Measuring the initial stage of dropwise condensation using atomic force microscopy

Abstract

The initial stage of dropwise condensation is known to nucleate and grow at the sub-micrometer scale. However, conventional optical microscopy and environmental scanning microscopy are unable to measure sub-micrometers due to their limited spatial resolution. Here, an atomic force microscope with a spatial resolution of a few nanometers was customized for precise control of the condensation environment to measure the initial stage of dropwise condensation. It was used to analyze the capillary forces exerted on the cantilever tip as a function of relative humidity. It was also measured that droplet density increases with the increasing surface roughness of the silane coating, which is often used for condensation. This was observed to be due to silane agglomerates acting as direct droplet nucleation sites. This study not only explains the mechanism of droplet nucleation on hydrophobic coatings but also presents a new method for measuring the initial condensation phenomenon. Furthermore, it provides insight into higher heat transfer performance by controlling the initial stage of condensation to suppress flooding.