A Rollin film, named after Bernard V. Rollin, is a 30
nm-thick liquid film of
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
in the
helium II
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. It ...
state. It exhibits a "creeping" effect in response to surfaces extending past the film's level (
wave propagation
Wave propagation is any of the ways in which waves travel. Single wave propagation can be calculated by 2nd order wave equation ( standing wavefield) or 1st order one-way wave equation.
With respect to the direction of the oscillation relative to ...
). Helium II can escape from any non-closed container via creeping toward and eventually evaporating from capillaries of 10
−7 to 10
−8 meters 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 vortices that continue to rotate indefinitely. Superfluidity occurs in two ...
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 conduct heat. It is commonly denoted by k, \lambda, or \kappa.
Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
.
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 physicist and Nobel laureate. He exploited the Hampson–Linde cycle to investigate how materials behave when cooled to nearly absolute zero and later to liquefy heliu ...
. The Onnes effect is enabled by the
capillary force
Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces l ...
s 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, the 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 electronic bond; they are comparatively weak and th ...
. 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 densities is cooled to temperatures very close to absolute zero (−273.15 °C or −459.6 ...
. 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
*
Second sound
Second sound is a quantum mechanical phenomenon in which heat transfer occurs by wave-like motion, rather than by the more usual mechanism of diffusion. Its presence leads to a very high thermal conductivity. It is known as "second sound" because t ...
References
*
*
*
External links
Video of the property in action* Video
(Alfred Leitner, 1963, 38 min.)
Helium
Bose–Einstein condensates
Fluid mechanics
Superfluidity
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