Abstract
Motion of condensed water or water-ethanol mixture drop on a cold copper surface with gradient of contact angle was studied for the purpose of applicability of surface tension gradient driven capture/removal of the small liquid droplets on the substrate. In this work, a new method of generating a gradient surface has been developed that painting self-assembled monolayers (SAMs) of hydrophobic perfluoroalkylthiols and hydrophilic mercaptoalcohols on the electrochemically reduced oxide-free varying roughness copper surface. The wettability gradient of the surface could be manipulated in a wide range with a Wenzel-relation. Ethanol-water mixture vapor condensation test on the hydrophilic, hydrophobic and wettability gradient copper surfaces was carried out. The results of the vapor condensation on the gradient copper surfaces were shown that numerous liquid droplets nucleate and grow by coalescence with surrounding drops, then the droplet rapidly moved from the perfluoroalkylthiol SAMs covered hydrophobic side to the mercaptoalcohols SAMs covered hydrophilic ones. This phenomenon could be useful for passively enhancing heat transfer including two-phase flow.
