First fluid knots created in the lab: To investigate, Dustin Kleckner and William Irvine of the University of Chicago, Illinois 3D-printed strips of plastic shaped into a trefoil knot and a Hopf link. Crucially, the strips had a cross section shaped like a wing, or hydrofoil (see picture).
Next, the researchers dragged the knots through water filled with microscopic bubbles. Just as a wing passing through air creates a trailing vortex, the acceleration of the hydrofoils created a knot-shaped vortex that sucked in the bubbles. The result was a knot-shaped flow of moving bubbles – the first fluid knot created in a lab – which the team imaged with lasers.