Atmosphere-mediated and roughness-variation-induced biphilic surfaces

Biphilic surfaces show great promise for water harvesting, dehumidification, and condensation applications. Capable of manipulating the nucleation and growth of the condensate droplets while maintaining low surface-condensate adhesion, a carefully designed biphilic surface not only allows for coalescence-induced droplet jumping, but also shows good flooding-resistance. However, the complex and costly fabrication, usually consisting of incorporating hydrophilic spots onto a hydrophobic substrate, hinders the wide-spread utilization of biphilic surfaces. Here we use laser ablation followed by thermal oxidation to fabricate CuO hierarchical biphilic surfaces on Cu substrates. The fabricated surfaces do not need additional chemical modification, as the prepared samples spontaneously transitioned from superhydrophilic to superhydrophobic due to the deposition of the airborne volatile organic compounds (VOCs). Interestingly, the dependence of the nanowire features on the curvature of the micro-topography enables local roughness variation and wetting gradients on a three-dimensional curved surface. The hilltops of the micro-cones are hydrophilic due to the presence of sparse and short nanowires, while in the valleys and ridges, the dense high-aspect-ratio nanowires together with the adsorbed airborne VOCs results in local superhydrophobicity. The as-prepared surfaces are globally superhydrophobic showing a water advancing contact angle above 160°. Condensation experiments under optical microscopy reveal that condensate prefers to nucleate at the hilltop hydrophilic areas, while droplets forming within the hydrophobic valleys show good mobility. Durable dropwise condensation and coalescence-induced droplet jumping are achieved on the CuO hierarchical surface, with ≈ 80% enhancement of the water harvesting rate over the conventional Cu hydrophobic surfaces in atmospheric water vapor condensation conditions.