Droplet Impacting a Cantilever: A Leaf-Raindrop System

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago,Chicago, Illinois 60607, USA(Received 17 October 2014; revised manuscript received 12 March 2015; published 30 April 2015)Previousstudies showthat air pollution and wind erosion, which damage a leaf’sepicuticular wax layer,can change leaf surface properties from hydrophobic to hydrophilic. However, the dynamic response of adamaged leaf to a raindrop impact has not been investigated and could clarify the direct influence ofchanges in wettability on early leaf abscission. In this article, we investigate how leaves with differentsurface properties respond to falling raindrops, viewing this as a unique system of coupled elasticity anddrop dynamics. An elastic beam with tunable surface wettability properties is used as a simple leaf model.We find that wettable beams experience much higher torque and bending energy than nonwettable beams.This is because a drop sticks to a wettable beam, while a drop falls off a nonwettable beam. An analyticalmodel using momentum balance and simple cantilever beam theory quantifies the bending energy andtorque experienced by wettable and nonwettable beams. The results elucidate the potential damage causedby raindrops impacting a leaf as a function of its surface wettability and are correlated with environmentalfactors contributing to premature changes of leaf surface properties.