Target slinging of droplets with a flexible cantilever

Abstract

Control of the directional bounce of droplets impacting solid surfaces is crucial for many agricultural and industrial applications. However, for the universal impact process of raindrops on plant leaves, little is known about how the highly coupled and complicated fluid–structure interaction controls the postimpact motion of droplets and endows the leaves with tenacious vitality. Here, we report a leaf-like superhydrophobic cantilever to flexibly bounce droplets with well-defined directionality and controllability. Through theoretical modeling and three-dimensional fluid–solid coupling simulations, we find that the flexible cantilever significantly relieves the impacting forces of raindrops to reduce droplet fragmentation and enhance water repellency. The results further uncover the scaling relations of the droplet bouncing direction with respect to Weber number and cantilever stiffness. By this technique, the seemed disorganized postimpact movements of droplets are programmable and predictable, achieving the goal of where to point and where to hit automatically. This work advances the understanding of natural droplet impact phenomena, opens a new avenue for delicately controlling liquid motion in space with soft materials, and inspires a plethora of applications like soft robots to transport materials and energies, monitor plant growth as well as predict pathogen transmission in plants.