Transient behavior of droplet growth and dropwise condensation characteristics on SAM coated hydrophobic surfaces

Abstract

The present study aims to experimentally analyze the transient characteristics of droplet growth and heat transfer during a dropwise condensation process on hydrophobic surfaces. A pure copper (purity 99.9 wt. %) substrate was coated with self-assembled monolayer (SAM) of n-octadecyl mercaptan to make the two different hydrophobic surfaces which contact angles are 124° and 153°. In-situ measurements of droplet size and distributions were conducted during the dropwise condensation process. A K2 lens (long distance microscope lens) was used to capture images of condensated droplets. A humidifier was employed in controlling humidity in a chamber. Surface temperature was measured by using RTD (resistance temperature detector) sensors and a data logger. From the results, as the droplet grows with time, we observed three distinct regimes such as the homogeneous pattern of small droplets, the coalescence dominant regime, and coalescence between large droplets. In particular, the coalescence dominant regime shows the highest rate of droplet growth and the smallest area covered by droplets due to the droplet mobility. Moreover, the temperature of copper surface increase with decreasing wettability in the coalescence dominant regime where the liquid area fraction substantially decreases. This result would be useful in understanding the transient droplet growth mechanism and controlling heat/mass transfer on the surfaces. © 2015 Begell House Inc.. All rights reserved.