Science & Technology

Watch the Weird Way Dandelion Seeds Float in a Wind Tunnel

Dandelions aren't exactly the world's most popular flower; they're more likely to end up in lawn trimmings than in a beautiful bouquet. But when the blossoms shrivel and go to seed, something magical happens: dozens of white, fluffy parachutes take flight on the breeze, some traveling great distances from their starting place. According to new research, dandelion seeds are even more impressive than we give them credit for. They fly in a way that was previously unknown to science.

We'll All Float On

Scientists already knew that dandelion seeds were impressive flyers. They've been known to travel more than 100 kilometers (62 miles), even journeying over water to take root in faraway lands. They achieve this thanks to the very thing that makes them beautiful: a bundle of snowy white bristles known as the pappus. Like a skydiver's parachute or Mary Poppins' umbrella, the pappus provides drag, slowing the seed's descent and helping it catch a ride on the next updraft.

But plenty of seeds are equipped with drag enhancers: think of the propeller shape of maple seed pods or the batlike wings of the Javan cucumber seed. They don't travel nearly as far as the humble dandelion. Why?

To find out, researchers from the University of Edinburgh gathered up a bunch of dandelion seeds, a wind tunnel, a camera, and — what else? — lasers. By letting a single seed float in the vertical wind tunnel and illuminating the air around it with laser light, they were able to use long-exposure photography and video to analyze the fluid dynamics of the moving air.

The Wind Beneath My Ring

What they found was surprising: The tiny spaces between the bristles let just enough air through to create a long, spinning bubble right above the pappus that the researchers call a separated vortex ring. As air flows up through the pappus, it hits the outside of the separated vortex ring and starts to follow it along the outside — up, over, down, and around again in a vertical cyclone. Fast-moving air has lower pressure, so the separated vortex ring acts as a region of low pressure right above the pappus, helping keep it aloft. You can watch it happen in the clip below, posted by David Breslauer.

But dandelion seeds aren't just built for lift — they're also built for stability. The scientists used X-ray scans and microscopes to figure out exactly how each pappus was put together and determined that they each contain 100 filaments, on average, and are 92 percent porous — that is, they're made up of 92 percent empty space. The scientists tested their own versions of dandelion seeds with varying amounts of porosity and found that 92 percent was the sweet spot. Any more or less, and the separated vortex ring broke down.

This is huge, since engineers are trying to build smaller and lighter electronics, and once you're small and light enough, you have to play by different flight rules. "These seeds we're talking [about are] a fraction of a milligram, so the forces and the weights involved are so much smaller," Cathal Cummins, an applied mathematician at the University of Edinburgh and first author on the study, told Popular Science. "The dandelion has managed to create a parachute which is virtually entirely empty space. Our research is suggesting that basically, less is more."

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Written by Ashley Hamer November 6, 2018

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