Influence of wettability design on moist air condensation: Superhydrophobic, unidirectional wettability gradient and patterned surfaces

Abstract

In this study, three engineered aluminum surfaces with distinct wettability characteristics, namely plain superhydrophobic, unidirectional wettability gradient, and patterned wettability, were fabricated and systematically evaluated for moist-air condensation under controlled conditions. The plain superhydrophobic surface was created by chemical etching and hot water treatment, then functionalized with 1H,1H,2H,2H-perfluorooctyl trichlorosilane. The wettability gradient surface was produced using a gradual substrate elevation method comprising chemical etching, hot water treatment, and trichlorosilane coating. A combination of chemical etching, hot-water treatment, masking, coating, and plasma etching produced the patterned surface. Surfaces were characterized using atomic force microscopy (AFM) for roughness, SEM for morphology, and FT-IR for chemistry. Wetting behaviour was assessed by measuring the equilibrium contact angle and hysteresis of a water droplet. Moist air condensation experiments were conducted at defined subcooling and relative humidity levels, and all quantitative results are reported as mean values with uncertainty based on the standard deviation. Surface performance was evaluated under orientation-matched, regime-specific conditions to isolate intrinsic wettability effects from gravitational drainage. The unidirectional wettability gradient surface exhibited a 36% higher condensation rate than a comparable superhydrophobic surface in the horizontal pendant (downward facing) configuration, whereas the patterned vertical surface had a 51% higher condensation rate than the plain vertical superhydrophobic surface under identical gravity assisted conditions. Results reveal that condensation depends on surface energy gradients and gravity. Wettability affects droplet transport differently in capillary and gravity regimes. Higher subcooling and humidity increase the rate of condensate collection. The work provides a framework for designing wettability to optimize droplet transport and water collection.