by James Buckland
Rollin Film is a helium-specific flavor of the Onnes Effect, a physical phenomena in which superfluids climb the walls of their containers, due to their high surface tension and zero viscosity overcoming the effects of gravity.
Fluids typically are restrained to the bottom of their container, due to the force of gravity pulling them down. For particularly narrow spaces, such as the roots and capillaries of trees, fluids can often exhibit capillary action, which relies on adhesive forces between the fluid and the walls of the container, as well as cohesive forces within the fluid itself. This cohesive force, known as surface tension, often forms a meniscus; a curve within the fluid itself, caused by its attraction or repulsion to the walls of the container. Of course, this capillary action is limited: it can’t climb too high, because of gravity.
But this capillary action is also limited by the viscosity of the fluid — how malleable it is, in a sense. Higher viscosity means more resistance to deformation — peanut butter, tar, and glass are incredibly viscous, whereas water, milk, and blood have a very low viscosity. The viscosity of a fluid is another important factor in determining how high its meniscus reaches.
However, when a fluid is cooled below a specific temperature (for Helium, this is called the lambda point, around 2 degrees Kelvin), it becomes a superfluid: it loses almost all heat capacity, and many of the atoms condense to the lowest possible energy. This state of superfluidity has some unusual side effects: very low liquid density, zero entropy, and zero viscosity, among other things.
Because the viscosity was one of the things preventing further capillary action, a superfluid will demonstrate a notable increase in capillary action — virtually unbounded. Thus, a Rollin Film is a thin film of supercooled liquid helium which will gradually climb the walls of whatever container it resides in, unhindered by gravity.