Facile fabrication of extreme-wettability contrast surfaces for efficient water harvesting using hydrophilic and hydrophobic silica nanoparticles

Freshwater shortages threaten ecosystems and human communities around the world. Inspired by the ability of cacti and desert beetles to collect water from the atmosphere, we successfully fabricated a wettability contrast surface with efficient water harvesting using a simple and low-cost process. A solution of isopropyl alcohol and a mixture of hydrophobic and hydrophilic silica nanoparticles was sprayed on a laminating film of ethylene vinyl acetate and polyethylene terephthalate, after which sandpaper was laid on the dry-coated film for hot-press lamination. After peeling the film from the sandpaper, the fabricated surface exhibited random wettability contrast patterns with nano-microstructures formed by nanoparticles and the sandpaper surface. The highest water-harvesting efficiency of the prepared surface was approximately 446.7 mg/cm2/h when the mass ratio of the hydrophobic to hydrophilic silica nanoparticles in the spraying solution was 75/25 due to the optimal performance between the generation of water nucleation, coalescence, and the movement of water droplets. An analysis of the effect of ultraviolet irradiation and adhesive damage on the surface revealed that the efficiency of water collection was nearly unchanged in the face of the ultraviolet irradiation and decreased slightly after the tape test. The proposed fabrication method can make flexible and wettable contrast surfaces at large scales and low costs. In addition, superhydrophobic, or nearly superhydrophilic surfaces can be fabricated with the same process by using solely hydrophobic or hydrophilic silica nanoparticles. Therefore, this process is versatile and can be applied for creating surfaces with a range of wettability properties.