Superhydrophobic surfaces exhibit large contact angle (> 150°) and small hysteresis (< 5°) which facilitate liquid transport and are expected to enhance condensation heat transfer on the surfaces. By growing short carbon nanotubes (CNTs) on an array of microposts etched on a silicon wafer, we formed a two-tier multiscale texture mimicking the surface structure of lotus leaves. Compared to one-tier microtexture which energetically favors the Wenzel state, the two-tier texture with microano-scale roughness favors the Cassie state, the desired superhydrophobic state. Using an environmental scanning electron microscope (ESEM), we investigated moisture condensation on the fluoropolymer-coated two-tier texture and we have observed continuous dropwise condensation on the engineered superhydrophobic surface. However, in a customer-designed vapor chamber our condensation measurements indicate that a film layer of condensate in Wenzel state was formed on the textured surface. In particular, due to the filmwise condensation, the condensation heat transfer coefficient of the lotus-leaf-like surface is lower than that of a smooth hydrophobic surface especially under high heat flux situations.
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