Section 6.7 Superfluidity
A superfluid is a fluid (liquid or a gas) with zero viscosity. In other words, a flow in a superfluid will never slow down. Superfluids are an example of a Bose condensate, and the liquid must be composed of identical molecules that are bosons (with spin \(s = 0\text{,}\) 1, 2, …). Helium-4 is the most common fluid used in studies of superfluidity. It is necessary to cool the system down to a temperature \(T\) below a (small) critical value \(T_c\text{,}\) since molecular motion associated with thermal energy knocks molecules out of the Bose condensate unless the system is very cold.
The lack of viscosity for a superfluid can be understood qualitatively by considering how viscous dissipation works. In a flowing liquid, molecules of the liquid moving with the flow will occasionally strike the side of the container or other liquid molecules and recoil in some other direction. All of the molecules in the fluid undergo these collisions, and the whole fluid eventually slows down to a stop as the energy is gradually converted from motion along the flow into motion in random directions, which is just thermal motion. For a superfluid, however, a significant fraction of the molecules in the system are crammed into the same quantum state, the Bose condensate. An individual molecule in the condensate will not easily exchange energy with the container, since the molecule would have to leave the Bose condensate if it were to move in a direction different than the flow. Superfluidity has not achieved practical status yet because of the fact that the critical temperatures are typically only a couple of Kelvin.
Recently, large atom Bose condensation has been observed experimentally for the first time in gasses. In this context it's called Bose-Einstein condensation, since Einstein predicted this possibility. This only happens at temperatures in the millionths of a Kelvin range, so this was not easy to accomplish. Recent Nobel-Prize winning advances in laser cooling techniques finally enabled physicists to reach these temperatures.