| Self-assembled monolayers (SAMs) formed by adsorption of 16-mercaptohexadecanoic acid and 1-octadecanethiol onto gold-coated-copper substrates were used to investigate the dropwise condensation (DWC) heat transfer of steam at atmosphere pressure. A durability test was conducted. Although hydrophobic SAMs increase the heat transfer coefficient by nearly an order of magnitude from that of film condensation, it was found that DWC using 1-octadecanethiol SAMs as a promoter is a dynamic process in that the heat transfer coefficient changes with time and heat flux. These results were reconfirmed by an integrated study using fourier transform infrared spectroscopy (FT-IR) and spectroscopic ellipsometry: the monolayers of 1-octadecanethiol become less crystalline with time, causing the monolayers thickness and heat transfer coefficient to decrease; the rate that SAMs become less crystalline (decrease in thickness) depends on the heat flux passing through the surface, resulting in faster decrease of heat transfer coefficients with increasing heat flux. There were also some indications that, as SAMs were partly removed (leaving patches of the bare gold), contaminants from steam spontaneously adsorbed onto these high energy sites, resulting in a slightly higher heat transfer coefficient at a later stage of condensation. |
