When things come to the quantum world, common sense does not work at all. One thing that bolsters the above statement is superfluid helium-4. It has properties that are completely absurd and does not make sense at all.
Superfluid helium-4 is the liquid state of helium-4, isotopic helium with 2 neutrons and 2 protons, at a temperature below 2.18 Kelvin (more than 270-degree centigrade below zero) and normal pressure. We know that any substance liquefies and finally solidifies on cooling. Helium-4 also liquefies on cooling. But, when we cool helium-4 to a temperature below 2.18 Kelvin, its atom attains a state called Bose-Einstein Condensate (BEC), also called the fifth state of matter and shows a completely different property.
BEC is a state in which all the particles in a system are in the same quantum state, like all the tenants of a building gathering in a ground floor. In this state, all these particles behave like one. This behaviour gives rise to some amazing properties.
Properties of Superfluid Helium-4
Superfluid helium-4 is named superfluid because of its properties which are actually super. This state is analogous to superconductivity, in which the resistance becomes zero whereas in this case viscosity becomes zero. Owing to the amazing state, there are properties which ordinary liquid don’t have.
Ability to Resist Solidification at Atmospheric Pressure
We normally think that when we cool something it solidifies. Water solidifies at 0 degree Celsius, even hydrogen, the lightest matter, solidifies at 259 degree Celsius but it does not work for helium – 4 at normal pressure. Helium – 4 is the only matter that resists solidification at absolute zero at normal pressure.
This property is due to the noble and light nature of helium-4. So, it is almost impossible to confine helium atoms and make them immobile (rigid), which is the case in solid objects.
Zero Viscosity – Flowing Through the Smallest Pores
Just like zero resistance in case of superconductivity, superfluids have zero viscosity. Viscosity is liquid’s resistance to deformation, the frictional force between two liquid layers. Glycerine has high viscosity than water so that an iron ball falls faster in water than in glycerine due to low viscosity, low friction.
Due to this property, it can pass through the smallest pores possible. It can easily leak through non-glazed ceramic objects which are solid and can hold other liquids without leaking. For instance, if you are passing water through a net with small holes, it does not pass smoothly, water gathers at middle and falls but with superfluids the passage is smooth.
There is no such friction between the layers because all the atoms are in a single state and they are not sliding over one another. Helium-4 acts as a Boson rather than Fermion due to presence of paired particles. And Bosons can occupy a single state, making all the atoms as one single system and giving rise to superfluidity. They all superimpose and give rise to a single state. In such case there is no sliding over one another, they are all acting as one. There is absolute perfection.
Climbing Walls, Defying Gravity
Liquid helium-4 can climb walls of the container. Generally, liquids can climb walls of the container to some extent due to their surface tension. Viscosity resists them to climb further and in vessels with the broad surface, the liquid cannot climb the walls. But, liquid helium-4 can climb walls of its container against gravity. Its motion is only limited by a critical velocity which is large enough. It also shows a fountain effect, rising through the walls like a fountain with a pressure of 0.692 bar.
Why Not Helium-3 but Helium-4?
Another isotopic helium is helium-3. It does not show superfluid behaviour in such temperature, it requires a lot smaller temperature. This is because helium-4 has bosonic nature while helium-3 is a fermion. Hence, helium-4 can transit to BEC state while Helium-3 cannot. For it to obtain such minimum energy state, the temperature must be very very close to absolute zero.