Wind-dispersed plants have evolved ingenious ways to lift their seeds1,2. The common dandelion uses a bundle of drag-enhancing bristles (the pappus) that helps to keep their seeds aloft. This passive flight mechanism is highly effective, enabling seed dispersal over formidable distances3,4; however, the physics underpinning pappus-mediated flight remains unresolved. Here we visualized the flow around dandelion seeds, uncovering an extraordinary type of vortex. This vortex is a ring of recirculating fluid, which is detached owing 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 to be able to recapitulate the flow behaviour of the pappus. The porosity of the dandelion pappus appears to be tuned precisely to stabilize the vortex, while maximizing aerodynamic loading and minimizing material requirements. The discovery of the separated vortex ring provides evidence of the existence of a new class of fluid behaviour around fluid-immersed bodies that may underlie locomotion, weight reduction and particle retention in biological and manmade structures.The flight of dandelion seeds is enabled by an extraordinary vortex ring, which was revealed by the visualization of the flow around the seed.

中文翻译：

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分离的涡环是蒲公英飞行的基础

风散植物已经进化出巧妙的方式来提升它们的种子1,2。常见的蒲公英使用一束增强阻力的刷毛（绒毛），有助于将种子保持在高处。这种被动飞行机制非常有效，可以使种子传播到很远的距离3,4；然而，支撑冠毛介导飞行的物理学仍未解决。在这里，我们可视化了蒲公英种子周围的流动，揭示了一种非凡的漩涡。该涡流是一圈循环流体，由于流经冠毛而分离。我们假设圆盘状几何形状和冠部的孔隙率是能够形成分离涡环的关键设计特征。孔隙度梯度是使用微加工圆盘测量的，并且发现具有相似孔隙率的圆盘能够概括冠部的流动行为。蒲公英冠部的孔隙率似乎经过精确调整以稳定涡流，同时最大限度地增加空气动力载荷并最大限度地减少材料需求。分离涡环的发现提供了证据，证明在流体浸没的物体周围存在一类新的流体行为，这可能是生物和人造结构中运动、减轻重量和粒子保留的基础。蒲公英种子的飞行是由非凡的涡环，这是通过种子周围流动的可视化揭示的。同时最大限度地提高空气动力载荷并最大限度地减少材料要求。分离涡环的发现提供了证据，证明在流体浸没的物体周围存在一类新的流体行为，这可能是生物和人造结构中运动、减轻重量和粒子保留的基础。蒲公英种子的飞行是由非凡的涡环，这是通过种子周围流动的可视化揭示的。同时最大限度地提高空气动力载荷并最大限度地减少材料要求。分离涡环的发现提供了证据，证明在流体浸没的物体周围存在一类新的流体行为，这可能是生物和人造结构中运动、减轻重量和粒子保留的基础。蒲公英种子的飞行是由非凡的涡环，这是通过种子周围流动的可视化揭示的。