Tunable Water Harvesting Surfaces Consisting of Biphilic Nanoscale Topography.

Water scarcity has become a global issue of severe concern. Great efforts have been undertaken to develop low-cost and highly efficient condensation strategies to relieve water shortages in arid regions. However, the rationale for design of an ideal condensing surface remains lacking due to the conflicting requirements for water nucleation and transport. In this work, we demonstrate that a biphilic nanoscale topography created by a scalable surface engineering method can achieve an ultra-efficient water harvesting performance. With hydrophilic nano-bumps on top of a superhydrophobic substrate, this biphilic topography combines the merits of biological surfaces with distinct wetting features (e.g., fog-basking beetles and water-repellent lotus), which enables a tunable water nucleation phenomenon, in contrast to the random condensation mode on their counterparts. By adjusting the contrasting wetting features, the characteristic water nucleation spacing can be tuned to balance the nucleation enhancement and water transport to cope with various environments. Guided by our nucleation density model, we show an optimal biphilic topography by tuning the nanoscale hydrophilic structure density, which allows a ~349% water collection rate and ~184% heat transfer coefficient as compared to the state-of-the-art superhydrophobic surface in a moisture-lacking atmosphere, offering a very promising strategy for improving the efficiency of water harvesting in drought areas.