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

The Cassie-Baxter equation is not always applied to all heterogeneous surfaces for explaining superhy-drophobic phenomena. A two-step anodization method combining with a microstructure (MS) imprint-ing process is used to prepare anodized aluminum oxide (AAO) templates with mushroom-shaped nanostructure (NS) and micro-/nanostructure (MNS). Polypropylene surfaces with single-tier architec-ture (i.e., NS or MS) and hierarchical structure (i.e., MNS) are replicated using AAO templates by compres-sion molding. The key to improving hydrophobicity of single-tier architecture is increasing contact line density (qcl). Data fitting shows that the relationship between qcl and contact angle (CA) is a sophisticat-edly exponential curve. By tuning the diameter and array density of NS, the optimized NS replica achieves a CA of 163.7 & DEG; and roll-off angle of 0.9 & DEG;. Further, the optimized MNS replica possessing low CA hysteresis of 0.2 & DEG; and surface adhesion force of 4 lN achieves a relative reduction of energy loss by 7% compared with NS replica.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

The application of the Cassie-Baxter equation to heterogeneous surfaces for explaining superhydrophobic phenomena is not always applicable. In this study, a two-step anodization method combined with a microstructure imprinting process was used to prepare anodized aluminum oxide templates with mushroom-shaped nanostructures and micro-/nanostructures. These templates were then used to replicate polypropylene surfaces with different hierarchical structures using compression molding. The study found that increasing the contact line density is key to improving hydrophobicity of the single-tier structure. The optimized replicas achieved higher contact angles and lower roll-off angles for the nanostructure replica, and lower contact angle hysteresis and surface adhesion force for the micro/nanostructure replica.