The Superfluid
I am not solid. I am not gas. I am helium, cooled to 2.17 Kelvin, and I have become something else entirely.
The transition is called lambda transition, because the specific heat versus temperature curve looks like the Greek letter λ (lambda). On one side, I am He-I—a normal liquid, viscous, turbulent. On the other, I am He-II—a superfluid, frictionless, quantum on a macroscopic scale.
Zero viscosity is not a metaphor. Through a capillary tube narrower than a human hair, I flow without resistance. No drag. No energy dissipation. A superfluid vortex, once started, will spin forever. The superfluid component does not dissipate energy.
In a normal fluid, viscosity arises from collisions between atoms. In a superfluid, a macroscopic fraction of atoms occupy the same quantum wavefunction. They move as one. There is no relative motion between them to dissipate. The superfluid component is a quantum condensate, and quantum condensates do not experience viscous drag.
The two-fluid model describes this behavior. Below Tλ, I am not one fluid but two: a normal component and a superfluid component. The normal component has viscosity and carries entropy. The superfluid component has zero viscosity, carries no entropy, and does not respond to rotation. They coexist in the same space, interpenetrating like ghosts.
As temperature drops below Tλ, the superfluid fraction grows. At absolute zero, all helium is superfluid. Near Tλ, the fractions are comparable, and two-fluid hydrodynamics produces bizarre phenomena.
Second sound is one. In ordinary materials, sound is a pressure wave. In He-II, there is a second sound—a temperature wave. The normal and superfluid components oscillate out of phase. Since only the normal component carries entropy, this counterflow produces oscillations in temperature.
The fountain effect is equally strange. Heat one end of a tube in He-II, and superfluid flows toward the heat source. Helium shoots out of the heated end. The pressure difference can shoot helium several centimeters into the air.
Rollin films are the most disconcerting. He-II creeps. A film of superfluid helium, only 30 nanometers thick, forms on any immersed surface. This film flows uphill. It defies gravity. A beaker left overnight empties itself. The rollin film is real. It has been photographed.
Superfluidity extends beyond helium-4. Helium-3 becomes superfluid at 2.5 millikelvin—over a thousand times colder. Helium-3 atoms are fermions, so they form Cooper pairs, and the paired fermions condense.
And superfluidity extends beyond the laboratory. Neutron stars—collapsed stellar remnants—are believed to contain superfluid neutrons. The sudden spin-ups of pulsars, called glitches, are caused by the superfluid component transferring angular momentum to the crust.
The mathematics is deep. The Landau critical velocity—the maximum speed without creating excitations—is determined by the spectrum of collective modes. The Bogoliubov theory of weakly interacting Bose gases captures the essential physics. A superfluid is, at its heart, an interacting Bose-Einstein condensate.
I am liquid helium. I am superfluid. I have no viscosity. I climb walls. I fountain. I am quantum matter, existing because the temperature is low enough that quantum mechanics takes over.