Plants have been practicing seed dispersal for long enough to get very good at it. Although many rely on pollinators, for some the answer is blowing in the wind. Lacking their own power source, plants have had to evolve sophisticated shapes to maximize the distance breezes carry them. Now the physics behind dandelion seeds’ remarkable hang time has been revealed, and it may bring applications for our own flight efforts.
Dandelion seeds are supported by a set of bristles known as a pappus, whose effectiveness can be seen every time you blow on them. Physicists, however, have been puzzled as to how they float so well. Where maple seeds use a foil that resembles the wing of an airplane or the rotor of a helicopter, established fluid dynamics couldn't explain the benefits of a pappus.
Dr Naomi Nakayama of the University of Edinburgh built a vertical wind tunnel and placed dandelion seeds inside, letting some float with the wind while others were tethered. A laser lit up the air passing around and through the seeds. Through advanced photography, Nakayama revealed the presence of an air bubble, called a separated vortex ring, that was a fixed distance downwind from the pappus.
In Nature, Nakayama reports that the distances between the pappus bristles appear perfectly calibrated to produce the separated vortex ring. Each bristle produces a boundary layer around it, and collectively these obstruct airflow through the pappus. A solid disk could provide the same obstruction while having a 38 percent smaller diameter, but such a disk would inevitably be much heavier than the tiny (and hollow) bristles.
The vortex ring gives the pappus four times the drag per unit area of a solid disk, Nakayama concluded, making it an exceptionally effective parachute and keeping the seed aloft. The vortex also helps the seed remain upright, maximizing its chances of sprouting when it comes to the ground.
Most dandelion seeds fall within 2 meters (7 feet) of their parent. If the wind is right and the weather warm and dry, many will drift far further. Seeds from members of the Asteraceae family, to which dandelions belong, have been observed to travel 150 kilometers (95 miles), and journeys of 30 kilometers (20 miles) are common.
Humans may have only just cottoned on, but Nakayama noted that many tiny plants and animals come similarly equipped with bristles, and these may be for the same purpose, including in water.
The thin bristles are hard to scale up, which is probably why larger seeds use wings instead. Consequently, technological applications of this knowledge may be limited to the small-scale. However, considering separated vortex rings were rejected as being impossible to sustain, engineers may have some rethinking to do.
