Experimental and Theoretical Investigation of Nucleation Site Density and Heat Transfer During Dropwise Condensation on Thin Hydrophobic Coatings

Dropwise condensation (DWC) has the potential to enhance heat transfer compared to filmwise condensation (FWC). The heat transfer rates achieved by DWC depend on the drop size distribution, which is influenced by nucleation processes of newly formed drops. In DWC modeling, the nucleation site density N_s is used as an input parameter to obtain the drop size distribution of small drops. However, due to the small scale of the condensate nuclei, direct observation is difficult, and experimental data on the nucleation site density are scarce. In the literature, values in the range of 10⁹ m⁻² to 10¹⁵ m⁻² can be found for N_s. In this paper, we report DWC experiments on SiO₂ and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES) thin hydrophobic coatings that show significantly different nucleation site densities. Nucleation site densities are estimated from high-speed imaging of small drops during initial condensation and from model calibration using established DWC theory. We have found the values for N_s to be in the range from $1.1×1010 m−2$ to $5.1×1011 m−2$ for the SiO₂ coating and $1011 m−2$ to $1013 m−2$ for the PFDTES coating. Our results show that there can be large differences in the nucleation site density under similar conditions depending on the surface properties. This underlines the importance of investigating nucleation site density specifically for each surface and under consideration of the specific process conditions used for DWC.