Gradient Wetting Transition from the Wenzel to Robust Cassie-Baxter States along Nanopillared Cicada Wing and Underlying Mechanism

Abstract

Gradient wettability is important for some living organisms. Herein, the dynamic responses of water droplets impacting on the surfaces of four regions along the wing vein of cicada Cryptotympana atrata fabricius are investigated. It is revealed that a gradient wetting behavior from hydrophilicity (the Wenzel state) to hydrophobicity and further to superhydrophobicity (the Cassie-Baxter state) appears from the foot to apex of the wing. Water droplets impacting on the hydrophilic region of the wing cannot rebound, whereas those impacting on the hydrophobic region can retract and completely rebound. The hydrophobic region exhibits robust water-repelling performance during the dynamic droplet impact. Moreover, a droplet sitting on the hydrophobic region can recover its spherical shape after squeezed to a water film as thin as 0.45 mm, and lossless droplet transportation can be achieved at the region. Based on the geometric parameters of the nanopillars at the hydrophilic and hydrophobic regions on the cicada wing, two wetting models are developed for elucidating the mechanism for the gradient wetting behavior. This work directs the design and fabrication of surfaces with gradient wetting behavior by mimicking the nanopillars on cicada wing surface.