Lagrangian coherent structures in spore dispersal around fluttering leaves*

Plant pathogens such as rust spores cause ongoing issues for agricultural practices. Past studies have shown that liberation of rust spores from leaf surfaces can result from vortices induced by impacting droplets. New experimental findings on oscillatory substrates like a fluttering leaf suggest that periodic shedding of counter-rotating vortex tubes can enhance particle mixing and spatial transport. The navigation routes of shed particles from sheared boundary layers 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.