Superfluid: Quantum Fluid at Near Absolute Zero
Thermodynamics & Statistical Physics arc, page 69 of 193. Back to The Pixelated Physics Universe
Who they are
Cool helium enough and part of it stops feeling friction. Below 2.17 K, liquid helium-4 becomes helium II, which pours through gaps no normal liquid can pass. In 1938 Kapitsa, and Allen and Misener, reported it; Tisza and Landau explained it as two fluids, one with zero viscosity. Its rotation comes only in quantized vortices. Superfluid on a pixel-art unisex tee; equation, symbols and sources in the panel below.
Superfluid is a character of the Thermodynamics & Statistical Physics arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.
The physics
Landau showed that a fluid can flow with no friction at all as long as it moves too slowly to create any excitations; that sets a critical speed. In superfluid helium, circulation also comes in whole-number multiples of h/m, so it can only swirl in quantized vortices.
$$ \begin{gathered} v_c = \min_{p}\frac{\varepsilon(p)}{p} \\ \oint \vec v_s\cdot d\vec\ell = n\,\frac{h}{m_4} \end{gathered} $$
| Symbol | Meaning | SI unit |
|---|---|---|
| $v_c$ | Landau critical velocity | m/s |
| $\varepsilon(p)$ | excitation energy at momentum p | J |
| $p$ | excitation momentum | kg·m/s |
| $\vec v_s$ | superfluid velocity | m/s |
| $n$ | integer winding number | 1 |
| $h$ | Planck constant | J·s |
| $m_4$ | mass of a helium-4 atom | kg |
Sources
- Kapitza (1938) Viscosity of Liquid Helium below the λ-Point, Nature 141, 74
- Allen & Misener (1938) Flow of Liquid Helium II, Nature 141, 75
- NobelPrize.org — Pyotr Kapitsa, Physics 1978, facts
- NobelPrize.org — Lev Landau, Physics 1962, facts
- Vinen (1961) Detection of single quanta of circulation in liquid helium II, Proc. R. Soc. A 260, 218
- Encyclopaedia Britannica — Superfluidity
- Tisza (1938) Transport Phenomena in Helium II, Nature 141, 913
Crossovers
Superfluid meets Vortia & Antivortia (BKT Transition). Vortia and Antivortia's transition governs thin helium films, where vortex pairs unbind and superflow is lost. Superfluid's bulk helium II shows the same quantised vortices in three dimensions.
Superfluid meets Cryoselle (Dilution Fridge Qubit). Cryoselle's dilution fridge runs on helium: a mixture of helium-3 and helium-4, where superfluid helium-4 is an inert background for helium-3 to "evaporate" into. That is how she reaches 20 mK.
Wear the lore
- Superfluid – Quantum Fluid at Near Absolute Zero Physics T-Shirt
- Related infographic poster: The Fountain Effect – Plate 065 Physics Poster
Concept keywords: Quantum Fluid at Near Absolute Zero, Superfluid, Thermodynamics & Statistical Physics, physics lore, Pixelated Physics