For the first time ever, scientists have tied water into a knot. It's not the kind of knot sailors would be familiar with, but it's a knot nonetheless. These knots have closed loops with no ends to untie, sort of like trefoil knots or Hopf links. It's an achievement that has eluded scientists for nearly a hundred years, but now they've finally done it in the lab.
Indeed, physicists have suspected that something like this is possible for quite some time, both for fluids and plasmas (in both classical and quantum realms). Given the prominence of knots in other aspects of science, like mathematics and biology, the potential for a fluid knot (linked and knotted vortex loops in particular) was taken very seriously.
Writing in New Scientist, Jacob Aron explains how it was done:
Mathematicians have shown that just as knots in string can't be untied no matter how much you prod and pull them, fluid knots should also never unravel – even though the particles that make up the fluid will be circulating around. But this non-unravelling property only applies if the knot is made of a theoretical "ideal fluid", one that has no viscosity – in other words, no resistance to flow. How a knot in a real fluid such as smoke or water would evolve is unknown, as is whether these structures exist in nature or in the plumes created by machines such as aircraft.
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.
Once formed, the knots move, rotate and eventually appear to dissipate, though whether the vortices completely unknot, unlike in ideal fluids, or somehow preserve the knottedness but in a more diffuse form remains an open question.
As the researchers note in their study, "This work establishes the existence and dynamics of knotted vortices in real fluids."
Moving forward, the physicists will seek to understand if similar effects can happen in other vortices, including those that come off of aircraft wings. This effect may also play a role in superfluids (frictionless quantum fluids).