HOME |
Phase transition from Normal liquid He I to Superfluid He II |
Liquid He shows strong bubbling at a boiling temperature (about 4.2 Kelvin, at 1 atm).
It is well known that the boiling temperature of water at the top of a mountain becomes lower than 100 ･C. For example, at the summit of Mt. Fuji with a height of 3776 m and an atmospheric pressure of 638 hPa, water boils at about 63 ･C. Thus, evacuation with a vacuum pump results in a lowering of the boiling temperature. At a pressure of about 37 Torr (about 50 hPa. Note that the indication of pressure in the video deviates somewhat from this number), the boiling temperature reaches about 2.17 K, where Normal liquid He I transforms to the Superfluid He II.
Since Superfluid He II has no viscosity, heat is quickly absorbed by the surrounding He II and transported to its surface, resulting in evaporation of He atoms from the liquid surface. Then, the bubbling does not occur in the interior of liquid He II, which means that the liquid He boiling strongly above 2.17 K suddenly stops boiling at 2.17 K, giving rise to a calm, water-like, -surface of He II below 2.17 K.
In this video, liquid He is stored in a small container mounted inside of a somewhat larger vessel to reduce heat leak. Liquid nitrogen with the boiling temperature of 78 K is contained between these dewars to further reduce the heat flow into the liquid He. Because of bubbling of liquid N2 and a limited resolution of the video (due to file compression), the feature of the transformation from Normal liquid He I to Superfluid He II is not clear enough.
