PURPOSE: To show the super fluid properties including the lambda transition and the consequences of zero viscosity.
DESCRIPTION: A nested double dewar system is made to hold liquid helium and liquid nitrogen cryogens, as shown in the photo at left. The liquid helium is pumped down to the lambda point. At the lambda point transition it is observed to become quiescent, and the super fluid state is achieved. The super fluid properties can be shown.
The creep effect, or self siphoning, is one consequence of zero viscosity. A small glass bucket is dipped into the LHe and then pulled out. The LHe in the bucket will drip out until it is entirely empty.
The fountain effect is a consequence of zero viscosity and also shows that heat can raise the LHe out of the super fluid state. A glass fountain with a super leak is used. The super leak is a sinter that is packed with a fine powder. A normal liquid cannot pass through the super leak. The fountain device is shown in the photos below. Inside the fountain are small black balls. Heat in the form of IR light is directed onto these black balls. The heat will cause the super fluid to change to a normal fluid. Once the LHe has returned to the normal fluid state it cannot pass through the super leak and fountains out the top of the fountain. The pressure to generate the fountain is derived from the Brownian motion of the super fluid helium molecules. As they randomly move a few will pass through the super leak. Once inside the chamber some of these molecules will become normal and hence can no longer pass through the super leak. This is how the over-abundance of normal helium is accumulated which ultimately causes the pressure which supports the fountain.
These demos are particularly useful in the first year advanced introductory physics course to show the difference in the physical properties of Bosons and a Fermions. The populations of particles with allowed energy states is changed into particles with excluded energy states. This leads to Bose-Einstein statistics and Fermi-Dirac statistics. This demo can also be done in the astronomy course to demonstrate one of the properties of star matter found during the lifetime of a star.
EQUIPMENT: Super fluid helium apparatus, as photographed.
SETUP NOTES: The photos below show the best set up for the camera and the lighting. Note that the point source is the best light to use for both the creep effect and the fountain effect. The point source is used as a back light, as shown in the photos. No fluorescent light is needed.
The left and center: Fountain Effect Apparatus. Right: The baffles with the creep effect bucket and the fountain.
Updated by JZ in 11/5/2005