The condensation heat transfer performance of various thermal applications can be dramatically enhanced when condensing droplets that have low thermal conductivity efficiently depart from heat transfer surfaces of heat exchangers. On a superhydrophobic surface, when two adjacent condensing droplets coalesce into one, the excess surface energy of the droplets will be converted into kinetic energy that drives the merged droplet to jump from the surface, promoting the condensation heat transfer performance of heat exchangers. However, the improvement of droplet jumping height to further enhance the heat transfer performance of those thermal applications remains a challenge due to the formation of Wenzel or partial-Wenzel state droplets that cause substantial interfacial adhesion between droplets and superhydrophobic surfaces. Besides, the condensation is restricted by the energy barrier between the superhydrophobic surface and water vapor, limiting the heat transfer performance.

In this study, a biphilic surface consisting of globally superhydrophobic and locally hydrophilic wettability is created to address the issues of Wenzel or partial-Wenzel state droplet formation and interfacial energy barrier. The surface structure of the biphilic surface (i.e., the density of hydrophilic areas) is optimized by a novel droplet jumping theory that can theoretically predict the droplet jumping height on the biphilic surface based on the classical nucleation theory, heterogeneous surface wetting mechanism, and energy conservation. With the optimized biphilic surface, the droplet jumping height can be improved by 28% as compared with that on the superhydrophobic surface, leading to an excellent water self-removal property and condensation improvement. Consequently, the water collection rate on the optimized biphilic surface can be improved by 61% and 273% as compared with that on the superhydrophobic surface and a normal copper plate, respectively, under an atmospheric condition. With improved condensation performance, the heat flux on the biphilic surface can be enhanced by 43% and 139% as compared with that on the superhydrophobic surface and the normal copper plate, respectively. The effects of surface orientation and air pressure on droplet jumping on the biphilic surface are also considered and experimentally verified in this droplet jumping theory. In addition, the contact electrification effects of the jumping droplets on the biphilic surface are also discovered in this study, allowing the droplet to jump 137% higher than that on the superhydrophobic surface, further enriching the application prospect of the biphilic surface.

To verify the application feasibility of the biphilic surface, a biphilic heat exchanger is assembled with 6 heat transfer tubes coated with biphilic nanostructure, showing 29% and 38% improvement in convective heat transfer coefficient as compared to that in a typical copper heat exchanger and superhydrophobic heat exchanger, respectively. In addition, if the biphilic surface is applied to phase-change thermal diodes, the thermal rectification ratio can be improved by 244% on the thermal diode with a biphilic surface as compared to that with a superhydrophobic surface due to the improved phase-change performance. By applying an electric field to the biphilic thermal diode, this improvement can be incredibly larger than 1000%. In summary, the study in this thesis created a comprehensive droplet jumping theory that can well predict the experimental results on the biphilic surface, contributing to the development of droplet jumping physics. Practically, this study provides new insights in promoting the heat transfer performance in heat changers with condensation effects, benefiting various thermal applications.