Condensation heat transfer on superhydrophobic surfaces
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Introduction Vapor condensation is a ubiquitous phenomenon occurring in nature.1–5 We observe this process in our daily lives, such as on a hot summer day when water accumulates on a cold drink container or on a humid day when fog forms. In industry, vapor condensation is an essential process in power generation,6 thermal management,7 water desalination,8,9 and environmental control.10 For example, the thermal efficiency of the steam cycle, typically responsible for a major fraction of a nation’s power production, is directly linked to condensation heat transfer performance. Meanwhile, in heating, ventilating, and air conditioning (HVAC) systems, which account for ≈20% of the total energy consumption in developed countries,11 the accumulation of condensed water on thermal components can lead to performance degradation and increased costs. Furthermore, condensation on glass strongly influences the transmittance of light into greenhouses, resulting in a potential 40% decrease in solar energy entering the greenhouse during the winter.12 In all of these industrial systems, vapor condenses on a surface rather than directly in the vapor phase due to the reduced energy barrier for droplet nucleation.13 However, the vapor typically forms a thin liquid film because of the high surface energy of industrial components (i.e., clean metals such as copper, aluminum, stainless steel). While this mode known as filmwise condensation14 (Figure 1a) is quite common,
the formation of a liquid film is not desired due to the large resistance to heat transfer. Meanwhile, if a surface is coated with a low-energy non-wetting “promoter” material (i.e., long chain fatty acid, wax, polymer coating, self-assembled monolayer),15–19 or if it naturally adsorbs hydrocarbons and impurities on its surface from the surroundings (as in the case of gold, silver, and chromium),20–22 the vapor forms discrete liquid droplets ranging in size from microns to millimeters.23–25 This process is known as dropwise condensation26 (Figure 1b). The progressive removal of these condensing droplets by gravity at length scales comparable to the capillary length (≈2.7 mm for water)27–29 helps refresh the surface for re-nucleation and allows 5–7 times higher heat transfer performance when compared to the filmwise mode.30 Since its discovery eight decades ago, dropwise condensation on common heat transfer materials has been a topic of significant interest,30,31 with a focus on creating non-wetting surfaces via promoter coatings for easy droplet removal. While robust coatings still continue to be a challenge and require more development,30 recent advancements in nanofabrication have allowed for the development of superhydrophobic surfaces,32 on which nearly spherical water droplets form with high mobility and minimal droplet adhesion. In addition, the role of surface structuring on wetting characteristics have been studied in detail33–35 to enhance condensation performance by reducing droplet departure sizes to below that of the capillary
Nenad Miljkovic, Massachusetts Institut
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