Exceptional anisotropic superhydrophobicity of sword-lily striated leaf surface and soft lithographic biomimicking using polystyrene replica

Abstract

Herein, we report unusually high anisotropic superhydrophobicity, unidirectional self-cleaning, and biomimicking of adaxial sword-lily (Gladiolus hortulanus) leaf comprising three distinct levels of surface textures. Observably, the static anisotropic wetting and rolling of water droplets are more favourable in the parallel (or, striation) direction than in the perpendicular direction. Inspired from such water repellency of the sword lily leaf surface, here bio-mimicked polystyrene (PS) leaf construct is developed through a soft lithographic technique. Considering different water droplet sizes (4–10 μl) on natural lily leaf and bio-mimicked PS construct surfaces, the respective parallel (θ||) and perpendicular (θ⊥) water contact angles (WCAs) stand at, θ|| ∼143°–147°, θ⊥ ∼156°–169°; and θ|| ∼130°–139°, θ⊥ ∼142°–145°. Moreover, the specimens under study exhibit roll-off angles ranging, α|| ∼8°–23° (α⊥ ∼16°–41°) and α|| ∼21°–49° (α⊥ ∼40°–55°) along parallel (and perpendicular) directions; respectively. A noticeable difference in α⊥ and α|| values can be ascribed to the profound three-phase contact line (TCL) pinning along the perpendicular direction taking advantage of striation as means of barrier. The roll-off angles can also alter due to a variation in the droplet volume. The unusual anisotropic superhydrophobicity and unidirectional droplet roll-off can be attributed to the entrapped air within the micro-nano texture beneath the water droplet along with the pinning effect in the perpendicular direction caused by the striated heights.