Turning hierarchically micro-/nanostructured polypropylene surfaces robustly superhydrophobic via tailoring contact line density of mushroom-shaped nanostructure

Abstract

The Cassie–Baxter equation is not always applied to all heterogeneous surfaces for explaining superhydrophobic phenomena. A two-step anodization method combining with a microstructure (MS) imprinting process is used to prepare anodized aluminum oxide (AAO) templates with mushroom-shaped nanostructure (NS) and micro-/nanostructure (MNS). Polypropylene surfaces with single-tier architecture (i.e., NS or MS) and hierarchical structure (i.e., MNS) are replicated using AAO templates by compression molding. The key to improving hydrophobicity of single-tier architecture is increasing contact line density (ρcl). Data fitting shows that the relationship between ρcl and contact angle (CA) is a sophisticatedly exponential curve. By tuning the diameter and array density of NS, the optimized NS replica achieves a CA of 163.7° and roll-off angle of 0.9°. Further, the optimized MNS replica possessing low CA hysteresis of 0.2° and surface adhesion force of 4 μN achieves a relative reduction of energy loss by 7% compared with NS replica.