Superhydrophobic hierarchical three-level structures on 3D polypropylene surfaces

Abstract

Abstract Hierarchical micro–micro–nanostructures were fabricated on planar and curved polypropylene surfaces by injection molding. The metal mold insert was microstructured with a microworking robot and nanostructured with an anodization process. 3D surface curvature was based on the simple bending of the mold insert into convex and concave dome-shaped formations. Three-level structural hierarchy was demonstrated to enhance superhydrophobicity compared to two-level structures. The third structural level on top of two lower structural levels decreased the contact area between the water droplet and polymer surface, which resulted in the high contact angle (CA) of 170°, low CA hysteresis of 5° and easily sliding droplets. Microstructures can be considered to protect other hierarchical levels, and the three-level-structured surface is less susceptible to lose its superhydrophobicity compared to two-level-structured surfaces. Structuration of four length scales was realized on the curved surfaces when the curvature in millimeter scale was combined with the surface structures in two microscales and nanoscale.