IPLS/BioP Seminar: Dr. Naomi Nakayama (Imperial College London)

Abstract: Hairs, whiskers, stems, tentacles… the living world is full of slender cylinders of various scales. Why are they so prevalent? We have started to answer this question, with an iconic example of hairy structures from nature: the dandelion fluff. The fluff (botanically named pappus) is a bundle of about 100 filamentous organs, which is positioned above the seed. The pappus is dogmatically believed to act as a parachute to lift the seed, but with over 90% empty space, the fluid dynamic mechanism of a parachute cannot apply to pappus. By visualizing the air flow movement around the pappus in a bespoke vertical wind tunnel, we have found a previously unobserved type of whirlwind (vortex). It is a ring-shaped vortex that is physically separated from the pappus but stays at a constant distance downstream of the body, which helps the seed stay afloat. To identify the key design features of the pappus structure in generating the separated vortex ring, we fabricated 2D disks with varying porosity and showed that the 90% porous disk could recapitulate the flow behavior. The pappus morphology is not fixed; it reversibly closes when the surrounding air is high in moisture. This morphing appears to enable preferred dispersal when the air is dry and wind is strong, while keeping the seeds within the water-rich microenvironment. There is a hydrostatic actuator at the base of pappus underlying this morphing. We are dissecting what structural features are key drivers of the morphing, using deformation analysis and 2D finite element method modelling.

Host: Yanlan Mao

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