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When you're basically a little floof ball, flying is hard.
To resist the wind, the dandelion seeds create a strange whirlwind in the air just above them. The new way of moving in the air, described on October 17 in Nature, solves a long – standing question about how the seeds stay in the air.
The flight of dandelion seeds is not unlike that of Mary Poppins: quite charming, but inexplicable in physics, until now. When a gust of wind pulls a seed out of the plant's blurred head, a fluffy structure called pappus keeps the seed in the air before it finally falls to the ground. The structure, which extends from the seed, is composed of tiny filaments in the shape of hair, which makes it essentially empty.
"It's a strange structure," says co-author Naomi Nakayama, a biophysicist at the University of Edinburgh. "Nobody really knew how he could fly."
So Nakayama and his colleagues dug in the weeds. High-speed mathematical and video simulations have revealed that pappus filaments act together as a uniform sheet or parachute and create a drag, a force that neutralizes gravity. The air also circulates around the pappus and is sucked into the area just above it. This air forms a swirling bubble that the researchers call a separate vortex ring, which adds to the drag.
Swirlin 'and Whirlin'
Dandelion seeds are attached to a fuzzy structure called pappus (seen from top to bottom on the left). The pappus creates a swirling air bubble in the area of low pressure above the structure (visible on the right with a seed in a wind tunnel). The pappus is the key to doing this job. The spacing of its filaments allows just enough air to hold the vortex stable, thus helping the seed float.
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As for the vortex rings, the dandelion is strange. Normally, these air bubbles remain attached to an object or are totally separated and disappear. But the dandelion bubble separates and hangs over the seed. "When you show it to a dynamic, it impresses," says Cathal Cummins, co-author of the study, a postdoctoral researcher at the Nakayama lab.
The bubble stays in place over the seed because the filament parachute leaks. A little air passes through the pappus and a pressure gradient is formed when the pressure is low above the filaments and the pressure below them, which keeps the bubble stable. This low pressure is also what sucks air that circulates around the filaments to form the bubble in the first place.
The way dandelion moves in the air is four times more effective at generating drag resistance – and therefore fighting gravity – than a structure more like a parachute. Other seeds with fuzzy external extensions or extensions may also use these odd vortex rings to fly through the air.
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