Rollin film

A Rollin film, named after Bernard V. Rollin, is a 30nm-thick liquid film of helium in the helium II state. It exhibits a "creeping" effect in response to surfaces extending past the film's level (wave propagation). Helium II can escape from any non-closed container via creeping toward and eventually evaporating from capillaries of 10nm or greater. Rollin films are involved in the fountain effect where superfluid helium leaks out of a container in a fountain-like manner. They have high thermal conductivity. The ability of superfluid liquids to cross obstacles that lie at a higher level is often referred to as the Onnes effect, named after Heike Kamerlingh Onnes. The Onnes effect is enabled by the capillary forces dominating gravity and viscous forces. Waves propagating across a Rollin film are governed by the same equation as gravity waves in shallow water, but rather than gravity, the restoring force is the van der Waals force. The film suffers a change in chemical potential when the thickness varies. These waves are known as third sound. The thickness of the film can be calculated by the energy balance. Consider a small fluid volume element which is located at a height from the free surface. The potential energy due to the gravitational force acting on the fluid element is , where is the total density and is the gravitational acceleration. The quantum kinetic energy per particle is , where is the thickness of the film and is the mass of the particle. Therefore, the net kinetic energy is given by , where is the fraction of atoms which are Bose–Einstein condensate.