Physical Acoustics

One of the remarkable properties of helium II, the liquid phase of helium that exists between 0° and 2.172°K, is that a certain fraction of it can flow without any viscosity. As a result of this, helium I I can flow quite freely through very narrow superleaks, or it can flow from beaker to beaker via the thin film that is adsorbed upon the walls. I t is also possible to excite propagating waves in such a film, analogous to long waves on the surface of a shallow body of water. These waves are called third sound.