939 |pages = 219–230 |author = Rollin, B. V. |author2 = Simon, F. |bibcode=1939Phy.....6..219R}}</ref> As a result of this creeping behavior and helium&nbsp;II's ability to leak rapidly through tiny openings, it is very difficult to confine. Unless the container is carefully constructed, the helium&nbsp;II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate. Waves propagating across a Rollin film are governed by the same equation as [[gravity wave]]s in shallow water, but rather than gravity, the restoring force is the [[van der Waals force]].<ref>{{cite web |author = Ellis, Fred M. |url = http://fellis.web.wesleyan.edu/research/thrdsnd.html |title = Third sound |publisher = Wesleyan Quantum Fluids Laboratory |date = 2005 |access-date = 2008-07-23 |archive-url = https://web.archive.org/web/20070621202145/http://fellis.web.wesleyan.edu/research/thrdsnd.html |archive-date = 2007-06-21 |url-status = live }}</ref> These waves are known as ''[[third sound]]''.<ref>{{Cite journal|doi = 10.1103/PhysRev.188.370 |title = Hydrodynamics and Third Sound in Thin He II Films |journal = Physical Review |volume = 188 |issue = 1|date = 1949 |pages = 370–384|author = Bergman, D.|bibcode = 1969PhRv..188..370B }}</ref><!-- "van", see cite itself and [[Talk:Van der Waals#Van should be capitalized unless preceded by first name]] rebuttal -->