Superfluidity: Flow through walls

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Superfluidity: Flow through walls
Superfluidity: Flow through walls

Flow Through Walls

In the winter, the lifeguard often blows up barriers made of washed-up ice floes because the water that flows in behind them is dammed up and threatens to flood the area. Such a thing would not be necessary in the quantum mechanical world of helium: there it simply flows through the obstacle. A little experiment while drinking lemonade: We dip the straw into our drink, hold our thumb on top, draw a little liquid out of the shower – and let the lower end of the straw freeze in a bucket full of ice. If a small lump of ice has now formed at the bottom of the tube, we can remove the thumb from the top without fear of loss. The lemonade that isn't frozen at the top stays in the stalk - it can't get through the solid plug of ice at the bottom.

What lemonade is impossible, some ultra-cold liquids are possible, according to reports from laboratories around the world. A team led by the Japanese Satoshi Sasaki from the French Universities 6 and 7 in Paris and the Center National de la Recherche Scientifique CNRS has now taken a closer look at this phenomenon.

Instead of lemonade, Sasaki and his colleagues used helium for their experiments. At high pressure and extremely low temperatures of a few degrees above absolute zero, this noble gas becomes liquid and finally solid. If the pressure is further increased or the temperature reduced even more, then superfluid or supersolid helium is formed. The resulting liquid suddenly loses all internal flow friction: a whirlpool in the broth would flow and flow and flow. The situation is similar with the supersolid substance: If it were allowed to slide along a surface, the movement would find no stopping.

The scientists now collected liquid helium in a total of thirteen glass tubes and sealed the lower end with supersolid helium. Then they saw what happened. At ten, nothing happened. The plugs held tight. In three samples, however, the liquid leaked into a catch basin provided for this purpose. On closer inspection, the experimenters found that the cannulas, which were sealed, were sealed with a clean, flawless ice crystal: a so-called monocrystal. The permeable tubes, on the other hand, were closed with clumps of ice, which apparently consisted of several crystallites whose interfaces were separated from one another by so-called grain boundaries. The material was nevertheless compact through and through - similar to solid steel or iron, which also consist of microscopically small and firmly bonded crystalline particles. And yet the superfluid helium managed to penetrate this obstacle.

The scientists now suspect that liquid exchange occurs along these grain boundaries. Because the more of them there were to be found, the faster the glass tubes emptied. In any case, they were able to rule out that the superfluid helium ran between the ice plug and the inner edge of the glass. A tube sealed with a perfect monocrystal would then also have to leak. Because of the uniform flow behavior, the experimenters were also able to read that the liquid was in fact a superfluid substance. A normal liquid would empty the tube after an exponential progression.

The researchers don't want to speculate on exactly how the process works. After all, they have not yet succeeded in specifically creating multi-crystalline ice caps with defined grain boundaries. But they seem to be hot on their heels for the mystery of the haunted liquid that goes through walls.

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