In physics, superfluidity is a property of fluids where they have zero viscosity or are frictionless. A substance displaying this property is superfluid. Superfluids flow without loss of kinetic energy. In the lab, superfluids form in some substances at cryogenic temperatures, not much above absolute zero.
Properties of Superfluids
Superfluidity results in some strange phenomena that are not observed in ordinary liquids and gases.
- Some superfluids, such as helium-3, creep up the walls of the container, flow over the side, and eventually escape the container. This creeping behavior (film flow) actually does occur in a few normal fluids, such as alcohol and petroleum, but due to surface tension.
- Superfluids can pass through the walls of containers that hold liquids and gases.
- Stirring a superfluid produces vortices that continue to spin indefinitely.
- Turning a container of a superfluid does not disturb its contents. In contrast, if you rotate a cup of coffee, some of the liquid moves with the cup.
- A superfluid acts like a mixture of a normal fluid and a superfluid. As the temperature drops, more of the fluid is superfluid and less of it is an ordinary fluid.
- Some superfluids display high thermal conductivity.
- Compressibility varies. Some superfluids are compressible, while other have low compressibility (e.g., superfluid helium) or no compressibility (superfluid Bose Einstein condensate).
- Superfluidity is not associated with superconductivity. For example, superfluid He-3 and He-4 are both electrical insulators.
Examples of Superfluids
Superfluid helium-4 is the best studies example of superfluidity. Helium-4 transitions from a liquid into a superfluid just a few degrees below its boiling point of -452 °F (-269 °C or 4 K). Superfluid helium-4 looks like a normal clear liquid. However, because it has no viscosity, once it starts flowing, it continues moving, past any obstacles.
Here are other superfluidity examples:
- Superfluid helium-4
- Superfluid helium-3
- Some Bose Einstein condensates as superfluids (not all, though)
- Atomic rubidium-85
- Lithium-6 atoms (at 50 nK)
- Atomic sodium
- Possibly inside neutron stars
- Superfluid vacuum theory considers a vacuum as a type of superfluid.
Credit for the discovery of superfluidity goes to Pyotr Kapitsa, John F. Allen, and Don Misener. Kapitsa and, independently, Allen and Misener observed superfluidity in the isotope helium-4 in 1937. A helium-4 atom has integer spin and is a boson particle. It displays superfluidity at much higher temperatures than helium-3, which is a fermion.
Helium-3 only forms a boson when it pairs with itself, which only occurs at temperature near absolute zero. This is similar to the electron pairing process that results in superconductivity. The 1996 Nobel Prize in Physics was awarded to the discoverers of helium-3 superfluidity: David Lee, Douglas Osheroff, and Robert Richardson.
More recently, researchers have observed superfluidity in ultracold atomic gases, include those of lithium-6, rubidium-87, and sodium atoms. Lene’s Hau’s 1999 experiment with superfluid sodium slowed light and eventually stopped it.
At present, there aren’t many practical applications of superfluids. However, superfluid helium-4 is a coolant for high-field magnets. Both helium-3 and helium-4 find use in exotic particle detectors. Indirectly, researching superfluidity aids in understanding how superconductivity works.
- Annett, James F. (2005). Superconductivity, superfluids, and condensates. Oxford: Oxford Univ. Press. ISBN 978-0-19-850756-7.
- Khalatnikov, Isaac M. (2018). An introduction to the theory of superfluidity. CRC Press. ISBN 978-0-42-997144-0.
- Lombardo, U.; Schulze, H.-J. (2001). “Superfluidity in Neutron Star Matter”. Physics of Neutron Star Interiors. Lecture Notes in Physics. 578: 30–53. doi:10.1007/3-540-44578-1_2
- Madison, K.; Chevy, F.; Wohlleben, W.; Dalibard, J. (2000). “Vortex Formation in a Stirred Bose–Einstein Condensate”. Physical Review Letters. 84 (5): 806–809. doi:10.1103/PhysRevLett.84.806
- Minkel, J.R. (February 20, 2009). “Strange but True: Superfluid Helium Can Climb Walls“. Scientific American.