Posted by Pranav Kakhandiki| Oct 21, 2018, 5:30:00 PM
Viscosity is defined as the resistance of a liquid to flow, or in other words, a liquid’s “thickness”. Some liquids have greater viscosities than others, which leads to different applications of these very different materials. Oil, for instance, has quite a low viscosity, which is why it is often used as a lubricant. Honey, contrarily, has a relatively high viscosity. However large or small this value of viscosity may be, it is positive, meaning that some constant force is required to move the liquid. But what if there was a liquid whose value of viscosity was equal to zero? These are called superfluids.
Superfluids exhibit many interesting properties which appear like they defy the laws of physics. No viscosity results in no cohesion force between molecules, which allows atoms of a superfluid to “flow” through almost any medium. For instance, if a superfluid were to be held in a cup, it would be able to pour out of the bottom, because the lack of friction between molecules. As long as the molecules are small enough to fit through the empty space between bonds of the cup, a superfluid would drain out completely. The never-ending fountain is another prime example of an interesting property of superfluids Since they exhibit no viscosity, if a fountain consisted of a superfluid, no kinetic energy would be lost in the process, so the fountain would keep flowing forever.
Superfluidity is most commonly shown in Helium-4 (2 neutrons, 2 protons), and only occurs at temperatures below 2.65˚ K. So although the never-ending fountain shown above may seems like a perpetual motion machine, the energy required to keep a superfluid at such a low temperature far exceeds the kinetic energy of the actual system. Although a lot about why superfluidity occurs is still unknown, the commonly accepted theory is that the total spin(angular momentum) adds up to zero, causing the net force between molecules to be zero. The low temperature is required to slow the general speed at which the molecules rotate at. In other liquids, such as water, the net angular momentum does not add up to zero, so forces between molecules create cohesion, thus giving the liquid a viscosity. There is still a lot to be learned about superfluids, from proving exactly how a liquid with zero viscosity can possibly exist to discovering real world applications.