Abstract
Heat transfer performance of dropwise condensation is higher than filmwise one due to droplet removal from the condensed surface. In order to further enhancement of its performance, removal of microscale droplet is a key issue. However, condensation mechanism at microscale on hydrophobic-hydrophilic hybrid surface is not understood, although all surfaces are consisted with those combinations. In this study, we conducted condensation experiments on a graphite surface at 0℃ and 550 to 600 Pa by using environmental scanning electron microscope (ESEM). Nanoscale step-terrace structures on graphite surface are obtained by using an atomic force microscope (AFM) before ESEM experiments. It was found that condensed droplets with diameter of 150 to 300 nm are lined up along step edges at over 150 nm intervals. In addition, we found that most droplets are on the steps of height of 1 nm and shorter droplet intervals are induced by higher steps and shorter terrace width. Our results were analyzed by an extended nucleation theory and we found that water molecules attracted on steps play an important role for droplet condensation.
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