Growth dynamics during dropwise condensation on nanostructured superhydrophobic surfaces

Condensation on superhydrophobic nanostructured surfaces offers new opportunities for enhanced energy conversion, efficient water harvesting, and high performance thermal management. Such surfaces are designed to be Cassie stable, which minimize contact line pinning and allow for passive shedding of condensed water droplets at sizes smaller than the capillary length. In this work, we investigated in situ water condensation on superhydrophobic nanostructured surfaces using environmental scanning electron microscopy (ESEM). The "Cassie stable" surfaces consisted of silane coated silicon nanopillars with diameters of 300 nm, heights of 6.1 μm, and spacings of 2 μm, but allowed droplets of distinct suspended (S) and partially wetting (PW) morphologies to coexist. With these experiments combined with thermal modeling of droplet behavior, the importance of initial growth rates and droplet morphology on heat transfer is elucidated. The effect of wetting morphology on heat transfer enhancement is highlighted with observed 6× higher initial growth rate of PW droplets compared to S droplets. Consequently, the heat transfer of the PW droplet is 4-6× higher than that of the S droplet. To compare the heat transfer enhancement, PW and S droplet heat transfer rates are compared to that of a flat superhydrophobic silane coated surface, showing a 56% enhancement for the PW morphology, and 71% degradation for the S morphology. This study provides insight into importance of local wetting morphology on droplet growth rate during superhydrophobic condensation, as well as the importance of designing CB stable surfaces with PW droplet morphologies to achieve enhanced heat transfer during dropwise condensation.