The U.S. Department of Energy defines a superfluid as the ‘property of a liquid in which it has zero-resistance for flow, and thus flows without loss of kinetic energy’. It was first observed by physicist Heike Kamerlingh-Onnes when he cooled helium-4 to temperatures close to absolute zero [1].
This means that when the substance (helium-4) was cooled to extremely low temperatures, it moved without friction. Why is that significant? Friction is what makes a liquid stop spinning when you stir it. The friction between the liquid and the walls of a container is what keeps the liquid in that container. So, if the liquid stops experiencing friction, it can ‘climb out’ of its container — a phenomenon that has been observed multiple times, specifically with helium-4.
(Note: Helium-4 is an isotope [2] of helium, which contains two protons and two neutrons in its nucleus. Hence the number 4.)
How does it work?
This occurrence is explained by observing helium-4 (in its liquid state) at an atomic scale, which means the laws of quantum mechanics govern it. The first principle is that the energy of atoms and molecules at any given point, is at a discrete energy level. The particle can jump from one energy level to another; it cannot have an energy value between the levels.
The second principle states that as temperatures decrease, the atoms will move into lower energy levels. Eventually, though, there will be a bottom line — where all the atoms of the fluid will reach the same, low energy level. This is where the third principle comes in; it mentions that when the atoms reach the same energy level, they start to behave in unison.
Why does this principle matter? It is well known that the atoms or molecules in a fluid move randomly, crashing and colliding with each other (which is what causes the friction!). However, because of the third principle stated above, the atoms of helium-4, due to their low-energy state, start moving together in an organized manner, much like a coordinated marching troop. This allows them to ‘climb out’ as mentioned before.
Why Helium-4?
There are three main reasons why Helium-4 is mainly used and studied:
- Helium-4 has two neutrons, two protons, and two electrons — a configuration that makes it a boson. This enables the atom to stay in the same energy state as its neighbors, thus allowing for unison in behavior.
- It becomes a superfluid at 2.17 Kelvin [3] above absolute zero (correct to two decimal places). This temperature is much easier to model as compared to the one required for Helium-3, which is about 0.003 Kelvin.
- Helium-4 is much more naturally abundant than Helium-3. There’s barely any Helium-3 in the atmosphere, so it’s much easier to source and conduct experiments with Helium-4.
Research suggests that helium-3 can also enter a superfluid state, but that requires a much deeper dive. In any case, a press release by The Nobel Prize, detailing helium-3’s properties, has been linked in the sources for this article.
Superfluidity is a very fascinating concept, and it could be the push for many other areas in science that haven’t been fully explored. So far, it has been linked to Superconductivity; some scientists believe it could contribute to the ‘Theory of Everything’, an ambitious theory that looks to explain, well, everything.
- Medha Gopalakrishnan
Glossary:
- Absolute zero — The lowest temperature theoretically possible.
- Isotope — A form of an element that has the same number of protons and electrons, but a different number of neutrons.
- Kelvin — A unit of temperature; most commonly used in scientific measurements.
Sources:
- https://www.sciencedirect.com/topics/physics-and-astronomy/superfluidity#definition
- https://science.osti.gov/bes/Highlights/2017/BES-2017-12-n#:~:text=Superfluidity%20is%20the%20odd%20property,than%20%2D270%20degrees%20Celsius).
- https://www.nobelprize.org/prizes/physics/1996/press-release/#:~:text=But%20it%20was%20not%20until,Nobel%20Prize%20in%20Physics%201962).
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