Wind-dispersed plants have evolved ingenious ways to lift their seeds 1,2. The common dandelion uses a bundle of drag-enhancing bristles (pappus) to help keep their seeds aloft. This passive flight mechanism is highly effective, enabling seed dispersal over formidable distances 3,4 ; however, the engineering underpinning pappus-mediated flight remains unresolved. Here, we have visualized the flow around dandelion seeds, uncovering an extraordinary type of vortex. This vortex is a ring of recirculating fluid, which is detached due to the flow passing through the pappus. We hypothesized that the circular disk-like geometry and the porosity of the pappus are the key design features that enable the formation of the separated vortex ring. The porosity gradient was surveyed using microfabricated disks, and a disk with a similar porosity was found able to recapitulate the flow behaviour of the real pappus. The porosity of the dandelion's pappus appears to be tuned precisely to stabilize the vortex, while maximizing the aerodynamic loading and minimizing the material requirement. The discovery of the separated vortex ring signals the existence of a new class of fluid behaviour around fluid-immersed bodies that may underlie locomotion, weight reduction, and particle retention of biological and manmade structures. Dandelions (Taraxacum officinale agg.) are highly successful perennial herbs, which can be found in temperate zones all over the world 5. Dandelions, like many other members of the Asteraceae family, disperse their bristly seeds using the wind and convective updrafts 6,7. Most dandelion seeds likely land within 2 m 8,9 ; however, in warmer, drier and windier conditions, some may fly further (up to 20,000 seeds per hectare travelling more than 1 km by one estimate) 6,10. Asteraceae seeds routinely disperse over 30 km and occasionally even 150 km 3,4. Plumed seeds comprise a major class of dispersal strategies used by numerous and diverse groups of flowering plants, of which the common dandelion is a representative example. Plumed seeds contain a bundle of bristly filaments, called a pappus, which are presumed to function in drag enhancement (Fig. 1a-c). The pappus prolongs the descent of the seed, so that it may be carried farther by horizontal winds 11 , and it may also serve to orientate the seed as it falls 7,12. Dandelion seeds fall stably at a constant speed in quiescent conditions 2,13-15. For wind-dispersed seeds, maintaining stability while maximizing descent time in turbulent winds may be useful for long-distance dispersal 16,17. It is not clear, however, why plumed seeds have opted for a bristly pappus rather than a wing-like membrane, which is known to enhance lift in some other species (e.g., maples 1). Here, we uncover the flight mechanism of the dandelion, characterizing the fluid dynamics of the pappus and identifying the key structural features enabling its stable flight. To examine the flow behaviour around the pappus, we built a vertical wind tunnel (Fig. 1d, and M1), designed so that the seed ca...
Pressure-solution creep is one of the most common crustal deformation mechanisms, inducing changes in the porosity and permeability of rocks. For a variety of rock types undergoing pressure solution, it has been shown that the presence of phyllosilicates may significantly enhance the rate of the pressure-solution process. In this experimental investigation, we present 4dimensional (three dimensions + time) X-ray microtomographic data that contrast deformation by pressure-solution of a pure NaCl aggregate with that of a mixture of NaCl and biotite. The results show that for mixed samples (NaCl+biotite), phyllosilicates induce a marked reduction in porosity and pore connectivity and contribute to an increase in the local strain rates by an order of magnitude over pure NaCl samples. At the same time, phyllosilicates do not induce strain localization in the sample. We discuss various possible explanations for these observations including a possible positive feedback between the porosity distribution and pressure solution. Our study yields novel insights into the local effects of phyllosilicates during pressure-solution creep and provides full 4-dimensional imaging and characterization of the coupled evolution of porosity and pore connectivity over previously unprecedented experimental time scales.
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