The micro/nano scale phase change phenomena become more and more important because the MEMS technology develops rapidly in the fields of electro- and bio-devices [1][2] and the MEMS enable us to control the surface wettability. In the dropwise condensation on the hydrophobic surface, the heat transfer coefficient is determined by the departing droplet size. In our previous paper, it was found that the droplets in radius around 7 μm made more significant contribution to the condensation heat transfer under the low-pressure conditions. That is, when the smaller droplets less than 7 μm cover the condensing surface, the higher condensing heat flux would be achieved than that of the ordinary dropwise condensation. However, it is still very difficult to keep the droplets to be continuous condensed within 7 μm at the surface.

A challenging work has been conducted to fabricate a droplets exclusion structure on the condensing surface for the purpose of the enhancement of condensation heat transfer in our previous experiment [3]. By using the MEMS technology, we made the hybrid-condensing surface with hydrophobic and hydrophilic patterns in order to remove the grown droplets effectively. It was found that the hybrid-surface has a possibility to increase the condensation heat transfer coefficient but its drainage-ability of the condensate has the limitation due to the occurrence of the flooding over the surface structures. In order to reduce the flooding at the hydrophobic area, in this study, the new design of the condensing surface has been proposed and the condensation heat transfer coefficient is evaluated.

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