Amazing properties result from this suppression of microscopic interaction. Foremost is the motivation of the term superfluid: in many experiments, fluid flow exists as one of the macroscopic quantum states mentioned above, and persists indefinitely without decay. This is a frictionless flow, or zero viscosity.
An accurate description of the fluid's behavior is with a model where
thermal agitation in the fluid is treated in the same way as black body
radiation for a vacuum. Instead of electromagnetic agitation, there
are acoustic modes called phonons and rotons that are quantized and thermally
populated. These excitations act as a second, gaseous like fluid,
distinct from the quantum state serving as their propagation medium.
In this "two fluid model", the superfluid (ground state medium) flows without
viscosity or thermal energy, and the normal fluid (excitation gas) flows
relatively independent as a regular fluid with drag and heat content.
The third sound film flow involves only the superfluid component within
the two fluid model. The normal component excitations are viscously
clamped to the substrate through scattering. As the superfluid wave
sloshes about, the normal component, containing the heat, is rarefied and
diluted resulting in temperature oscillations that are coupled to the thickness
oscillations. The relative amplitude of the temperature oscillations
can be as large as the thickness oscillations ( DT/T
= - Dh/h ) in an adiabatic limit, but is often
smaller due to thermal conduction within the film and to the substrate.
Pulsed time of flight and resonance methods have been successfully used,
with the latter the method of choice for accurate amplitude and attenuation measurement.
van der Waals potential ( PDF or xmcd )
Film - Vapor Equilibrium ( PDF or xmcd )
Third Sound Speed vs. Film Thickness ( PDF or xmcd )
Equations of motion including thermal and vapor coupling ( PDF or xmcd )