Bending, twisting and flapping leaf upon raindrop impact*

The dynamics of drop impact on soft surfaces has drawn a lot of attention for its application and natural examples like raindrop impact on a leaf. Previous studies have focused on categorizing the bending motion observed using cantilever beam theory, but the complex dynamic response shown by a leaf involving other degrees of motions like torsion and flapping remain yet to be understood. We experimentally try to characterize the response using hydrophobic Katsura leaves and decompose the different degrees of motion for detailed analysis. The decomposed motions were further characterized as dynamic responses to damped harmonic oscillators. The frequency, maximum amplitude and damping coefficient were analysed in a parametric study involving different drop speeds and impact locations. The experimental observations of these variables were found to be in good agreement with theoretical estimates of bending, torsion and flapping. Furthermore, the study might shed light on developing optimized designs of harvesting  raindrop energy using similar dynamic response of a leaf.