Have you ever found yourself lying on the lawn on a summer day, absentmindedly examining the dandelions there? The cheerful yellow flowers, bowing their heads under the weight of bumblebees feeding on their nectar. A nearby collection that has gone to seed, with dense pods at the base of long, delicate stems and a bundle of bristles at the top, called the pappus. You pick one and blow while making a wish, and the tiny little cloud of filaments scatter, potentially travelling over 100 km in warm, dry air.
Wait – 100 km? Just drifting on air currents? Science helps us to understand how the intricately delicate design of the pappus is the perfect aerodynamic structure for making sure the seeds get spread as far as possible on the wind, giving rise to the incredible success of this plant in the western world. For years it was believed that the pappus acts like a parachute, increasing the drag on the seed to keep it afloat for longer than it would without this structure attached. But recent studies have demonstrated that drag alone is not enough, and the precise design of 100 to 110 fine hairs per seed is critical to these long flights.
(Mara is substituting cattail seeds here for dandelions, given the season!)
In 2018, a team in Scotland reported beautiful experimental results showing that the airflow through these filaments creates swirling rings of air, called vortices, behind the seed as it travels. This creates a region of low pressure behind the pappus, increasing the lift and allowing the structure to stay aloft for incredible distances, way further than if the seed was attached to a more conventional parachute-like design. The following year, scientists in Switzerland mathematically modeled the airflow through the filaments of the pappus, confirming these experimental findings. They even found that the optimal number of hairs on the pappus for maximum flight stability is 100, precisely the typical number found on every dandelion seed in your lawn. Mother Nature is a brilliant physicist!
More details:
Experimental work: https://www.nature.com/articles/s41586-018-0604-2
Theoretical work: https://journals.aps.org/prfluids/abstract/10.1103/PhysRevFluids.4.071901
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