Applied Surface Science 258 (2012) 6416–6424 Contents lists available at SciVerse ScienceDirect Applied Surface Science jou rn al h om epa g e: www.elsevier.com/locate/apsusc Mechanism of wettability transition in copper metal foams: From superhydrophilic to hydrophobic Mahmood R.S. Shirazy a,∗ , Sonia Blais b , Luc G. Fréchette a a Department of Mechanical Engineering, Université de Sherbrooke, 2500 Boul. Université, Sherbrooke, J1K 2R1 Canada b Centre de caractérisation des matériaux (CCM), Université de Sherbrooke, 2500 Boul. Université, Sherbrooke, J1K 2R1 Canada a r t i c l e i n f o Article history: Received 29 August 2011 Received in revised form 2 March 2012 Accepted 13 March 2012 Available online 19 March 2012 Keywords: Heat pipe Wick Metal foam Wetting Hydrophilicity Organic compound a b s t r a c t 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 ambi- ent 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 char- acterization 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 hydropho- bicity 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 hydropho- bic 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 (-pinene). © 2012 Elsevier B.V. All rights reserved. 1. Introduction Wetting property of a porous material is a key property to determine the suitability of the material as a wicking element. A hydrophilic wick system is necessary to transport water in capillary-driven systems such as heat pipes and heat spreaders increasingly used for electronics cooling [1]. When one end of a heat pipe is connected to a heat source, such as a hot microproces- sor, heat is removed by evaporating water enclosed in the pipe. The vapor travels to the other end of the heat pipe where it condenses [2]. The water is pumped back to the hot end by the capillary action of the wicking material inserted in the pipe. It is shown that low wettability of the wicking material directly reduces the critical heat load that can be carried by a heat pipe [3]. The degree of wettabil- ity of a flat surface by a specific liquid can be shown by measuring the contact angle (CA) of a droplet of that liquid on the surface. For water as the liquid, a surface is called hydrophilic when CA < 90 ◦ , and hydrophobic when CA > 90 ◦ . When the CA of water is larger than 150 ◦ , it is called superhydrophobic and when the CA of water is almost 0 ◦ , it is called superhydrophilic [4]. Among different materials used in engineering, copper is widely used in many industrial applications. Due to its high thermal ∗ Corresponding author. Tel.: +1 819 821 8000x62818; fax: +1 819 821 7163. E-mail address: reza.shirazy@Usherbrooke.ca (M.R.S. Shirazy). conductivity, moderate cost and good thermal contact, copper is a major candidate to make thermal management components for electronics cooling. Copper based porous materials such as sintered copper powder, have been used as the heat transfer medium in cap- illary driven systems such as flat heat pipes and vapor chambers for electronic cooling [5,6]. Copper open cell metal foams are among more recent copper based porous materials which offer a high per- meability, high surface area and high capillary pumping capacity. These properties make them interesting candidates as the wicking material in flat heat pipes and thermal spreaders [7]. Copper metal foams are superhydrophilic as they leave the pro- duction line. The high surface wettability combined with their porous structure yields high capillary forces which is favorable for a wicking material. A water droplet spreads on their surface immediately and is pulled into their porous structure due to high capillary forces. But after exposure to room ambient air, they grad- ually lose their superhydrophilic property and eventually turn into hydrophobic surfaces; a water droplet remains on the surface and does not penetrate inside the foam. Moreover, the measured con- tact angle is more than 90 ◦ (∼136 ◦ ) for a copper metal foam left in the room ambient for several days. A contact angle of more than 90 ◦ implies a hydrophobic surface which is not favorable for heat pipe applications. The same phenomenon also happens for sintered cop- per powder wicks while exposed to ambient air. In order to restore the hydrophilicity of the foams and use them as wicking material, a hydrogen reduction process is typically done on the foams right 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.03.052