(English)

Normal liquid Helium: Low penetrability through "super leak"(484 kB)（requires QuickTime） Normal liquid He (He I) stored in a glassware sealed by a filter of the dense fine white powder (alumina powder), called a "superleak", demonstrates that He I penetrates the superleak very slowly because of a finite (i.e. normal) viscosity. This is very different from the case of superfluid He II. Phase transformation from normal liquid He I to superfluid He II(420 kB)（requires QuickTime） Liquid He shows strong bubbling at its boiling temperature of 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. Similarly, 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 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 as clear as desirable. Superfluid He II without viscosity below 2.17 K : penetrates easily through the superleak(156 kB)（requires QuickTime） This video demonstrates the remarkable feature that He II passes through the superleak instantly when the glassware is pulled up from the liquid surface of He II, which can be compared with the case of the normal liquid He I. He II does not have the viscosity that prevents He I from rapidly penetrating the superleak. More precisely speaking, it is well known that He II behaves as a mixture of two different kinds of fluids; normal fluid and superfluid. In the following, we use the terminology He I for the normal fluid and He II for the superfluid. Fountain of the superfluid He II(344 kB)（requires QuickTime） When the electric current is changed through the heater in the superfluid He II located above the white superleak, it is seen that the height of the He II fountain follows the current strength (i.e. the heating power). When the electric current increases the temperature of the He II, a part of the He II transforms into He I. As a result, He II flows from below the superleak into the upper region with the heater to compensate the difference in the chemical potential between the two parts separated by the superleak. Since the He I cannot rapidly penetrate down through the superleak, the liquid He overflows vigorously, like a fountain. Superfluid He II climbing walls and escaping from a container(404 kB)（requires QuickTime） 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 (the very 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 viewpoint then moves to the closed, tapered end of the container, where the surrealistic 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.

After Rika museum (Science museum) Vol. 11, "Liquid Helium -Superfluidity and Very low temperatures-"NHK Educational Co. Ltd.

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