After a century of studying their tangled mathematics, physicists can tie almost anything into knots, including their own shoelaces and invisible underwater whirlpools.
At least, they can now thanks to a little help from a 3D printer and some inspiration from the animal kingdom.
Two US researchers have effectively created “smoke rings” in water and knotted them together for the first time.
The gnarly feat paves the way for scientists to experimentally study twists and turns in a range of phenomena — ionized gases like that of the Sun’s outer atmosphere, superconductive materials, liquid crystals and quantum fields that describe elementary particles.
Physicists had long believed that a vortex could be twisted into a knot, even though they’d never seen one in nature or the even in the lab.
Determined to finally create a knotted vortex loop of their very own, physicists at the University of Chicago designed a wing that resembles a delicately twisted ribbon and brought it to life using a 3D printer, reports the Science.
Creating a knot in a fluid bears little resemblance to tying a knot in a shoelace, say Dustin Kleckner and William Irvine, physicists at the University of Chicago in Illinois.
The entire three-dimensional (3D) volume of a fluid within a confined region, such as a vortex, must be twisted.
Kleckner and Irvine have now created a knotted vortex using a miniature version of an aeroplane wing built with a 3D printer.
This development, reported today in Nature Physics, opens the path to new understandings in fields as diverse as cosmology, meteorology, fluid dynamics and turbulence.
Australian physicists praised the work and said the ability to now create knotted vortices would allow researchers to study properties such as their structure and stability.
Capturing images of the knot was another technical tour-de-force.
Fluid dynamicists often use coloured dye to trace the motion of fluids, but Kleckner and Irvine injected tiny gas bubbles into the water that were drawn to the centre of the knotted vortex by buoyancy forces.
A high-speed laser scanner capable of producing CT-scan views of the fluid at 76,000 frames per second enabled the researchers to reconstruct the 3D arrangement of the bubbles, thus revealing the knots, writes Nature.
“The authors have managed a remarkable achievement to be able to images these vortex knots,” says Mark Dennis, an optical physicist at the University of Bristol, UK, who has made knotted vortices from light beams2.
The new study, he adds, transforms abstract notions about physical processes involving knots into testable ideas in the laboratory.
Traditionally to create vortex in water scientists have forced a burst of fluid through a hole.
In this paper, the researchers used a round wing structure with angled edges that when accelerated through water generates either linked rings or a “trefoil” – a single ring that ties itself into a knot.
Irvine says until now energy and momentum have been considered the key to understanding fluid dynamics but there is a “lot of interest and suggestion that the degree of knottedness in fluids will help us understand fluid flow”.