For a plant species to survive and proliferate, it must disperse its seeds. A seed is more likely to thrive away from its parent plant because there is less competition for resources. Spreading helps the population find new environments, be less vulnerable to predators and pathogens, and boost its genetic diversity.

1 et al. , Science 324, 1438 (2009). 1. D. Lentink, 1438 (2009). https://doi.org/10.1126/science.1174196 Seeds, though, do not propel themselves; rather, they hitch rides on animals, flowing water, or gusting wind. For travel by wind, seeds use one of two mechanisms: wings or plumes. Winged seeds, like those from maple trees, generate lift by using a stable leading-edge vortex as they fall.Plumed seeds, such as those of dandelions, amplify drag by using a bundle of filaments known as a plume or pappus. The additional drag prolongs the seed’s descent and increases the chance that a breeze can arrive to carry it away.

2 et al. , Nature 562, 414 (2018). 2. C. Cummins, 414 (2018). https://doi.org/10.1038/s41586-018-0604-2 Whereas some researchers assumed that pappus-mediated flight functioned in the same way as a parachute, Cathal Cummins, Ignazio Maria Viola, Naomi Nakayama, and their coworkers at the University of Edinburgh suspected that something else might be going on. They used a vertical wind tunnel to look at the flow around a falling dandelion seed and observed above the pappus a hovering ring of circulating air known as a separated vortex ring (SVR).Such fluid flow had been considered theoretically, but until now was presumed too unstable to be physically realized.

During a seed’s flight, the researchers found, aerodynamic interactions between filaments enhance the drag on the pappus so much that the pappus generates four times as much drag as a solid membrane of the same surface area. To investigate the source of the drag, they fabricated artificial dandelion seeds with different pappus geometries. Not every artificial pappus generated an SVR, but the researchers found that real dandelion seeds are optimized to do just that.

A new type of flow Section: Choose Top of page ABSTRACT A new type of flow << Artificial plumes Generating drag References CITING ARTICLES 3 et al. , Nature 388, 252 (1997). 3. S. B. Field, 252 (1997). https://doi.org/10.1038/40817 A vortex that is generated in the wake of a falling object has two stagnation points at which the fluid velocity goes to zero. For a solid body, like a disk or a parachute, one of those points is attached to the surface of the object. A vortex creates a region of low pressure behind the object that retards its fall through the air. If the vortex becomes unstable, it separates from the surface and moves upward as another vortex forms in its place. That sequence of events creates an oscillating flow in the wake of the object and causes it to tumble chaotically like a falling coin instead of descending smoothly like a parachute. Unlike a solid body, a dandelion pappus is filamentous, so air can flow through it in addition to going around it. To investigate how that difference might affect the flow around the whole seed, Cummins built a vertical wind tunnel. He added smoke as a tracer and used laser light to image a two-dimensional vertical cross section of the three-dimensional flow. From the light scattered in the imaging plane, the paths of the smoke particles could be reconstructed and used as a proxy for the airflow. 1a 1b When Cummins placed the dandelion seeds in the wind tunnel and tuned the flow such that the seeds hovered at a fixed height, he observed the formation of an SVR. In the images, it looked like a vortex bubble was hovering above the pappus, as shown in figure. Unlike in the case of a solid disk, shown in figure, the stagnation point sits above the pappus; it can’t be attached because air is flowing through the filamentous structure. However, the separated vortex does not move away from the seed. Instead, it stays at a fixed height above the pappus and creates a low-pressure region that stabilizes the seed’s flight. When the researchers set out to study dandelion-seed flight, they didn’t expect to find an SVR. “It was a surprise,” says Nakayama. But they did have an inkling that they would see interesting flows. “We kind of knew that something cool was going on, so we were definitely keen to see the fluid behavior from the beginning.” And their intuition proved right: The SVR, with its steady detached flow, had never been seen before. The mechanism they uncovered is “effectively a new way of flying,” says Viola.

Artificial plumes Section: Choose Top of page ABSTRACT A new type of flow Artificial plumes << Generating drag References CITING ARTICLES Nakayama, a biomechanics researcher, was always interested in studying plumed-seed flight for its implications in plant ecology and habitat establishment. However, it was not until she moved to the University of Edinburgh that she could tackle the problem. There, she teamed up with her “partner in crime” Viola, a fluid mechanist, and with Cummins, an applied mathematician studying biological flows. 2 After observing the SVR above the dandelion pappus, Nakayama, Viola, and Cummins wanted to further investigate seed flight and vortex formation. Natural dandelion seeds are remarkably uniform in their geometry. To explore a range of seed geometries, the researchers needed to replicate the delicate structures. Enrico Mastropaolo, a microfabrication expert at Edinburgh, became the fourth crucial collaborator on their team. He used photolithography and microfabrication techniques to make artificial silicon seeds with pappi ranging from solid to highly filamentous. One of his replica seeds is shown in figure 3 The researchers found that a pappus’s ability to form a stable SVR depended on two factors: its porosity, which is defined as the empty fraction of a circle enclosing the pappus, and the ambient Reynolds number, which describes the relative importance of inertial forces to viscous forces in the flow around the seed; for a seed fixed in the wind tunnel, the Reynolds number is directly proportional to the wind speed. Figureshows the measured dependence of stable vortex formation on porosity and Reynolds number. At each porosity, the SVR became unstable above a critical Reynolds number, and that value increased with porosity. All of the freely flying natural seeds that the researchers tested generated stable SVRs. Their measured Reynolds numbers were just below the critical value when they were falling at their terminal velocity (about 39 cm/s). At 92%, the porosity of a real pappus could hardly be higher, but if it were any lower, the resulting vortex would be unstable. Dandelion seeds have very similar porosities and weights wherever they come from, notes Nakayama. “A lot of things in biology tend to be so variable,” she says, but for dandelions, “when you look at their design, their structure, the variations are quite tight. And that tightness is necessary to hit the sweet spot in how this flight mechanism works.”