Droplets on engineered surface roughness exhibits superhydrophobic properties. Condensate droplets in Cassie state may have larger contact angles and lower dynamic hysteresis to gain superior mobility and high condensation heat transfer coefficient. In this article, research investigations have been focused on visualization of dropwise condensation in high aspect ratio (HAR) roughness structures. Both single and hierarchical HAR structures are developed for visualization of dropwise condensation in an open moisture environment. Experimental visualization is performed though combining high-speed Sensi-Cam and Nikon microscope to zoom in condensation area or look through a large condensed droplet. Experimental images indicate that condensate droplets co-exist in both Wenzel and Cassie states within the single HAR roughness structure. Condensation site density reduces with increase of the roughness structure depth due to air/non-condensable gas (NCG) barrier. In contrast, only Cassie state droplets are observed on and in the hierarchical HAR structures. Less condensation droplets in deep structure may be attributed to the reduced surface energy that inhibits the generation of condensation sites. Under droplet visualization denotes that large droplets hold in Cassie state on both the HAR roughness structures. Inner droplet flow driven surface tension may play a critical role in heat transfer of the dropwise condensation.

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