A Rollin film, named after
Bernard V. Rollin, is a -thick liquid film of
helium
Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...
in the
helium II
Superfluid helium-4 (helium II or He-II) is the superfluid form of helium-4, the most common isotope of the element helium. The substance, which resembles other liquids such as helium I (conventional, non-superfluid liquid helium), flows without ...
state. It exhibits a "creeping" effect in response to surfaces extending past the film's level (
wave propagation
In physics, mathematics, engineering, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. '' Periodic waves'' oscillate repeatedly about an equilibrium (resting) value at some f ...
). Helium II can escape from any non-closed container via creeping toward and eventually evaporating from capillaries of or greater.
Rollin films are involved in the ''fountain effect'' where
superfluid
Superfluidity is the characteristic property of a fluid with zero viscosity which therefore flows without any loss of kinetic energy. When stirred, a superfluid forms vortex, vortices that continue to rotate indefinitely. Superfluidity occurs ...
helium leaks out of a container in a fountain-like manner. They have high
thermal conductivity
The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1.
Heat transfer occurs at a lower rate in materials of low ...
.
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
Heike Kamerlingh Onnes (; 21 September 1853 – 21 February 1926) was a Dutch Experimental physics, experimental physicist. After studying in Groningen and Heidelberg, he became Professor of Experimental Physics at Leiden University, where he tau ...
. 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
In molecular physics and chemistry, the van der Waals force (sometimes van der Waals' force) is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical elec ...
. 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
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
In condensed matter physics, a Bose–Einstein condensate (BEC) is a state of matter that is typically formed when a gas of bosons at very low Density, densities is cooled to temperatures very close to absolute zero#Relation with Bose–Einste ...
. Minimizing the total energy with respect to the thickness provides us the value of the thickness:
[Huang, K. (2017). A superfluid universe. World Scientific.]
See also
*
Zero sound
*
Second sound
References
*
*
*
External links
Video of the property in action* Video
(Alfred Leitner, 1963, 38 min.)
Helium
Bose–Einstein condensates
Fluid mechanics
Superfluidity
{{physics-stub
de:Rollin-Film