What is a Superfluid?

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  • Written By: Michael Anissimov
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  • Last Modified Date: 25 March 2017
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A superfluid is a phase of matter capable of flowing endlessly without energy loss. This property of certain isotopes was discovered by Pyotr Leonidovich Kapitsa, John F. Allen, and Don Misener in 1937. It has been achieved at very low temperatures with at least two isotopes of helium, one isotope of rubidium, and one isotope of lithium.

Only liquids and gasses can be superfluids. For example, helium's freezing point is 1 K (Kelvin) and 25 atmospheres of pressure, the lowest of any element, but the substance begins exhibiting superfluid properties at about 2 K. The phase transition occurs when all the constituent atoms of a sample begin to occupy the same quantum state. This happens when the atoms are placed very close together and cooled down so much that their quantum wave functions begin to overlap and the atoms lose their individual identities, behaving more like a single super-atom than an agglomeration of atoms.

A limiting factor on which materials can exhibit superfluidity and which cannot is that the material must be very very cold (less than 4 K) and remain fluid at this cold temperature. Materials that become solid at low temperatures cannot assume this phase. When cooled to very low temperatures, a superfluid-ready set of bosons, atoms with an even number of nucleons, forms into a Bose-Einstein condensate, a superfluid phase of matter. When fermions, atoms with an odd number of nucleons such as the helium-3 isotope, are cooled down to a few Kelvin, this is not sufficient to cause this transition.

Because only bosons can readily become a Bose-Einstein condensate, fermions must first pair up with each other in order to become a superfluid. This process is similar to the Cooper pairing of electrons that occurs in superconductors. When two atoms with odd numbers of nucleons pair up with each other, they collectively possess an even number of nucleons and begin to behave like bosons, condensing together into a superfluid state. This is called a fermion condensate, and emerges only at the mK (milliKelvin) temperature level rather than at a few Kelvins. The key difference between atom pairing in a superfluid and electron pairing in a superconductor is that the atomic pairing is mediated by quantum spin fluctuations rather than by phonon (vibratory energy) exchange.

Superfluids have some impressive and unique properties that distinguish them from other forms of matter. Because they have no internal viscosity, a vortex formed within one persists forever. A superfluid has zero thermodynamic entropy and infinite thermal conductivity, meaning that no temperature differential can exist between two superfluids or two parts of the same one. They can also climb up and out of a container in a one-atom-thick layer if the container is not sealed. A conventional molecule embedded within a superfluid can move with full rotational freedom, behaving like a gas. Other interesting properties may be discovered in the future.

Most so-called superfluids are not pure, but are in fact a mixture of a fluid component and a superfluid component. The potential applications of superfluids are not as exciting and wide-ranging as those of superconductors, but dilution refrigerators and spectroscopy are two areas where they have found use. Perhaps the most interesting application today is purely educational, showing how quantum effects can become macroscopic in scale under certain extreme conditions.


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Post 14

You know that if you actually create a super fluid big enough for a macro study, and it can't be stopped, we would have basically created something whose molecules would be vibrating so slowly, and be so cold, that it would basically explode if contained, because it's like dry ice in a water bottle. It'll keep expanding, until it explodes its container, hence destroying whatever it's in, like a big government building, or a particle collider. Would they be stupid enough to put a superfluid in there?

Post 13

I'm a junior student, and our teacher asked us to research about an example of Bose Einstein Condensate, but she said that no scientist ever discovered something like that. Still, she is making us research it. How can I find what I want to find? I'll sure flunk in Chemistry class.

Post 12

@anon48700 - Since they're still kind of obscure and since really most of them are made from the same thing -- helium -- I couldn't find four superfluids. Here's my list of the three I did manage to locate along with their uses.

Sorry if the uses seem a bit cryptic; judging by my reading on this interesting substance, it's mostly used for really technical and scientific activities.

1. Superfluid helium-4: used as a quantum solvent in spectroscopic work (studying the interactions between radiated energy and matter). The technique name is a mouth full, too: Superfluid Helium Droplet Spectroscopy.

2. Superfluid helium: used to cool the Infrared Astronomical Satellite (IRAS) that was launched in the early 1980s. Kept in liquid form and flushed through its own system to cool the satellite's components.

3. Liquid helium-3: used to enable quantum gyroscopes to make extremely precise measurements.

Post 11

I think superfluids are really fascinating substances. In particular I'm curious if they have any conductance ability with electricity; they are a liquid, after all.

Did you know that superfluids will hold still if you rotate the container holding them slowly enough? This is an indication of their zero viscosity. When you rotate the container fast enough that the superfluid inside reaches the speed of sound for it, it will suddenly rotate rapidly the same way you turned the container.

Another bizarre and interesting behavior of superfluids is the creep factor. Technically it's called the "Rollin film". The article refers to superfluids creeping out of their containers in a one-molecule-thick film, and it's quite true.

Rollin film

refers particularly to a layer of helium II superfluid that creeps out of an uncovered cup that contains the superfluid. It will creep over the edges and drip down the bottom of the cup, then repeat the process. That means that, drop by drop, the superfluid will eventually move itself out of its container.

I think that's both fascinating and a bit freaky. I know it behaves this way due to physics of some kind but it kind of makes it seem alive.

Post 10

@seHiro - Yes, super fluids work -- and do not freeze -- in outer space. In fact, super fluid materials only become super fluids when cooled to extremely low temperatures, so outer space and vacuum are good places for a super fluid.

Super fluids are already in use in space situations; they are sometimes used to cool satellites' camera equipment when they are sent up into space.

I think the original question that noticepair had was more about what kind of motion or behavior a super fluid would have if you released it into the vacuum of space. Since it has zero viscosity and can move with zero energy transferred in the process, a super fluid would likely do absolutely nothing

in outer space.

The only problem with that idea is that releasing it into space would create a motion -- and since the super fluid never loses a motion once it starts, it would eternally move whichever way the release pointed it, in theory.

Post 9

@noticepair - That's exactly what I thought when I read this, too -- "wouldn't this work great for outer space?"

Because super fluids have an infinity of thermal conductivity potential and, as the article notes, zero thermodynamic entropy, that means to me that a super fluid takes on the temperature of whatever area it is in and can does not cool down or heat up more than that temperature.

This means that it would be precisely the same temperature as outer space if it was used in space in a vacuum. That really makes me curious whether it is possible to freeze a super fluid -- is it? It behaves more like a gas than a liquid -- would it even freeze if you tried? If not, it could behave in extremely interesting ways in a vacuum.

Post 8

Could the superfluid be used to push a turbine to power electricity? Or would it just flow right around the the turbine, not even touching it?

Post 7

Perpetual motion is useless. By definition it is a completely separate system from everything else, and thus can do no work.

Post 6

could anyone give me four superfluids and their uses?

Post 5

Could these superfluids be used (theoretically) in perpetual motion devices? If superfluids are frictionless, then you have overcome the biggest boundery in perpetual motion.

Post 4

I am curious about how a superfluid might behave in a vacuum under conditions of zero gravity.

Post 3

Under normal pressure, helium never freezes. It only freezes at higher pressures.

The reason it doesn't freeze is due to helium's high zero-point energy (a quantum mechanical value which is the energy it has at absolute zero, which is inversely proportional to the mass squared), this high ZPE means that forming bonds to form a solid is only favored at higher pressures.

Post 2

dfl287 - i would have 2 say no because if it has no loss of energy then it's basically trying to stop something that can not be stopped.

Post 1

Could one create a superfluid sample large enough to be useful for macroscopic work, like say passing information in some theoretically sealed container from one point to another with no loss in energy from point a to point b?

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