Theoretical and experimental study of spontaneous adsorption-induced superhydrophobic Cu coating with hierarchical structures and its anti-scaling property

Some experimental studies have proven that micro/nano structured coatings achieve superhydrophobicity in air, without low-energy modification. However, it remains an issue how comprehensively explain the reason for changes in wettability. Herein, a hierarchically (nano-submicron-micro) structured Cu coating was fabricated on pipeline steel substrate by one-step electrodeposition. Notably, the superhydrophilic hierarchically structured Cu coating transforms to superhydrophobicity after stored in air for 15 days without chemical modification, with water contact angle of 151° and roll off angle of 3°. Both the microstructure and the chemical composition were characterized to understand the wettability transition mechanism. The fresh hierarchically structured Cu coating exists various defects, with high surface energy, which lead to superhydrophilicity. After the Cu coating stored in air, hydroxylation contributes to hydrocarbons adsorption, resulting in superhydrophobicity. The adsorption kinetic curve model shows that the hierarchical structure promotes hydrocarbon adsorption, which prominently reduces the transition time from superhydrophilicity to superhydrophobicity. The three-level wetting model is constructed to analyze the wetting state when water contacts the Cu coating, which verifies that it is the stable Wenzel-Cassie-Cassie wetting state. Moreover, the superhydrophobic Cu coating maintained anti-scaling property after immersing at 70 °C for 4 h. The anti-scaling behavior and mechanism of the superhydrophobic Cu coating were analyzed by both nucleation and wetting theories. The Cu coating also shows excellent self-cleaning property, water droplet impact resistance, and chemical stability. The superhydrophobicity of the Cu coating also maintained in weak acid and base solutions for 12 min. This study enriches and develops the theory and the technology in the field of wetting, and provides technical support and theoretical basis for the development of superhydrophobicity without low-energy modification.