Dropwise condensation mechanism on smooth hydrophilic surfaces

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The objective of this work is to identify the mechanism of dropwise condensation on a smooth solid surface. We investigate the stable dropwise condensation that occurs at a droplet growth rate of 1 µm/s in diameter on a gold-coated glass surface. Additionally, we present our observations on unstable dropwise condensation, i.e., degradation of dropwise condensation of steam on a gold surface. Surface Plasmon Resonance Imaging (SPRi) is used to evaluate the existence and structure of thin films and occurrence of initial nuclei during condensation. SPRi approach used in this study has lateral resolutions of 4–10 µm, thickness resolutions of 0.1–1 nm, and temporal resolutions of 200–10,000 frames per second (FPS). Visualization of the onset of stable dropwise condensation suggests droplets form at heterogeneous nucleation sites and that no film greater than a monolayer exists on the surface before the formation of droplets. Observation of the unstable dropwise condensation of steam shows the existence of water films that are several nanometers thick between droplets. This work shows that neither the nucleation theory nor film rupture theory can individually explain the physics of dropwise condensation. Therefore, there is a need for a more comprehensive theory that can explain the mechanism of dropwise condensation.

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Applied Sciences (Switzerland)