Tying a smoke ring in a knot sounds impossible - but University of Chicago physicists have done something similar by creating a vortex knot for the first time, in a container of fluid.
Vortex knots should, in principle, be persistent, stable phenomena. "The unexpected thing is that they're not," says Dustin Kleckner of UChicago's James Franck Institute. "They seem to break up in a particular way. They stretch themselves, which is a weird behavior."
In the end, this behavior culminates in 'reconnection events, with the loops elongating, circulating in opposite directions, moving toward each other and then colliding. Parts of the vortices then annihilate other parts, changing their configuration from linked or knotted into one that is unlinked or unknotted.
Understanding such knots could offer a way of untangling the complicated behavior of the electrically charged gas in plasma flows, for example, and for understanding the energy transport of complex flows in regular fluids and superfluids.
The team created the knot after designing and fabricating various hydrofoils on a 3D printer - and tried around 30 different shapes before they acieved success. When accelerated in a water tank at more than 100 g, hydrofoils leave behind bubble-traced vortex loops, whose dynamics the researchers recorded with a high-speed camera.
"The bubbles are a great trick because they allow you to see the core of the vortex very clearly," says assistant professor in physics William Irvine.
Next, Irvine and Kleckner hope to perform some of their experiments on a larger scale, to investigate whether increasing the size would make vortex rings more stable. They're also investigating the fine scale features of the vortices and whether "knottedness" is, or can be, conserved in fine-scale twisting of the vortex loops. "This is not something we presently know," says Kleckner.