Enhancement of condensation heat transfer on a microstructured surface with wettability gradient

Micro- and nanoscale phase-change phenomena are becoming increasingly important for heat transfer owing to the rapid development of microelectromechanical system (MEMS) technology in the fields of microsystems such as in advanced electronics or biomedical devices. In particular, many studies on dropwise condensation have been conducted based on progress of material science for hydrophobic or hydrophilic surfaces. Although majority of these studies achieved high heat transfer performance under low subcooling conditions, the heat transfer coefficient decreased as subcooling increased because of condensate removal limitations. Our previous study using a bi-philic surface with microgrooved structures failed in terms of flooding wherein several patterns were covered by a large liquid film because of poor drainage ability. In microsystems, gravity or vapor shearing forces are ineffective in enhancing droplet departure from the condensing surface as the spaces of the microsystem are limited. Thus, to enhance condensation heat transfer, the flooding and plugging phenomena need to be considered, and therefore, a novel condensing surface needs to be fabricated to remove the growing droplets from the condensing surface. Herein, we introduced a wettability gradient by gradually changing the pattern widths of the hydrophobic and hydrophilic surfaces. The groove width of the hydrophilic part was designed to increase as the condensate flowed downstream. The experiments were conducted under low steam pressure conditions; we found that the size of the droplets could be controlled by modifying the pattern width and the wettability gradient could be used to remove large droplets. The flooding was suppressed, and the heat transfer coefficient was enhanced by three times compared with the results of the microstructured condensing surface with a straight pattern. These results showed that the wettability gradient effectively enhanced condensation heat transfer.