Superfluid helium-4

Some liquids act in strange ways.

When helium gets very cold, it becomes a superfluid. This liquid can crawl up walls. It can even flow through tiny holes.

This happens because it has no friction. It can move past any surface easily. It can also move through a cup. It will crawl up and over the top. This helps it find its own level. It is a very cool thing to see!

Helium is a common gas. But when it gets very cold, it changes. It becomes a superfluid. This is a special way for a liquid to act.

In this state, the liquid has zero viscosity. Viscosity is a word for how much a liquid resists flowing. Most liquids have it. Water is thicker than alcohol. But a superfluid has no resistance at all. It can flow past any surface without friction. It can even move through tiny pores in a container.

This happens because of a set of steps called Bose–Einstein condensation. This happens when helium atoms become bosons. Bosons are tiny particles that can all act as one. This occurs at a temperature of 2.17 K. That is very close to absolute zero.

Superfluids can also do strange things with heat. If you heat one spot, the liquid moves fast to carry the heat away. It can also crawl up the walls of a cup. It will flow over the top and down the outside. It does this to find its own level.

Superfluid helium-4 is a very strange form of liquid. It is a special state of the most common kind of helium. This liquid is known as helium II. It behaves differently than normal liquids like water or oil. One major difference is that it has zero viscosity. Viscosity is a measure of how much a liquid resists flowing. Because it has no resistance, it can flow past any surface without friction. This allows the liquid to move through tiny pores in a container.

This amazing way of working happens through a process called Bose–Einstein condensation. In this state, the helium atoms act together as one. This occurs when the liquid is cooled to a very low temperature. The specific point where this happens is called the lambda point. At this point, the temperature is 2.17 K. Helium-4 atoms are called bosons, which allows them to form this condensate. This is different from helium-3, which requires much colder temperatures to act this way.

Scientists have studied these strange properties for a long time. Heike Kamerlingh Onnes may have seen this change in 1911. He saw it on the same day he observed superconductivity in mercury. Later, Pyotr Kapitsa, John F. Allen, and Don Misener discovered superfluidity in 1937. Many other scientists added to this knowledge over the years. Hall and Vinen studied vortex lines in the 1950s. Rayfield and Reif found vortex rings in the 1960s.

There are many specific facts about how helium II moves. It can act like a mixture of two different parts. One part is a normal liquid, and the other is a superfluid. The superfluid part has zero entropy, which is a measure of disorder. If you heat one spot, the liquid moves at speeds up to 20 cm/s. This helps it carry heat away very quickly. The liquid can also crawl up the walls of a container. This is called a Rollin film.

We can use this liquid for many important jobs today. It is used in high-precision tools like gyroscopes. These tools help measure gravity. In 1983, the Infrared Astronomical Satellite used 73 kilograms of superfluid helium. This helped keep the satellite very cold. Scientists also use it in chemistry to study gas molecules. They use tiny droplets of the liquid to cool molecules down. This helps them see how molecules behave in a gas.

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.