Plasma polymer films for dropwise condensation of steam

Thin hydrophobic polymer films produced by radio frequency (rf) plasma decomposition of hexamethyldisiloxane (HMDSO) have been produced on flat metal (titanium, stainless steel, and copper/nickel) and silicon substrates. By using a statistical design methodology, the deposition conditions have been assessed for their control of the deposition rate, refractive index, water droplet contact angle, and longevity of dropwise condensation of steam. The films showing the best performance were those deposited at the slower rates, using low rf power density and high monomer flow rates, and having the lower refractive indices. There is some dependence on substrate material, with CuNi behaving worst, and titanium providing the best results of the three metals studied. This may be connected with the stability of an interfacial oxide layer. The steam immersion tests have already exceeded 7500 hours of continuous condensation: and many of the films are still producing excellent dropwise condensation. The reasons for these effects of preparation conditions and substrate material on performance are discussed with reference to the plasma chemical process.