SUMMARY

Condensation of water vapor is an everyday phenomenon which plays an important role in power generation schemes, desalination applications and cooling of power electronic devices. Dropwise condensation is a desirable mode of condensation in which small droplets regularly shed off the surface, thereby minimizing the thermal resistance to heat transfer across the condensate layer. While difficult to sustain, dropwise condensation has been shown to achieve heat and mass transfer coefficients over an order of magnitude higher than its filmwise counterpart. Superhydrophobic surfaces have been studied to promote dropwise condensation with mixed results; often these surfaces do not retain their properties when exposed to condensed droplets. Recently, nanostructured superhydrophobic surfaces have been developed that are robust to vapor condensation; however, these surfaces are still not ideal for condensation heat transfer due to the high thermal resistance of the trapped vapor layer and the small amount of contact area between each droplet and the substrate. In this work, first a comprehensive free-energy based thermodynamic model is developed to understand why traditional superhydrophobic surfaces often lose their properties when exposed to condensed droplets. The model is validated using data from existing literature and is then extended to analyze the suitability of amphiphilic (i.e. part hydrophobic/part hydrophilic) surfaces for condensation applications. Secondly, an amphiphilic surface identified by the model is fabricated and tested using environmental scanning electron microscopy and optical microscopy to probe the condensation behavior and compare the performance to that of a traditional surface. Observations from these experiments are used to propose a mechanism of coalescence that governs the temporal droplet size distribution on the amphiphilic surface and continually generates new droplet nucleation sites to enhance the rate of heat transfer.