Wettability Contrast in the Hexagonally Patterned Gold Substrate of Distinct Morphologies for Enhanced Fog Harvesting.

Inspired by the Stenocara beetle's hydrophobic-hydrophilic surface, we fabricated hexagonally patterned hydrophobic-hydrophilic surfaces consisting of silicon and gold regions using colloidal lithography and selective surface functionalization. We investigated surface wettability for different patterns (hexagonally ordered nanotriangles and nanoholes) and the influence of surface functionalization (octadecanethiol and 16-mercaptohexadecanoic acid/octadecyltrichlorosilane (MHA/OTS)). The as-prepared patterned substrates exhibit hydrophilicity, which transforms to hydrophobicity after surface functionalization. The MHA/OTS functionalization results in maximum enhancement in the contact angle (114 ± 0.4°) with the least contact angle hysteresis (19 ± 2°). Fog harvesting studies show that the patterned substrate has a higher water collection rate, a factor of 1.32, than the nonpatterned substrates. A further enhancement in water collection (almost twice) is observed with selective functionalization. The patterned (nanohole) and functionalized (MHA/OTS) substrate facilitates rapid falling of droplets at a frequency of 20 mHz and an average droplet mass of 15 ± 2 mg/cm. Furthermore, it yielded a maximum water collection rate of 1051 ± 132 mg cm h. This work provides valuable insights into the influence of surface wettability and morphology for fog harvesting applications.