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I'm trying to teach myself about superfluidity and I'm slightly confused on the ''two-fluid'' description. From what I understand, the superfluid is considered to be a mixture of two fluids, a superfluid component and a normal fluid component.

Are these two fluids a true ''mixture'' or are is it just a model to break the behavior of the one real fluid into its two different aspects? Could we separate them from each other as if they were truly separate fluids?

For example, if it were truly a mixture, we could make it do something that only a superfluid could do, such as pass through an extremely small opening. This would separate the fluids and leave us with just the superfluid component. Would this remaining ''pure''-superfluid still be described by the two-fluid model?

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In the end they are the same particles. So even when you pass the fluid through a small opening on the other side the particles become again normal (until an equilibrium is reached). – Fabian Dec 21 '12 at 21:12
Your separation method relies on the state of the superfluid component being unchanged upon passing through the small opening. However, unless the fluid on the other side of the opening were under the same conditions, upon passing through the opening the fluid would see a very different environment (pressure, temperature) and would cease to be a superfluid. If it is passing into a chamber with the same properties, then you are not separating the superfluid at all. Instead, superfluidity simply allows exchange between the two chambers that would not occur for normal fluids. – KDN Dec 22 '12 at 15:44

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