Insights into the flight of dandelion seeds are revealed in a paper published in Nature this week. By investigating the physics underlying the flight of the dandelion, the authors report the discovery of a new class of fluid behaviour around fluid-immersed bodies.
Dandelions use a bundle of bristly filaments, called a pappus, to help keep their seeds aloft for dispersal. The pappus prolongs the descent of the seed, so that it may be carried farther by horizontal winds, and may also serve to orientate the seed as it falls. However, it is not clear why plumed seeds, such as the dandelion, have a bristly pappus rather than a wing-like membrane, which is known to enhance lift in some other species (for example, maples).
Naomi Nakayama, Ignazio Maria Viola, and colleagues constructed a vertical wind tunnel to visualize the flow around freely flying and fixed dandelion seeds. Using long-exposure photography and high-speed imaging, the authors discovered a stable air bubble - called a vortex ring - that is detached from the seed body, yet steadily remains a fixed distance downstream of the pappus. They found that the porosity of the dandelion’s pappus appears to be tuned precisely to stabilize the vortex, and that the pappus delivers more than four times more drag per unit area compared with a solid disk. The authors argue that this makes the plumed design far more efficient than a wing-like membrane for the dispersal of lightweight seeds.