Fabrication of micro-nano gradient wettability surface by nanosecond laser and thermal oxidation

Surfaces with gradient wetting properties are crucial for alleviating water scarcity and efficient recycling, as it can be used for directed transport and collection of water. However, a major challenge is how to efficiently manufacture multi-scale micro-nano structures to increase surface wetting gradients. In this study, a composite processing that combines nanosecond laser oblique incidence and thermal oxidation to fabricate microgroove-nanowire (CuO) hierarchical structures with gradient wettability is proposed. Laser oblique incidence scanning is employed to fabricate microgroove structures with gradient geometric dimensions and chemical composition on the Cu surface, which in turn induced gradient wetting properties and facilitated the directed movement of droplets. The effect of scanning times on gradient structure and its wetting properties is also discussed in detail. To further enhance the directional flow distance of droplets, dense CuO nanowires are grown on the surface of microgrooves through thermal oxidation treatment, forming a micro-nano dual scale structure. The growth mechanism of nanowires is revealed, and the effects of thermal oxidation temperature and duration on nanowire growth and gradient wetting properties are discussed in detail. The gradient in contact angles, in conjunction with the variation in energy barriers in different directions, leads to more pronounced anisotropic wetting. Compared to the single microgroove structures, the directional flow distance of microgroove-nanowire dual scale structure is increased by about 50%. The proposed laser and thermal oxidation composite process provides a new strategy for efficiently manufacturing micro-nano dual scale structures and further enhancing surface gradient wetting performance.