Dustin Kleckner sez, "Scientists tie vortex rings (smoke rings, basically) into knots using 3D printed wings. Includes high speed video, also in 3D. In addition to being very cool, they are also related to knots and braids that appear in places like the sun's surface. Full disclosure: I'm one of the scientists that did the research."
The duo overcame their experimental difficulties by designing and fabricating various hydrofoils (wings used in water) on a 3-D printer. They tried approximately 30 different shapes before they successfully created the desired vortices. 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," Irvine said.
Vortex loops could untie knotty physics problems [U Chicago Press Release]