How to tie a knot in a liquid
As early as the 19th century, physicists discovered that stable nodes could exist in liquids. Thanks to special quantum vortices, this also works in real systems.
Liquids are generally too changeable for complex structures such as knots to remain stable in the long term. But there are exceptions, as a team led by Toni Annala from Aalto University in Finland has now demonstrated: In some fluids, you can knot a vertebra in such a way that it won't come loose again. However, you don't have to worry about choking on an unexpected knot in your coffee. As the team reports in a paper accepted for publication in the journal "Physical Review Letters", this only works in liquids that exhibit macroscopic quantum effects. In addition to Bose-Einstein condensates, this also includes certain liquid crystals. There, the vortices cannot penetrate themselves and thus loosen the knot.
Vortices in liquids are essentially rotating tubes, and knots can be formed from them under certain conditions. Such a knot is a closed loop that crosses itself in such a way that it cannot be distorted into a simple ring - for example, the cloverleaf knot that you can actually create in coffee. However, such knots quickly disintegrate again. On the one hand, the vortices can simply penetrate themselves and loosen the knot, and on the other hand, they quickly disintegrate into many simple, unconnected ring vortices. As early as the 19th century, however, experts realised that such knots would be mathematically stable in an ideal fluid without friction.
Until now, however, no real fluid was known in which vortices could be permanently knotted. Annala's team has now been able to show that such fluid nodes can be realised if quantum vortices are used. These so-called topological vortices are not mechanical vortices, but linear perturbations in the structure of a highly ordered fluid, where it makes a difference whether you go round them clockwise or anti-clockwise. The trick is that a quantity called vorticity is conserved in such systems, analogous to the conservation of energy in classical physics. In their publication, the working group demonstrated that there are knots in real, albeit exotic, fluids that can only be solved by violating this conservation parameter. The next step for the experts is to actually tie a knot in a Bose-Einstein condensate.
Spectrum of Science
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