- #1
BobGom
- 11
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I'm looking for an explanation for the fountain effect in superfluids using the 'two fluid model'. However the explanations I've come across are not very satisfactory
From the NASA website
http://cryo.gsfc.nasa.gov/introduction/liquid_helium.html
But why does superfluid Helium flow from cool areas to warm areas? Unless I'm missing something that it is not a property of all fluids is it? The wiki article on Helium says
But I don't see why the system needs to maintain the same fraction of superfluid helium. Surely at higher temperatures there is a lower equilibrium fraction of superfluid.
From the NASA website
http://cryo.gsfc.nasa.gov/introduction/liquid_helium.html
Here is the two fluid model explanation of the fountain effect. When the heater in the tube is turned on, the liquid helium in the tube begins to warm up. Since superfluid helium flows from cool areas to warm areas, superfluid helium flows into the tube through the porous plug. Normal fluid is too viscous to flow out through the porous plug. Therefore, when the tube fills with liquid helium, the only way out for the normal fluid is to squirt out the hole in the top.
But why does superfluid Helium flow from cool areas to warm areas? Unless I'm missing something that it is not a property of all fluids is it? The wiki article on Helium says
In the fountain effect, a chamber is constructed which is connected to a reservoir of helium II by a sintered disc through which superfluid helium leaks easily but through which non-superfluid helium cannot pass. If the interior of the container is heated, the superfluid helium changes to non-superfluid helium. In order to maintain the equilibrium fraction of superfluid helium, superfluid helium leaks through and increases the pressure, causing liquid to fountain out of the container.
But I don't see why the system needs to maintain the same fraction of superfluid helium. Surely at higher temperatures there is a lower equilibrium fraction of superfluid.