Influence of different parameters of preparing self-assembled monolayers on copper surfaces in the dropwise condensation heat transfer: an experimental study

Dropwise condensation (DWC) is one of the phase change heat transfer regimes, which due to its high capacity has received much attention from investigators. To produce DWC, it is necessary to have hydrophobic surfaces. Low surface energy coatings have been usually used to produce hydrophobic surfaces. These coatings due to their low thermal conductivities must have the minimum thickness. Therefore, self-assembled monolayers coatings like stearic acid or 1-octadecanethiol coatings (with thicknesses as high as a few nanometers) have been used in the literature. To create hydrophobic coatings by these materials, the surfaces are immersed in hydrophobic solutions with particular concentrations and in a specific time. There is not any general investigation on the effects of the time and concentration of these coatings on DWC in the literature. In this study, first, a proper apparatus for DWC experiments is designed and fabricated. Then, the effects of concentration and immersing time of the two mentioned materials on DWC phenomena have been studied. Between four coating times and solution concentrations of 1-octadecanethiol, concentration of 0.025M at coating time of 30min increases DWC heat transfer coefficient by about 4-7 times with respect to filmwise condensation in different subcooling temperatures. Concentration of 0.0025M at coating time of 1hour of stearic acid also increases the heat transfer coefficient by about 3-4 times. As well as the 1-octadecanethiol coating has a more uniform structure and higher heat transfer coefficient with respect to stearic acid coating.