two-fluid model
/ TOO-FLOO-id MOD-ul /
Imagine a single glass of liquid that somehow contains two liquids at once, pouring through each other freely without ever mixing — one of them sticky and ordinary, the other slippery and frictionless. That strange image is exactly how physicists picture a superfluid below its transition, and it explains its weirdest tricks with surprising ease.
The two-fluid model describes superfluid helium as a blend of two interpenetrating parts: a normal component that carries all the heat and behaves like any sticky liquid, and a superfluid component that flows with zero viscosity and carries no heat or disorder at all. They are not two separate substances but two aspects of the same atoms; as you cool the liquid, the superfluid fraction grows from nothing at the transition to nearly everything near absolute zero, while the normal fraction shrinks to nothing.
The model matters because it turns a baffling collection of effects — fountains, frictionless leaks, heat waves — into one tidy, predictive picture, and it remains the everyday working language for superfluids. The honest caveat is that it is a picture, not a literal claim that the liquid is split in two; you cannot scoop out a jar of pure superfluid component, because the split is a way of bookkeeping the motion, not two physically separable fluids.
The two-fluid picture predicts a strange 'second sound' in superfluid helium — not a wave of pressure like ordinary sound, but a wave of temperature, where the normal and superfluid components slosh in opposite directions while the total density barely moves.
Second sound: a heat wave from the normal and superfluid parts sloshing oppositely.
Only the normal component carries entropy and heat, so a tube that admits superfluid but blocks the normal part lets through frictionless liquid that is, in effect, perfectly cold — the basis of the fountain effect.