Dropwise condensation has shown the ability to increase condensation heat transfer coefficients by an order of magnitude over filmwise condensation. In standard dropwise condensation, liquid droplets forming on a sub-cooled nonwetting surface are removed from the surface by gravitational forces when the droplets reach a critical mass. The dependence on gravity for liquid removal limits the utilization of dropwise condensation in low gravity aerospace applications and horizontal surfaces. Presented in this study is a novel passive mechanism to remove droplets from a condensing surface using a surface energy gradient (wettability gradient) on the condensing surface. The wettability gradient creates a difference in contact angle across droplets condensing on the surface. The difference in contact angle across the droplets causes motion of the droplets to regions of increased wettability, without relying on additional forces. The movement of droplets away from the surface prevents flooding and allows for the condensation of new droplets on the surface. This paper presents an overall description of the wettability gradient mechanism and experimental condensation data acquired on surfaces with wettability gradients. A mechanism for creating the wettability gradients is also described, which involves varying the surface concentration of hydrophobic molecules through a self-assembled monolayer process.

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