What is super-fluidity of Helium ?

As we all know that, if metals cooled at super low temperature (Critical temperature), they show superconductivity as I have explained in my previous blog; same way if we cool down the Helium gas at super low temperature then it shows "Super-fluidity". If we cool down the Helium gas at 4.2 K (-268.8)°C, its physical state change to liquid. If we cool it down further at about 2.19 K (-270.81°C) then suddenly liquid Helium changes its property into super-fluidity.

Unlike other gases, if we cool down the Helium at critical low temperature, it does not follow “gaseous then liquid and the solid state sequence”. Instead, even if we cool it down at 0 K, it remains in liquid state instead of becoming solid.

Due to this property of Helium, it is used as a super coolant in cryogenic experiment like Magnet Resonance Imaging (MRI), Nuclear Magnetic Resonance (NMR) spectroscopy, particle accelerators, Large Hadron Collider, Superconducting Quantum Interference Device (SQUID), Electron Spin Resonance spectroscopy (ESR), superconductors, strong-field magnetic separator, toroidal field superconducting magnets for fusion reactors and other cryogenic research.

Up to 2.19 K temperature, Helium behaves as a normal fluid, called as “Helium-1” and below that temperature, when Helium shows super-fluidity, is known as “Helium-2”. The temperature at which the Helium shows the Super-fluidity is known as “Lambda Temperature (ƛ)”. At 2.19 K, the nature of graph between Specific heat and temperature of Helium is like ƛ (Lambda), so it is called as “Lambda Temperature”.

Graph between Specific Heat and Temperature of He-4

Every liquid possess the property called as “Viscosity”. Viscosity is the property of a fluid which resists the relative motion between two layers of the fluid that are moving at different velocities or speed. In simple words, viscosity means friction between the molecules or layers of fluid.

Oil has more viscosity and water has comparatively less viscosity. That is the reason why water flows smoothly on the tilted surface and oil does not. We can see the same effect in capillary tubes also. Water can easy enter in capillary tube whereas oil does not.

When Helium acquires super-fluidity, its viscosity becomes zero and is can climb inside the capillary tube easily. It is like superconductivity. Superconductive metals have zero electrical resistance, same way super-fluid Helium has zero viscosity.

If we keep the Helium-2 in a glass jar then it will flow in upward direction along the wall of jar (Opposite to gravity) and then come out of the jar (As shown in figure). Suppose, if rotate that glass jar at high speed then it will not affect the Helium-2 present inside it. It means the Helium-2 will not show any movement. Other fluids rotate along with the jar but due to zero viscosity, Helium-2 will not rotate along with the jar.

He-2 Flowing along with the wall and opposite to gravity

Helium-2 has higher thermal conductivity compared to normal helium (Approx. 30 million times higher than normal Helium).

Helium-4 is an isotope of Helium having 2 protons and 2 neutrons in its nucleus and 2 electrons revolving around the nucleus. That is why He-4 atom behaves like a Boson Particle and follows Bose-Einstein statistics. So, at extreme low temperature, Helium-2 follows Bose-Einstein condensation.