Springboard Droplet Bouncing on Flexible Superhydrophobic Substrates

Abstract

Droplet impact on rigid, superhydrophobic surfaces follows the well-known spreading, recoil, and lift-off behavior at lower impact speeds (a), and splashing at higher impact speeds (b). The contact time tc of these bouncing droplets is independent of the impact speed, and difficult to control. Using high speed imaging (9500 fps) of water droplets impacting superhydrophobic substrates with stiffness 0.5 to 7630 N/m (rigid), we were able to show that substrate flexibility can reduce contact times. Upon impact on a flexible substrate, the droplet excites the substrate to oscillate at the membrane or cantilever natural frequency (d). The oscillation accelerates the droplet upwards, initiating early droplet lift-off at the edges of the droplet close to the point of maximum spreading (c). Droplets fully lift off before fully recoiling, i.e. in a pancake shape. We call this phenomenon the springboard effect. Contact times are reduced by up to 50% compared to rigid substrates.