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Superfluid He II climbing up wall and overflowing from a container |
There is no friction (viscosity) between the Superfluid He II atoms. However, the attractive interaction between the atoms of the container walls and the nearby He atoms is still there.
As an analogy, consider the water molecules in a glass cup. In this case, however, the strong attractive interaction between the water molecules (origin of viscosity) acts as a counter-weight, pulling downwards the molecules at the edge of the surface, being pulled up by the attractive (hydrophilic) interaction with the atoms in the glass wall. Therefore, water molecules are effectively prevented from climbing up the wall and flowing out of the container.
However, in the case of the Superfluid He II, this is precisely what happens: pulled up by the atoms in the wall, it overflows the small container and falls down as a liquid drop onto the surface of the outer container. We cannot observe directly the layer of the He II because it is too thin. The speed of the flow can be estimated to reach more than several tens of meters per second.
In this video, a view around the open side of the small container appears first: note carefully the position of the liquid surface, which locates slightly below the container edge. Then, overflow will not occur in the case of a normal liquid. The camera view point then moves to the closed, tapered end of the container, where the seemingly unreal feature is demonstrated that the Superfluid He II has escaped over the edge of the container and drops from the peaked bottom about every 6 seconds.
