Dual dimensional nanostructures with highly durable non-wetting properties under dynamic and underwater conditions

Non-wetting states with high durability under both dynamic and underwater conditions are very desirable for practical applications of superhydrophobic surfaces in various fields. Despite increasing demands for this dual stability of non-wetting surfaces, studies investigating both the impact dynamics and underwater stability are very rare. In the current study, we performed water droplet impact dynamics and underwater stability studies using ZnO/Si hierarchical nanostructures (HNs) as a model system. The effects of the surface structure on the non-wetting states under dynamic conditions were first studied by comparing various surface structures, such as ZnO nanowires (NWs), Si microposts (MPs), ZnO/Si HNs with controlled MP interspacings, and lotus leaf (LL). The growth of ZnO NWs on Si MPs drastically improves the non-wetting properties of Si MPs under dynamic conditions. The transition of wetting states from the Cassie-Baxter state to the Wenzel state occurs on ZnO/Si HNs as the impact velocity increases. Measurement of the critical We number during transition enables us to determine the important parameters of wetting pressure using a simple model. Moreover, compared to Si MPs, ZnO NWs, and LL, our ZnO/Si HNs exhibit dramatically increased air pocket lifetimes under underwater conditions, which is due to the enhanced capillary pressure originating from the dual dimensional hierarchical structure. Our study indicates that optimally designed hierarchical surfaces have remarkably high durability non-wetting states under both dynamic and underwater conditions, expanding the potential application of non-wetting surfaces.