Biomimetic hybrid PAM-ZnO fog-harvester with cacti-driven wettability gradients for atmospheric water harvesting

Atmospheric water harvesting provides a passive method for generating freshwater in regions with persistent water scarcity. Inspired by the water-collection strategies of cacti, a hierarchical polyacrylamide zinc oxide (PAM/ZnO) composite was developed for efficient water harvesting in arid and semi-arid regions. Incorporation of ZnO nanoparticles within a PAM matrix creates a hydroxyl-rich active site and nanoscale roughness. This lowers the energy barrier for heterogeneous nucleation, mimicking the hydrophobic/philic gradient and spike structure found in cacti spines. These topographies endorsed rapid droplet nucleation, directional coalescence, and transport towards the base. Structural and chemical investigations using XRD, SEM, FTIR, EDX, and XPS confirm the uniform distribution of ZnO, robust polymer-oxidation interfacial bonding, and the development of multiscale roughness, all essential for efficient condensation. Wettability studies indicate a progressive reduction in the water contact angle with increasing ZnO concentration, consistent with enhanced surface hydrophilicity and increased nucleation density. Under controlled fog–flow conditions, the composite exhibits a 6.10 g cm − 2 h − 1 $6.10\mathrm{g}{\mathrm{cm}}^{-2}{\mathrm{h}}^{-1}$ water capture rate, demonstrating improved performance over the pristine PAM system. External stimuli indicate how moderate heating and sunlight further accelerate the evaporation-condensation cycle, increase thermal-induced vapor flux, and maintain continuous droplet collection. The integration of biomimetic spike geometry, ZnO-mediated micro-nano texturing, and stimuli-responsive surface activation establishes a robust materials platform capable of high–efficiency passive water harvesting and provides a scalable strategy for development in low–humidity and high-irradiance climates.