Credit: Nature Video

Dandelion seeds fly using a method that researchers thought couldn’t work in the real world, according to a study1 published on 17 October in Nature.

When some animals, aeroplanes or seeds fly, rings of circulating air called vortices form in contact with their wings or wing-like surfaces. These vortices can help to maintain the forces that lift the animal, machine or seed into the air.

Researchers thought that an unattached vortex would be too unstable to persist in nature. Yet the light, puffy seeds of dandelions use vortices that materialize just above their surfaces and lift the seed into the air.

Up in the air

Dandelion seeds bear filaments that radiate out from a central stalk like the spokes on a bicycle wheel, a feature that seems to be the key to their flight. Many insects harbour such filter-like structures on their wings or legs, suggesting that the use of detached vortices for flight or swimming might be relatively common, says study co-author Naomi Nakayama, a plant scientist at the University of Edinburgh, UK.

Researchers were curious about how these bristly seeds stayed in the air because they looked so different from the wing-like seeds of other plants, such as maples. Those structures act like the wings of a bird or aeroplane, generating pressure differences above and below the wing to fly. To find the answer, Nakayama and her colleagues put dandelion seeds in a vertical wind tunnel and used a laser to illuminate particles that helped to visualize the airflow around the seed.

That’s when they saw the vortex floating above the seeds. The amount of open space between the seed’s spokes seems to be the key to the stability of these detached vortices, says study co-author Cathal Cummins, an applied mathematician at the University of Edinburgh. Pressure differences between the air moving through the spokes and the air moving around the seed creates the vortex ring.

Previous studies have found that dandelion seeds always have between 90 and 110 bristles, says Nakayama. It’s “scary consistent”, and that consistency turns out to be very important.

﻿Just right

When the team designed small silicon discs to imitate these spokes, they produced models with a range of openings: from solid discs to ones that were 92% air, like the structures on the dandelion seeds. When the researchers tested these model seeds in their wind tunnel, they found that only the discs that best approximated dandelion seeds could maintain the detached vortex.

If the number of openings in the discs was even 10% off of those in dandelion seeds, the vortex destabilized. The seed looks inefficient for flight because it has so much open space, says Nakayama, but these openings are what allow the unattached vortex ring to remain stable.

It’s great to see an analysis of something we see every day but didn’t fully understand, says Richard Bomphrey, a comparative biomechanist at the Royal Veterinary College in Hatfield, UK. “To discover that there were aerodynamic mechanisms that we didn’t already know — despite the fact that we can fly things at Mach 9 — is always exciting.”