Stable dropwise condensation observed on a hierarchically structured superhydrophobic surface incorporating micro-domes

With global temperatures increasing and air conditioning becoming more prevalent, the need to limit its electricity consumption and improve its efficiency is greatly increased. However, air conditioning heat transfer is often impeded by condensed atmospheric water accumulating on cooling coils, potentially leading to degraded cooling performance and the growth of pathogens that may endanger human health. In order to improve air conditioning heat transfer performance, surfaces with superior dropwise condensation and more rapid condensate-shedding capability are desirable. Inspired by the exceptional droplet shedding performance of natural plant leaf surfaces, we have developed a fabrication process to create hierarchical surfaces composed of arrays of micro-domes or -pillars that are then coated with a nanoporous, fluorosilanized zinc oxide film. In this paper, we characterize the performance of these surfaces when exposed to supersaturated moist air. We found that the most desirable condensation performance was provided by one specific surface, which featured a micro-dome array with diameters and spacings of 30 mu m. This surface repeatably offered stable dropwise condensation over 30-min experimental durations. Surfaces with other dome separations, surfaces with square-tipped micropillars, and flat surfaces with only the nanoporous coating all showed less stable dropwise condensation and tended to become flooded with condensate as time progressed. The fabrication, testing and image analysis techniques described here could help in the development of optimized surfaces for heat exchangers operating in moist air.