Wettability-Tuned Polydopamine-Modified Surfaces for Enhanced Dropwise Condensation and Water Harvesting

Low-surface-energy superhydrophobic surfaces facilitate droplet removal but often exhibit a high nucleation energy barrier, hindering water-harvesting efficiency. To address this limitation, appropriately increasing surface hydrophilicity offers a promising strategy to enhance droplet nucleation while preserving sufficient droplet mobility. In this study, the surface wettability of SiO2 nanoparticle-textured superhydrophobic substrates (SHB) was systematically regulated by controlling the polydopamine (PDA) reaction time. Increasing the PDA reaction time gradually enhanced surface hydrophilicity, resulting in asynchronous transitions in sessile-droplet wetting and condensation mode. The Cassie-to-Wenzel transition of sessile droplets occurred at a PDA reaction time of 9 min. In contrast, the transition from dropwise condensation (DWC) to filmwise condensation (FWC) was delayed until a PDA reaction time of 11 min. The PDA10 substrate (the surface prepared at a PDA reaction time of 10 min) exhibited the optimal condensation performance, achieving enhanced droplet nucleation without excessive loss of droplet mobility. Compared with the PDA0 (SHB) substrate, the PDA10 substrate increased the condensate mass collection rate by 62.0% and the heat transfer coefficient by 61.4%. These findings demonstrate that temporal control of PDA modification enables regulation of distinct wetting and condensation transition thresholds, providing a design strategy to improve condensation-based water harvesting and heat transfer.