Effect of biphilic pattern type, size and wettability ratio on atmospheric water collection

Water scarcity is intensifying worldwide, driving the development of sustainable atmospheric water-harvesting strategies. Nature-inspired biphilic surfaces, integrating hydrophilic and hydrophobic areas, provide a promising solution by promoting controlled condensation and directional droplet transport. Although several biphilic pattern types have been proposed in the literature, there is no systematic study evaluating a series of different patterns in order to extract specific design guidelines for the biphilic patterning. In this work, we fabricate and evaluate surfaces with a series of different biphilic patterns, which exhibit different ratio of superhydrophobic/superhydrophilic area coverage and different working principle for drop capturing and collection in fog collection and drop nucleation and collection in dew collection. Biphilic patterning is realized on pre-designed, deterministic, micro-topographies (pillars, trapezoids, and honeycombs) which are fabricated using PDMS soft lithography. In fog collection, water collection rate (WCR) of superhydrophobic surfaces with pillars and trapezoids is improved compared to untreated PDMS, whereas after biphilic patterning, the WCR is not affected, emphasizing the importance of drop mobility in fog collection. In dew collection, performance is strongly dependent on the environmental conditions. At moderate conditions (relative humidity: 70% and temperature difference between the sample and the environment ΔT = 15 °C), biphilic surfaces with parallel stripes (width: 1000 μm, spacing: 2000 μm), which act as "water guides" driven by gravity and low surface coverage (35%), exhibit increased WCR by 156% compared to flat PDMS. From the fabrication point of view, the method presented here is PFAS-free and scalable, providing another pathway for the realization of water collection interfaces.