Rollin film
A Rollin film, named after Bernard V. Rollin, is a 30 nm-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 10 to 10 meters 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. Thickness of the film. The thickness of the film can be calculated by the energy balance. Consider a  small fluid volume element formula_1 which is located at a height formula_2 from the free surface. The potential energy due to the gravitational force acting on the fluid element is formula_3, where formula_4 is the total density and formula_5 is the gravitational acceleration. The quantum kinetic energy per particle is formula_6, where formula_7 is the thickness of the film and formula_8 is the mass of the particle. Therefore, the net kinetic energy is given by formula_9, where formula_10 is the fraction of atoms which are Bose–Einstein condensate. Minimizing the total energy with respect to the thickness provides us the  value of the thickness: