Supercooled Helium Does Things That Don’t Seem Possible

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We all know helium as a gas for blowing up balloons and making people talk like chipmunks. It’s also incredibly important as a coolant for medical devices and scientific instrumentation. However, what is less known is that helium has two different liquid states, one of which is truly strange, bordering on creepy.

Helium I occurs between 2.18 and 4.22 Kelvin (that is -270.97°C to -268.93°C ). It has a few quirks of its own. For one thing it is almost impossible to see, so that scientists float things in it just to be able to work out where the surface is. This is a result of being both transparent and barely slowing light at all compared to a vacuum, or the Earth’s atmosphere.

To understand what is going on you need to realize that all the fluids we normally encounter have viscosity. Particles within the fluid rub interact other as it flows, providing friction. Sometimes the viscosity is obvious, such as in honey. With water, we hardly notice it, but that doesn’t mean water isn’t viscous, just much less so than other liquids.

However, a “superfluid” has no viscosity at all. Not just a little, but absolutely none. In a striking parallel with superconducting materials, it turns out that the difference between little and none can be much larger than between a little and a lot. Helium II contains a mixture of superfluid and nonsuperfluid material. Freed entirely from frictional forces helium II can climb walls, squeeze through holes that block helium I, let alone anything else and conduct heat a million times better than helium I and hundreds of times more easily than elemental metals. Most extraordinarily it produces an endless fountain such as we see in the film.

All of these are the result of quantum mechanics – something that normally occurs on a scale far too small for us to see. Although a liquid rather than a supercooled gas, there are strong similarities to Bose-Einstein Condensate (BEC) http://www.sciencemag.org/content/269/5221/198.short, which is also a superfluid. BEC does things even stranger than Helium II, but is much harder to produce.

The wall climbing occurs in the form of a film 30 millionths of a millimeter thick, known as a Rollin film. The Helium creeps towards warmth where it can evaporate, even if that requires it to defy gravity to get there. It is an example of the capillary forces that cause water to rise up a very small tube, here taken to an astonishing degree.

Most containers we use have tiny holes in them, but liquids don’t get out because their viscosity prevents anything squeezing through. No viscosity – lots of leakage.

The thermal conductivity works in a very different way from heat conduction in most materials. Instead heat flows like sound through air, moving at 20m/s at temperatures around 1°K.

The fountain effect occurs when a chamber is seperated from some helium II by a barrier the superfluid can pass, but liquids not in the superfluid state cannot. Warming of the chamber causes the helium II to lose its superfluid state. A balance is maintained for the proportion of superfluid and nonsuperfluid helium on each side of the barrier, so extra superfluid passes through, increasing the pressure and producing a fountain. The effect was discovered by accident when experimenter Jack Allen shone his pocket torch on the equipment, warming the chamber enough to start the effect.

[Learn more here](http://web.archive.org/web/20050901062951/http://cryowwwebber.gsfc.nasa.gov/introduction/liquid_helium.html)

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“Supercooled Helium Does Things That Don’t Seem Possible”