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Dhananjay Ravikumar
Superfluidity is a very rare phenomenon that occurs at ultra cold temperatures. It is only observed in Helium because only Helium stays a fluid at that temperatures.
Say you have a container of gas. You keep removing energy from it. In other words, you are cooling it. You remove so much energy that now all the atoms go sit in the lowest energy state. Seeing that they are already in their lowest energy state, nothing else can now remove energy from it for eg: neither gravity nor friction. Which is why if you hold superfluid helium in a container, it will spontaneously flow out (creep along side the walls of the container). If you look at the image below, you can see a drop at the bottom. It is because of this sort of creeping that climbs up the inner wall, flows along the outer wall and collects at the bottom (center) as a drop.
It also displays other remarkable properties such as having no measurable viscosity. Viscosity is a fluid’s response to shear stress which can also be understood as a fluid’s internal resistance to flow. Naively said, this is how “sticky” a fluid is. For example, Oil is more “stickier” than air and hence more “viscous”. This viscous nature is important because it helps fluid molecules “stick” to other things and to each other. If for example air were not viscous, turning on the fan would have no effect since air molecules would not stick to the fan thus the fan cannot impart kinetic energy to the air and hence it cannot move and cool you.
Coming back to the topic on hand, the superfluid has no measurable viscosity, so if you put a fan inside a container of superfluid and have something else placed in the fluid, it shouldn’t feel the effect of the fan inside. But guess what? It does! This led us to believe that at a container of “superfluid” helium is actually a mixture of normal and superfluid components at non zero temperatures. The plot below shows concentrations with respect to temperatures. Ps/P is superfluid density/total density and Pn/P is normal liquid helium density/total density. You can see that the superfluid density/total density approaches 1 when temperatures approach absolute zero which means all of the Helium in your container will become superfluid only when you reach absolute zero and any higher temperatures will result in an appropriate fraction of normal Helium in the container. The normal Helium is viscous which is why anything else in the container will feel the effect of the fan spinning inside.
ref: Two Fluid Model of Superfluid Helium (He II, Superfluidity)
Some more amazing things about Superfluid He:
1.) The superfluid state is its own state of matter, the Bose Einstein Condensate. All normal matter is made of fermions (electrons , protons, neutrons are all fermions) In the sense that they obey fermi-dirac statistics which is what tells you that no to fermions that are exactly the same cannot occupy the same energy state at the same instant of time. (the real reason why accidents occour)
Bosons however, have no such restriction and rules allow any number of identical bosons to occupy the same energy state a valid example would be the laser (multiple photons occupy the same energy state in coherence)
An important characteristic that allows them to behave this differently is their spins. Fermions have fraction spins, Bosons have whole number spins.
Helium is obviously normal matter which means it starts out with having normal protons and electrons with spins +/- 1/2 but, when you approach cold temperatures, these fermions form pairs, creating whole number spins (+1/2,+1/2 = 1) (+1/2,-1/2 = 0) this means that these pairs start behaving like bosons which thus allows these pairs to flood a single energy state, causing Bose-Einstein condensation, which is it’s own state of matter
2.) Superfluid Helium has zero entropy!
3.) Helium never solidifies at normal atmospheric pressures.
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Superfluidity is a very rare phenomenon that occurs at ultra cold temperatures. It is only observed in Helium because only Helium stays a fluid at that temperatures.
Say you have a container of gas. You keep removing energy from it. In other words, you are cooling it. You remove so much energy that now all the atoms go sit in the lowest energy state. Seeing that they are already in their lowest energy state, nothing else can now remove energy from it for eg: neither gravity nor friction. Which is why if you hold superfluid helium in a container, it will spontaneously flow out (creep along side the walls of the container). If you look at the image below, you can see a drop at the bottom. It is because of this sort of creeping that climbs up the inner wall, flows along the outer wall and collects at the bottom (center) as a drop.
It also displays other remarkable properties such as having no measurable viscosity. Viscosity is a fluid’s response to shear stress which can also be understood as a fluid’s internal resistance to flow. Naively said, this is how “sticky” a fluid is. For example, Oil is more “stickier” than air and hence more “viscous”. This viscous nature is important because it helps fluid molecules “stick” to other things and to each other. If for example air were not viscous, turning on the fan would have no effect since air molecules would not stick to the fan thus the fan cannot impart kinetic energy to the air and hence it cannot move and cool you.
Coming back to the topic on hand, the superfluid has no measurable viscosity, so if you put a fan inside a container of superfluid and have something else placed in the fluid, it shouldn’t feel the effect of the fan inside. But guess what? It does! This led us to believe that at a container of “superfluid” helium is actually a mixture of normal and superfluid components at non zero temperatures. The plot below shows concentrations with respect to temperatures. Ps/P is superfluid density/total density and Pn/P is normal liquid helium density/total density. You can see that the superfluid density/total density approaches 1 when temperatures approach absolute zero which means all of the Helium in your container will become superfluid only when you reach absolute zero and any higher temperatures will result in an appropriate fraction of normal Helium in the container. The normal Helium is viscous which is why anything else in the container will feel the effect of the fan spinning inside.
ref: Two Fluid Model of Superfluid Helium (He II, Superfluidity)
Some more amazing things about Superfluid He:
1.) The superfluid state is its own state of matter, the Bose Einstein Condensate. All normal matter is made of fermions (electrons , protons, neutrons are all fermions) In the sense that they obey fermi-dirac statistics which is what tells you that no to fermions that are exactly the same cannot occupy the same energy state at the same instant of time. (the real reason why accidents occour)
Bosons however, have no such restriction and rules allow any number of identical bosons to occupy the same energy state a valid example would be the laser (multiple photons occupy the same energy state in coherence)
An important characteristic that allows them to behave this differently is their spins. Fermions have fraction spins, Bosons have whole number spins.
Helium is obviously normal matter which means it starts out with having normal protons and electrons with spins +/- 1/2 but, when you approach cold temperatures, these fermions form pairs, creating whole number spins (+1/2,+1/2 = 1) (+1/2,-1/2 = 0) this means that these pairs start behaving like bosons which thus allows these pairs to flood a single energy state, causing Bose-Einstein condensation, which is it’s own state of matter
2.) Superfluid Helium has zero entropy!
3.) Helium never solidifies at normal atmospheric pressures.
See Questions On Quora
Continue reading...
