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5323285 
Journal Article 
Long term testing for dropwise condensation using self-assembled monolayer coatings of n-octadecyl mercaptan 
Vemuri, S; Kim, KJ; Wood, BD; Govindaraju, S; Bell, TW 
2006 
Yes 
Applied Thermal Engineering
ISSN: 1359-4311 
26 
4 
421-429 
WOS:000233311700010 
Two different types of hydrophobic coatings have been analyzed experimentally for their ability to promote dropwise condensation (DWC). For any technique used for promoting dropwise condensation, the longevity of the coating is critical if it is to be used in any further applications. Previous studies using self-assembled monolayers of n-octadecyl mercaptan have not reported any information on the ability for promoting DWC beyond 500 h of experimentation. In the current research experiments were carried out using self-assembled monolayers for more than 2600 h of experimentation and it showed good dropwise phenomena. Stearic acid solution (SAM-1) and n-octadecyl mercaptan solution (SAM-2) were used to form an ultra-thin organic hydrophobic film on the surface. An oxide layer was initially formed on the substrate surface before coating the surface with monolayers. The oxide layer formed on the substrate surface tends to improve the bonding between the substrate and the monolayers which eventually improves the longevity of the coating. In general, a SAM system with a long-chain, hydrophobic group is nano-resistant, meaning that such a system forms a protective hydrophobic layer with negligible heat transfer resistance but a much stronger bond. When compared to complete filmwise condensation, the SAM-2 (n-octadecyl mercaptan) coating increased the condensation heat transfer coefficient by a factor of approximately 3 after 100 h of experimentation and by a factor of approximately 1.8 after 2600 It of experimentation for copper alloy surfaces, under vacuum condition (33.86 kPa). Lifetime of maintaining dropwise condensation is greatly dependent on the bonding of SAM coating to the condensing surface, form the experimental investigation it was evident that n-octadecyl mercaptan showed good DWC due to its covalent bonding with the substrate surface when compared to that of stearic acid which is bonded to the substrate surface by hydrogen bonding. Contact angles were measured for all the SAM coated surfaces before and after experimentation, respectively. (c) 2005 Elsevier Ltd. All rights reserved. 
dropwise condensation; organic coatings; monolayers