Superfluid helium-4

Superfluid helium-4, often called helium II, is a unique state of the most common helium isotope. It is a substance that defies our everyday understanding of how liquids behave. While normal liquids like water have internal friction, helium II flows without any friction at all. This lack of resistance allows the liquid to move past any surface effortlessly. It can even circulate through tiny pores in a container or over obstructions. It continues to move based only on its own inertia. This remarkable state is a visible manifestation of quantum mechanics on a large scale.

The transition to this state occurs through Bose–Einstein condensation. This process happens when helium atoms undergo a massive change in how they behave together. In helium-4, every atom is a boson, which is a type of particle with zero spin. Because they are bosons, the atoms can all occupy the same lowest energy state. This condensation occurs at a specific temperature called the lambda point. This point is exactly 2.17 Kelvin. This is much warmer than the condensation temperature for helium-3. Helium-3 atoms are fermions, which must pair up to act like bosons. That pairing requires much colder temperatures, around 2.5 millikelvin.

Scientists describe the behavior of helium II using a two-fluid model. This model suggests the liquid is actually a mixture of two distinct components. One component is a normal fluid that possesses all the standard properties of a liquid. The other is the superfluid component, which has zero viscosity and zero entropy. Entropy is a measure of the disorder within a system. As the temperature drops, the fraction of the superfluid component increases. At the lambda point, the superfluid density is zero. By the time the liquid reaches zero Kelvin, it is almost entirely superfluid.

The history of this discovery involves several important scientific milestones. Heike Kamerlingh Onnes may have observed the phase transition in 1911. This occurred on the same day he observed superconductivity in mercury. However, the superfluidity effect was officially discovered in 1937. This discovery was made by Pyotr Kapitsa, John F. Allen, and Don Misener. Later, researchers like Hall and Vinen studied quantized vortex lines in the 1950s. In the 1960s, Rayfield and Reif established the existence of quantized vortex rings. More recently, in 2006, scientists at the University of Maryland used solid hydrogen particles to visualize these vortices.

Superfluid helium-4 exhibits strange physical movements, such as film flow. Many liquids creep up walls due to surface tension, but helium II is different. Its flow in a thin layer is not restricted by viscosity. Instead, it is limited by a critical velocity of about 20 centimeters per second. This allows the liquid to climb up the walls of a container. It can flow over the top and down the outside in a siphon effect. This thin layer is known as a Rollin film.

Rotation also reveals the quantum nature of the fluid. If you rotate a container of normal liquid, the liquid rotates uniformly. A superfluid does not behave this way. If the container rotates slowly, the liquid stays perfectly still. Once it reaches a critical angular velocity, it forms quantized vortices. These vortices are not continuous, but exist at specific, "allowed" values. As the speed increases, more vortices form. They often arrange themselves into beautiful, organized patterns.

Today, this substance is vital for high-precision technology and research. It is used in gyroscopes to measure predicted gravitational effects. In 1983, the Infrared Astronomical Satellite used 73 kilograms of superfluid helium for cooling. It also serves as a quantum solvent in chemistry. Using superfluid helium droplet spectroscopy, scientists can study gas molecules. The droplets cool molecules to a nearly ground rovibronic state. This allows the molecules to behave as if they were in a gas phase. Finally, it helps create extreme cold in dilution refrigerators. By evaporating helium-3 into helium-4, scientists reach temperatures as low as 40 millikelvin.