As so many of us have done, whether in childhood or as adults, physicists recently blew on a bunch of fine, white dandelion seeds, and made a wish. It came true. They discovered a new, unique element of flight never before documented by scientists.
The dandelion—which derives its name from the French dent de lion, meaning lion’s tooth—may seem like a common weed to be eradicated by vigilant gardeners. But these plants, found in Asia, Europe, and the Americas, are officially classified as perennial herbs, with historical use in both food and medicine. Now, it turns out, they may soon inform human design.
A study published in the journal Nature on Oct. 17 (paywall), written by physicists from the University of Edinburgh in Scotland, reveals that dandelion seeds, which can fly for miles, travel unlike any other kind of seed previously studied. They do this with the help of a detached vortex that appears to propel and stabilize locomotion.
Basically, the dandelion seed floats through the air using a bundle of bristles atop a stalk, called a pappas. That structure acts like a parachute but looks more like the skeleton of an umbrella after the wind has ripped the protective fabric off. The pappas is made of filaments with large gaps between them that allows air to flow up through the bristles and carry the seeds far and wide, propelled by a floating vortex perfectly calibrated to the pappas.
By illuminating the air flow around the flying seeds with lasers, physicists were able to discern the unique mechanics of the pappas. They placed dandelion seeds in a vertical wind tunnel which kept them afloat indefinitely and used long-exposure photography and video to analyze the behavior of the air moving through the pappus bristles. That’s how they discovered the detached vortex ring, an air pocket with a stretched donut shape, floating stably above each pappus.
The porous dandelion pappus consistently contains 90 to 110 filaments (no more and no less). The number of filaments and the gaps between them appear to be tuned precisely to stabilize the locomotive vortex, the researchers say. The vortex floats above the seeds, and the amount of open space between the seed’s spokes are apparently the key to the stability of these detached vortices, study co-author and applied mathematician Cathal Cummins told Nature. Pressure differences between the air moving through the spokes and the air moving around the seed creates the vortex ring.
This design maximizes aerodynamic loading and minimizes material requirements, and the dandelion’s flight mechanism may help illuminate movement in other natural and artificial structures, the researchers say. “The discovery of the separated vortex ring provides evidence of the existence of a new class of fluid behavior around fluid-immersed bodies that may underlie locomotion, weight reduction and particle retention in biological and manmade structures,” the paper states.
In other words, the deceptively pesky and common dandelion is as mighty as its name implies. The “lion’s tooth” offers new clues on how to design things that fly. That’s not bad for the seeds of a plant so many believe is a weed.