Spontaneous Water Condensation on Surfaces with Nanoscale Chemical and Topographical Heterogeneity

Condensation is a phase transition in which water molecules transition from the vapor to the liquid phase. To initiate water condensation from undersaturated vapor onto a solid surface, the substrate must be cooler than the ambient temperature to facilitate this exothermic process. In this work, we report that undersaturated vapor condenses isothermally to form macroscopic droplets on the surface of porous nanocomposite films featuring nanoscale chemical and topographical heterogeneity. These porous nanocomposites are prepared by partially filling the interstices of randomly packed hydrophilic nanoparticles with a hydrophobic polymer. Through a combination of experimental and computational analyses, we demonstrate that as the vapor pressure increases, the pores are filled with capillary condensate. Beyond a specific vapor pressure, visible water droplets are formed on the film surface. The amount of water droplets is strongly dependent on the fraction of polymer within the film and is inversely proportional to the NP size, showcasing the role of polymer hydrophobicity and capillary forces, respectively. In addition, the amount of water droplet is proportional to the film thickness, which implies that the droplet formation on the surface is influenced more by the bulk structure of the film rather than that of the surface. Our findings could open new avenues inducing water condensation without energy consumption for water harvesting and other applications.