What Is Superfluid Helium?
Superfluid helium is formed when helium is cooled below its cryogenic temperature of 2.17 K, transitioning into a viscosity-free liquid state.
As helium’s temperature decreases, it first changes from gas to liquid. Further cooling leads to a secondary phase transition at the λ-point, resulting in superfluid helium.
This state is characterized by extremely weak interatomic forces, allowing helium atoms to penetrate tiny spaces and exhibit unique phenomena due to the absence of viscosity.
Both helium 3 and helium 4 can achieve superfluidity, although their respective λ-points differ.
Uses of Superfluid Helium
Superfluid helium, an excellent conductor of heat, is used primarily for cooling. Its theoretical infinite thermal conductivity makes it an ideal cooling medium.
In practice, superfluid helium is often used in place of liquid helium due to the high costs associated with maintaining its superfluid state.
One significant application is in cooling superconducting helical coils in MRI machines, where it generates stronger magnetic fields than liquid helium, thereby enhancing measurement precision.
It is also crucial in equipment used for measuring minute signals, where even minimal heat generation can impact accuracy, necessitating efficient cooling with superfluid helium.
Characteristics of Superfluid Helium
In its superfluid state, helium exhibits zero friction between atoms, leading to the absence of viscosity.
This lack of interaction between atoms allows them to move freely and penetrate narrow spaces. Consequently, superfluid helium can creep up container walls and spill over (superfluid phenomenon).
Just as an electric current in a superconductor can flow indefinitely once initiated, superfluid helium can flow endlessly once set in motion. Similarly, if rotated at high speed, it continues to rotate indefinitely.
Due to its non-viscous nature, it also flows at a constant rate in thin pipes regardless of pressure and does not achieve thermal equilibrium, allowing for highly efficient cooling systems via superheat conduction.
Maintaining cryogenic temperatures is crucial for retaining superfluid helium’s properties, which are lost if the temperature rises above its λ-point, posing a challenge in its application.
Currently, superfluid helium is mainly used for cooling, with ongoing research into harnessing its unique properties for other technologies.