Anisotropic water sliding radiative cooling emitter for atmospheric water harvesting

Water shortage is a major problem for human society in the 21st century. Radiative cooling-based atmospheric water harvesting (AWH) has emerged as a promising strategy to address this crisis. Hydrophilic surfaces are known to facilitate water vapor condensation, but a water film that adheres tightly to the radiative cooling emitter (RCE) will severely reduce water collection flux. Hydrophilic surfaces with low sliding angles are highly desirable for high-performance AWH, but achieving both sufficient hydrophilicity and a low sliding angle is inherently contradictory in conventional surface designs. In this work, we report a mass-producible silicon-based hydrophilic RCE with anisotropic low sliding angles. We fabricated composite microstructures consisting of grooves and pockets on a diamond-wire-cut silicon substrate by combining metal-assisted chemical etching with a rounding treatment. The structured surface was then modified <i>via</i> a specially designed localized hydrophilic treatment. The introduction of nano-pockets trapped a large number of air bubbles at the interface between water droplets and the RCE, which effectively reduced the water sliding angle. The as-prepared anisotropic RCE exhibited a water contact angle of 60.2° and an ultra-low water sliding angle of only 31°. Experimental results demonstrated that the proposed RCE enables efficient AWH for more than 17 hours per day, with a maximum water mass flux of 38.2 g m^-2 h^-1. This performance is 71% higher than that of a control group with a comparable emissivity.