To understand the physics behind dropwise condensation, a microscopy technique must be able to measure a sub-nanometer film at a high temporal resolution (>1,000 FPS). In this work, automated Surface Plasmon Resonance imaging (SPRi) is used as a tool to study the existence of a thin film between the dropwise condensate. SPRi is a label-free imaging technique that works based on the attenuated total internal reflection. SPRi can detect changes in the refractive index (RI) of the test medium in the thin region (<300 nm) above the sensing gold layer. The automated, angle-scanning SPRi instrument was developed by integrating linear and rotating motorized stages. This instrument improves conventional SPRi by enhancing the resolution of angle probing, increasing the speed of angle scanning, and minimizing the angle-dependent image artifacts. As a proof of concept, we visualized the three stages of coalescence at 10,000 FPS, including bridge formation, composite peanut-shape droplet formation, and the relaxation stage. Furthermore, we probed the solid-vapor interface during the dropwise condensation to evaluate the existence of a thin film on the substrate. The results of our visualization show that the area between droplets is covered by an adsorbed film with a thickness of a monolayer (0.275 nm) and a surface coverage of less than one (m2/m2). Moreover, the results reveal a dry region forms on the substrate when part of the substrate is exposed to ambient conditions due to the coalescence. The dry zone on the substrate has higher surface energy, as compared to the surrounding area. Therefore, the exposed area serves as a favorable site for vapor molecules to strike the surface and form new nuclei.

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