|MadSci Network: Physics|
Dear Andre You wanted to know why superfluid helium apparently runs uphill, for example, it climbs out of open containers. I’m pleased you found something useful on the Madsci site. ( http://www.madsci.org/posts/archives/feb98/888462465.Ph.q.html ) I did some web searching myself and I hope can take your understanding a little further. The generally accepted model for liquid helium, as proposed by Laszlo Tisza, is the “two fluid” model, a mixture of a normal liquid, “helium I” and a superfluid, “helium II”. As helium is cooled from its boiling point around 4 degrees K there is a transition from helium I to helium II at just above 2 K and at about 1K the composition is 100% helium II. Helium II has • no thermal energy ( no entropy ) • an exceptionally high thermal conductivity and • no viscosity The mixture of He I.and He II displays properties which are not necessarily intermediate. For example viscocity can be measurable but still very low. By analagy, a suspension of a hard latex in water will exhibit similar viscosity to water. What does this tell us about He flowing out of the vessels into which it is poured ? Firstly, we can say that the effect is not driven by surface tension. One of the properties of He II is that it exhibits virtually no viscosity. This indicates a lack of interaction between atoms in the liquid so it is unlikely that He II interacts strongly with the surface of the vessel. The low viscosity makes it easy for the liquid to move about ( superfluidity ) and explains why small influences may produce big effects. It can pass through very small holes and tubes which would block the transport of normal liquids including He I. He II does have a low surface tension generally but it does form thin films of liquid of 50 to 100 atoms thick on surfaces with which it is in contact. These films are also superfluid, allowing friction-free movement of liquid through them. He II appears to flow from colder to warmer regions. I have to confess that I couldn’t find an expalnation for this beyond the statement that “In small pores or thin films the application of a temperature gradient along the pores sets up a pressure differential” ( see http://mgravity.itsc.uah.edu/microgravity/micrex/exps/mas-spl2.html ). If two vessels containing helium II are connected by a narrow capillary and one of the two is heated a flow of helium toward the heated vessel will occur. I think that this can be explained by the fact that at the two different temperatures, there is a different ratio of He I /He II and that on the high temperature side there is a lower concentration of He II. While He II can easily flow through the capillary, He I with its higher viscosity is unable to do so. This results in a flow from cold to hot regions. This provides a reason why liquid helium climbs out of open containers. In this situation, the walls of the vessel above the bulk liquid would be expected to be at a higher temperature than the walls in contact with the liquid. The walls have on them a thin film of helium within which He I would flow with difficulty while He II will flow easily. Again this would result in flow in the direction of the higher temperature and out of an open vessel.. Another site, http://cryowwwebber.gsfc.nasa.gov/introduction/liquid_helium.html describes an experiment to show the “Thermo-mechanical” effect which amplifies this behaviour and results in a fountain. Conversely, a porous plug in the opening of a vessel can be used to contain liquid helium even in zero gravity. Here, the evaporation of He on the outside of the plug establishes a temperature gradient and a corresponding pressure gradient in the right direction to discourage flow through the plug. Sometimes you get lucky !
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