Physicists create knotted rings

Sophisticated 3D stencils make it possible to produce complex shapes from smoke or bubbles

A cloverleaf knot from a vortex tube: This model shows one of the complex ring shapes that the physicists first created. © Dustin Kleckner and William TM Irvine
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Producing simple smoke rings is easy, but so far even researchers have been biting their teeth on more complex structures such as knotted or entangled rings. Now, thanks to a clever template, two US physicists have for the first time succeeded in producing such vertebrae as cloverleaf-like nodes or two linked rings. Once produced, these structures promptly developed a completely unexpected behavior: they untangled themselves as if by magic, gradually, as the researchers report in the journal "Nature Physics".

"Tying shoelaces into a knot is a relatively simple affair, " explain Dustin Kleckner and William Irvine of the University of Chicago. But what they have tried is more like trying to knot a swirling tornado with itself. Because whether smoke ring or bubble field: All these phenomena consist of a kind of vortex tube, a flow field, which has the same shape as the concentric magnetic field, which forms around a current-carrying cable. According to theory, such flow fields should exist not only as rings or simple arcs, but also in more complex forms. For example, it is suspected that the magnetic field lines on the sun form knotted plasma hoses, and in quantum physics researchers also assume that knots are knotted.

From the wing to the complex 3D template

The problem with this: So far, such knot structures exist only in the imagination and in the calculations of physicists. Up to now, only simple smoke or bubble rings have been produced in the laboratory, no more complex shapes. "Normally you create a bubble ring by blowing a burst of ultrafine gas bubbles through a circular opening into a water tank, " the researchers explain. In order to form a knot out of the simple ring, they first had to develop a suitable 3D template that would bring the bubbles into the desired shape.

The physicists tested different template shapes until they finally found a working one - this is the one that finally works. © Dustin Kleckner and William TM Irvine

For this, the physicists initially started from a plastic ring, the hoop of which had a flared shape in cross-section and which initially produced ring-shaped vortex hoses. They then modified these underwater surfaces by making them either two interlocked rings using 3D printers, or a continuous, self-tied strip. Using these templates, the physicists then blew a bob of tiny gas bubbles into a water tank and recorded the developing structure using a high-speed camera. As hoped, three-lobed knots and interlocked rings actually formed - for the first time since the beginning of such efforts some hundred years ago.

Knots are solved as if by magic

Surprisingly, however, the physicists watched the further development of their bladder formations. "Simple rings behaved as expected: they just moved forward and barely changed their shape, " they report. The warped rings and three-lobed knots, however, would have behaved in unexpected ways: they began to rotate and deform. "Neighboring areas of the vortex tubes moved towards each other until they were almost parallel, then wound around each other and finally collided, " Kleckner and Irvine describe their observations. display

This is how the first cloverleaf knot created with the special template looked like microblisters. Dustin Kleckner and William TM Irvine

This process led to a fundamental shapeshifting - from the two linked rings as well as from the three-lobed knot, two separate, single rings remained at the end. This surprising resolution of the complex forms in favor of simpler ones gives the researchers an important insight into the physics of such vortex tubes, as they are theoretically also in the solar plasma, in ultracold Bose-Einstein condensates and other "exception "Matters" of matter occur.

But the experiment also raises new questions, as Kleckner and Irvine emphasize. So it is still completely unclear whether all nodes decay in the observed way, or whether there are also stable node shapes. Also, which mechanisms are ultimately responsible for the dissolution of the nodes, is still in the dark. "But our system now offers the opportunity to experimentally explore many aspects of these structures, " said the physicists. (Nature Physics, 2013; doi: 10.1038 / nphys2560)

(Nature Physics, 04.03.2013 - NPO)