Mechanism of wettability transition in copper metal foams: From superhydrophilic to hydrophobic

Hydrophilicity is a necessary condition for wicking materials such as copper metal foams used in heat pipes. Unfortunately hydrophilic copper metal foams become hydrophobic when exposed to room ambient air. This wettability transition is commonly explained by the formation of copper oxides on the surface; however copper oxides are known to be hydrophilic. An experimental study is conducted to explain this controversy. Effect of different atmospheres on the rate of hydrophilicity loss is studied by a novel approach which is to measure the spreading time of droplet on the foam surface. Also, surface characterization techniques such as XPS and TOF-SIMS are used to characterize the time dependent impact of the ambient air on the morphology and chemical composition of the copper metal foams. The hydrophobicity mechanism of copper metal foams is found to be dominated by surface chemistry and not its morphology (Wenzel type). Results show that oxidation is not the reason for this hydrophilic to hydrophobic change but rather, adsorption of volatile organic compounds (VOCs) on the copper foam's surface. This explanation is further supported by observing the same wettability change towards hydrophobicity when immersing hydrophilic copper metal foams in a liquid VOC (alpha-pinene). (C) 2012 Elsevier B.V. All rights reserved.