Editing Helium (section)

===Liquid phase===
{{Main|Liquid helium}}
[[File:Phase diagram of Helium-4-en.svg|thumb|upright=1.25|Phase diagram of helium-4. (Atmospheric pressure is about 0.1 MPa)]]
[[File:2 Helium.png|thumb|Liquefied helium. This helium is not only liquid, but has been cooled to the point of [[superfluid]]ity. The drop of liquid at the bottom of the glass represents helium spontaneously escaping from the container over the side, to empty out of the container. The energy to drive this process is supplied by the potential energy of the falling helium.]]

Helium liquifies when cooled below 4.2&nbsp;K at atmospheric pressure. Unlike any other element, however, helium remains liquid down to a temperature of [[absolute zero]]. This is a direct effect of quantum mechanics: specifically, the [[zero point energy]] of the system is too high to allow freezing. Pressures above about 25 atmospheres are required to freeze it. There are two liquid phases: Helium I is a conventional liquid, and Helium II, which occurs at a lower temperature, is a [[superfluid]].

====Helium I====
Below its [[boiling point]] of {{convert|4.22|K|C F}} and above the [[lambda point]] of {{convert|2.1768|K|C F}}, the [[isotope]] helium-4 exists in a normal colorless liquid state, called ''helium&nbsp;I''.<ref name="enc" /> Like other [[cryogenic]] liquids, helium&nbsp;I boils when it is heated and contracts when its temperature is lowered. Below the lambda point, however, helium does not boil, and it expands as the temperature is lowered further. <!-- clarifyme / The rate of expansion decreases below the lambda point until about 1&nbsp;K is reached; at which point expansion completely stops and helium&nbsp;I starts to contract again. / if it is below the lambda point, should not it be helium II?-->

Helium&nbsp;I has a gas-like [[index of refraction]] of 1.026 which makes its surface so hard to see that floats of [[Expanded polystyrene|Styrofoam]] are often used to show where the surface is.<ref name="enc" /> This colorless liquid has a very low [[viscosity]] and a density of 0.145–0.125 g/mL (between about 0 and 4 K),<ref name="crc6120">{{RubberBible86th|page=6-120}}</ref> which is only one-fourth the value expected from [[classical physics]].<ref name="enc" /> [[Quantum mechanics]] is needed to explain this property and thus both states of liquid helium (helium I and helium II) are called ''quantum fluids'', meaning they display atomic properties on a macroscopic scale. This may be an effect of its boiling point being so close to absolute zero, preventing random molecular motion ([[thermal energy]]) from masking the atomic properties.<ref name="enc" />

====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 -->