Superfluid Film Flow
Below the lambda point helium-4 becomes a superfluid that creeps as a thin film over any surface, a phenomenon both exploited and fought in cryogenic design.
The lambda transition
Liquid helium-4 undergoes a transition to a superfluid state at about 2.17 kelvin, the lambda point. In the superfluid state a component of the liquid flows without viscosity, carries no entropy, and can pass through microscopic channels that would block any ordinary fluid. This is a macroscopic quantum effect arising from Bose-Einstein condensation of the helium-4 atoms.
The creeping film
A striking consequence is the Rollin film. Superfluid helium-4 coats every surface in contact with it with a film about 30 nanometers thick, and this film flows to equalize levels and temperatures. Left uncontrolled, the film creeps up the walls of a vessel and evaporates where it is warm, carrying heat and mass to places it is not wanted.
A problem for the still
In a dilution refrigerator, superfluid film creep up the pumping line from the still wastes cooling power, because helium-4 arriving at the warm pumping stage evaporates and must be recondensed. Designers add an orifice or restriction of small diameter and a film-burner heater to evaporate the film at a controlled point before it climbs further.
Zero viscosity, finite consequences
The same properties make superfluid helium-4 an excellent thermal conductor through its second-sound mechanism, useful for spreading heat, and a challenge to contain. Understanding film flow is part of the practical craft of building any millikelvin apparatus that relies on the helium-3 / helium-4 mixture.
- Lambda point at 2.17 K marks onset of superfluidity
- Superfluid flows without viscosity and carries no entropy
- Rollin film about 30 nm thick creeps over all surfaces
- Film burners control unwanted creep in the still