Spore dispersal by elasticity-vortex coupling from a leaf upon raindrop impact

Plant pathogens such as rust spores cause significant agricultural losses in crops. Past studies have shown that liberation of rust spores from a fixed leaf surface can result from vortices induced by impacting droplets. In this present study, we find that an elastic fluttering leaf creates a periodic shedding of counter-rotating vortex tubes that enhance particle mixing and spatial transport. The dispersal trajectories of shed particles can be described by the centrifugation/expulsion from Lagrangian coherent vortices and the further expulsion of the outer conveyer belts formed by deforming vortex dipoles. We used both Lagrangian-averaged vorticity deviation (LAVD) calculations and Finite Time Lyapunov Exponents (FTLE) ridges to identify such elliptical and hyperbolic Lagrangian coherent structures (LCS) respectively, which emerge over 2D flow maps on the transverse cross-section of the leaf vibration, while investigating particle advection properties owing to such oscillatory, unsteady flow patterns. The vorticities are extracted from smoke visualization data and matched with theoretical predictions based on 2D velocity potentials commonly adopted in oscillatory airfoil theory. Dispersal statistics were collected for the dynamical process. In summary, the study visually captures vortical airflow patterns hidden in natural leaf vibrations, applies dynamics concepts to study its dry advection properties, and compares experimental data on transport with theoretical estimates.