An integrated study of dropwise condensation heat transfer on self-assembled organic surfaces through Fourier transform infra-red spectroscopy and ellipsometry

摘要

Self-assembled monolayers (SAMs) formed by adsorption of 1-octadecanethiol [CH_3(CH_2)_(17)SH] and 16-mercapto-hexadecanoic acid [CO_2H(CH_2)_(15)SH] onto gold-coated-copper substrates were applied to investigate the enhancement of dropwise condensation (DWC) heat transfer of steam at atmosphere pressure. A durability test was also conducted. Although hydrophobic SAMs increase the heat transfer coefficient by nearly an order of magnitude from that of film-wise condensation, it was found that DWC using octadecanethiol SAM as a promoter is a dynamic process in that the heat transfer coefficient decreases with time over 2h. These results were reconfirmed by an integrated study using Fourier transform infra-red spectroscopy (FT-IR) and Spectroscopic Ellipsometry. We found that the monolayer of octadecanethiol becomes less crystalline with time, causing the film thickness and heat transfer coefficient to decrease. There was also some indications that, as SAMs were partly removed (leaving patches of the bare metal), contaminant from steam spontaneous adsorbed onto these high energy sites, resulting in a slightly higher heat transfer coefficient at a later stage of condensation.