When helium is cooled to extremely low temperatures near absolute zero, it undergoes a phase transition to become a superfluid. In this state, it has zero viscosity, meaning it experiences no friction. Superfluid helium can escape its container by spontaneously creeping up the walls and flowing over the top in a thin film.
The Phase Transition at the Lambda Point
Liquid helium occupies a unique position in condensed matter physics because it refuses to freeze under ordinary atmospheric pressure, even when cooled arbitrarily close to absolute zero. The light mass of helium atoms, combined with the unusually weak van der Waals forces between them, allows quantum zero-point energy to overcome the forces that would otherwise lock the atoms into a solid crystal lattice. As a result, liquid helium remains fluid down to the lowest achievable temperatures unless subjected to substantial external pressure.
When liquid helium-4 is cooled under saturated vapor pressure to approximately 2.17 Kelvin, it undergoes an abrupt second-order phase transition. Above this critical temperature, the liquid is known as Helium I, behaving largely like a conventional, low-density fluid. Below this threshold, it enters a state called Helium II, or superfluid helium-4. The transition is named the lambda transition because the graph of the liquid's specific heat capacity plotted against temperature spikes sharply at the critical point, tracing a shape that closely resembles the Greek letter lambda.
The emergence of Helium II marks the boundary where quantum mechanical behaviors cease to be confined to atomic scales and begin to govern the macroscopic properties of the bulk liquid. In this low-temperature regime, the fluid displays macroscopic quantum coherence, completely losing its viscosity under specific conditions and exhibiting extraordinarily high effective thermal conductivity.
Creeping Films and the Rollin Effect
Among the most striking visual manifestations of superfluid helium is its ability to climb up and over the walls of its container, a behavior known as the creeping film or the Rollin film, named after physicist Bernard Rollin. When a solid vessel partially filled with Helium II is suspended above a bath of liquid helium, the liquid crawls upward against gravity along the interior walls, crosses the rim, coats the outside of the container, and drips continuously from the bottom until the vessel is completely emptied.
This climbing phenomenon is driven by the interaction between helium atoms and the solid container walls. Because of attractive van der Waals forces, a liquid naturally attempts to coat any solid surface it touches with an ultra-thin film, typically tens of nanometers thick. In an ordinary liquid, internal friction and viscosity quickly halt the upward flow, limiting the film to a static microscopic layer near the meniscus. In Helium II, however, the superfluid component possesses zero viscosity, meaning there is no shear resistance to oppose the continuous flow of the film.
Driven by surface forces and the tendency to minimize chemical potential, the thin film steadily moves across every available surface until the fluid levels on both sides of a barrier reach equilibrium. If an empty beaker is lowered into a bath of Helium II, the fluid climbs up the exterior and flows inward until the inner liquid level matches the outer reservoir. If the beaker is raised above the surface, the flow reverses, siphoning the liquid outward in a continuous frictionless creep.
The Two-Fluid Model
To reconcile the seemingly contradictory properties of Helium II—such as its ability to flow through microscopic pinholes without resistance while simultaneously exerting drag on vibrating objects suspended within it—physicists developed the two-fluid model. Independently proposed by László Tisza and expanded by Lev Landau, this phenomenological framework describes Helium II as an interpenetrating mixture of two distinct fluid components: a normal component and a superfluid component.
The normal fluid component behaves like a conventional liquid. It possesses ordinary viscosity, experiences friction when moving against solid surfaces, and carries the entirety of the thermal energy and entropy of the system. The superfluid component, in contrast, represents the macroscopic quantum ground state. It is characterized by zero viscosity, zero entropy, and the ability to flow completely free of dissipation through narrow channels and capillaries.
The relative proportion of these two components depends strictly on temperature. At the lambda point (2.17 Kelvin), the liquid consists entirely of the normal fluid component. As the temperature drops toward absolute zero, the fraction of the normal component steadily decreases while the superfluid fraction increases, approaching one hundred percent near absolute zero as thermal excitations such as phonons and rotons fade away.
The Fountain Effect and Second Sound
Because the superfluid component carries zero entropy, thermal differences across Helium II produce mechanical responses that have no analogue in classical fluid dynamics. A classic example is the thermomechanical effect, commonly called the fountain effect. If two vessels containing Helium II are linked by a narrow capillary or porous plug that blocks the viscous normal component, heating one vessel causes the superfluid component to flow rapidly toward the warmer region to dilute the higher entropy.
This rapid influx of frictionless superfluid builds up significant hydrostatic pressure inside the heated chamber. When the chamber is fitted with a narrow vertical nozzle, the resulting pressure forces a continuous, fountain-like jet of liquid helium upward out of the tube, powered purely by the applied temperature gradient without any mechanical moving parts.
The two-fluid nature of Helium II also fundamentally alters how thermal energy moves through the bulk liquid. In conventional materials, heat dissipates through thermal conduction, a slow diffusive process driven by molecular collisions. In Helium II, a temperature gradient instead produces an internal counter-flow: the normal fluid carries heat away from the source while the superfluid component rushes toward it. This counter-flow allows heat to travel as an undulating wave of temperature and entropy, a phenomenon known as second sound.
Quantized Vortices and Superfluid Rotation
The quantum constraints on Helium II lead to unusual behavior when the fluid is rotated. In classical fluids, rotating a container creates a smooth, continuous velocity gradient that forms a standard parabolic meniscus. A pure superfluid cannot rotate in this manner because the quantum mechanical phase of the fluid's macroscopic wavefunction must remain single-valued around any closed path, restricting the circulation of the superfluid velocity to discrete integer multiples of Planck's constant divided by the mass of a helium-4 atom.
At very low rotational speeds, the bulk superfluid remains entirely stationary relative to the laboratory, refusing to rotate along with the container walls. Once the rotation exceeds a critical speed, the fluid accommodates the angular momentum not through continuous rotation, but by forming a regular array of quantized vortex lines. Each vortex line is an atomic-scale whirlpool whose core represents a region where the superfluid density drops to zero.
Every individual vortex line carries exactly one quantum of circulation. When the rotation speed increases, the density of these vortex lines multiplies, forming an orderly triangular lattice. From a macroscopic perspective, the collective motion of millions of tiny quantized vortices averages out to mimic the parabolic surface profile of a classical spinning liquid, while remaining strictly quantized at the microscopic scale.
Discovery and Quantum Legacy
The journey to understanding superfluidity began in 1908 when Dutch physicist Heike Kamerlingh Onnes first succeeded in liquefying helium. Early observations revealed strange anomalies in density and expansion near 2.2 Kelvin, but the full nature of the phase transition remained elusive for nearly three decades. In the late 1930s, Pyotr Kapitsa in Moscow, working simultaneously with John F. Allen and Don Misener in Cambridge, conducted experiments measuring the flow of helium through tight slits, proving that the viscosity dropped to undetectable levels and coining the term 'superfluidity.'
Superfluid helium-4 provided the first macroscopic realization of Bose-Einstein statistical physics. Helium-4 atoms are composite bosons, possessing an integer net spin composed of two protons, two neutrons, and two electrons. This bosonic nature allows a significant fraction of the atoms to condense into a single, coherent quantum state below the lambda transition. While strong interactions between the dense liquid atoms prevent the system from forming an ideal Bose-Einstein condensate, the macroscopic quantum coherence remains the fundamental driver of its frictionless flow and wall-climbing capabilities.
Key takeaways
•Helium-4 transitions into a superfluid state (Helium II) below 2.17 Kelvin, exhibiting zero viscosity in narrow channels and extreme thermal conductivity.
•The wall-climbing Rollin film occurs because van der Waals forces pull a thin layer of helium up container walls, and zero viscosity allows frictionless flow toward equilibrium.
•Under the two-fluid model, Helium II acts as a mixture of a normal, viscous fluid that carries heat and a zero-viscosity, zero-entropy superfluid component.
•Rotation in a superfluid cannot occur smoothly; it is restricted to quantized vortex lines that each carry discrete units of quantum circulation.