Frictionless Flow
In 1937 Pyotr Kapitza in Moscow and John Allen and Don Misener in Cambridge independently discovered that liquid helium-4 below 2.17 K flows without any measurable viscosity. This temperature — the lambda point, named for the shape of the specific heat curve — marks a phase transition into a macroscopic quantum state. Below it, helium atoms undergo Bose-Einstein condensation into a single quantum wavefunction spanning the entire container.
The Two-Fluid Model
Lev Landau proposed that helium II behaves as if it were two interpenetrating fluids: a superfluid component with zero viscosity and zero entropy, and a normal component carrying thermal excitations (phonons and rotons). As temperature drops below Tλ, the superfluid fraction rises from 0% to nearly 100% near absolute zero. This model explains the fountain effect, second sound (temperature waves), and the anomalous thermal conductivity of helium II.
Quantized Vortices
When a superfluid is rotated, it cannot spin like a rigid body because the superfluid velocity field must be curl-free (irrotational). Instead, the rotation is accommodated by discrete vortex lines, each carrying exactly one quantum of circulation κ = h/m₄. These vortices have angstrom-scale cores and arrange themselves in a triangular lattice — directly analogous to magnetic flux vortices in type-II superconductors. This simulation visualizes the vortex lattice and counts vortices as you vary rotation speed.
Beyond Helium-4
Superfluidity appears in other systems: helium-3 becomes superfluid below 2.5 mK through fermionic Cooper pairing (Lee, Osheroff, and Richardson, Nobel Prize 1996), ultracold atomic gases form Bose-Einstein condensates at nanokelvin temperatures, and neutron superfluids flow in the interiors of neutron stars. The study of quantized vortices in these systems connects cryogenics to astrophysics, quantum computing, and fundamental tests of quantum mechanics.