Talk:Superfluid
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[edit] Moved from helium II :
This is going to be moved from the Helium II subsection of helium, for that section is information which applies to all superfluids, as we know them now:
Liquid helium below its lambda point begins to exhibit very unusual characteristics, in a state called helium II. Boiling of helium II is not possible due to its high thermal conductivity; heat input instead causes evaporation of the liquid directly to gas. The isotope helium-3 also has a superfluid phase, but only at much lower temperatures; as a result, less is known about such properties in the isotope helium-3.
Helium II is a superfluid, a quantum-mechanical state of matter with strange properties. For example, when it flows through even capillaries of 10-7 to 10-8 m width it has no measurable viscosity. However, when measurements were done between two moving discs, a viscosity comparable to that of gaseous helium was observed. Current 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.
Helium II also exhibits a "creeping" effect. When a surface extends past the level of helium II, the helium II moves along the surface, seemingly against the force of gravity. Helium II will escape from an vessel that is not sealed by creeping along the sides until it reaches a warmer region where it evaporates. It moves in a 30 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, B. V. Rollin. As a result of this creeping behavior and helium II's ability to leak rapidly through tiny openings, it is very difficult to confine liquid helium. Unless the container is carefully constructed, the helium II will creep along the surfaces and through valves until it reaches somewhere warmer, where it will evaporate.
In the fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a 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.
The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of copper. This is because heat conduction occurs by an exceptional quantum-mechanical mechanism. Most materials that conduct heat well have a valence band of free electrons which serve to transfer the heat. Helium II has no such valence band but nevertheless conducts heat well. The flow of heat is governed by equations that are similar to the wave equation used to characterize sound propagation in air. So when heat is introduced, it will move at 20 meters per second at 1.8 K through helium II as waves in a phenomenon called second sound.
—The preceding unsigned comment was added by Centrx (talk • contribs) 18:52, 27 July 2005.
[edit] Watch out for possible crank edits
I've noticed that a recent edit mentioning that superfluidity in connection with gravity is probably related to this website. I couldn't find any other connection, except a paper mentioning that superfluids behave mathematically much like space-time. —The preceding unsigned comment was added by Peter bertok (talk • contribs) 12:34, 7 October 2005.
[edit] Film behavior in real life?
As this article and Talk page discuss, superfluids do some odd things. I've never seen this in real life, though. How difficult is it to set up a scenario in which a superfluid will flow out of a cup, or easily flow through a porus material? Can this be done at a macro scale as a demonstration or does it need to be done in a sealed container with small volumes of superfluid? —BenFrantzDale
[edit] Minor Edit
I made a minor edit to the last claim in the article. At American Physical Society and other meetings, Moses Chan has described the possible observation of supersolid hydrogen. In his good judgment, he did not publish this finding. Last week, in a private communication, he withdrew this claim upon the completion of another set of tests. The results for supersolid helium still stand. —The preceding unsigned comment was added by 129.128.241.111 (talk • contribs) 15:16, 12 May 2006.
[edit] temperature gradient
"(It is thus impossible to set up a temperature gradient in a superfluid, much as it is impossible to set up a voltage difference in a superconductor.)"
This sentence is misleading. For example, if you put an ice cube and a hot iron on either side of a container with a superfluid, that is by definition a temperature gradient (whether or not there is a temperature gradient *inside* the fluid). However, heat conduction is limited by the speed of light, and thus temperature can't change instantaneously, and so temperature gradients must also exist inside superfluids - if only for an unmeasurable amount of time.
Similarly for a superconductor, put a battery's leads on either end of a superconductor. Again the speed of light is a limitation. Fresheneesz 03:37, 30 May 2006 (UTC)
- I would say that's a trivial distinction.
