Influence of the Wettability on the Boiling Onset B. Bourdon, † R. Rioboo,* ,† M. Marengo, ‡ E. Gosselin, † and J. De Coninck † † Laboratoire de Physique des Surfaces et des Interfaces, Universite ́ de Mons, Parc Initialis, Av. Copernic, 1, B-7000 Mons, Belgium ‡ Faculty of Engineering, University of Bergamo, Viale Marconi 5, 24044 Dalmine, Italy ABSTRACT: Experimental investigation of pool boiling is conducted in stationary conditions over very smooth bronze surfaces covered by a very thin layer of gold presenting various surface treatments to isolate the role of wettability. We show that even with surfaces presenting mean roughness amplitudes below 10 nm the role of surface topography is of importance. The study shows also that wettability alone can trigger the boiling and that the boiling position on the surface can be controlled by chemical grafting using for instance alkanethiol. Moreover, boiling curves, that is, heat flux versus the surface superheat (which is the difference between the solid surface temperature and the liquid saturation temperature), are recorded and enabled to quantify, for this case, the significant reduction of the superheat at the onset of incipient boiling due to wettability. ■ INTRODUCTION The current trend of miniaturization and of increase of functionality in electronic components leads to strong heating in materials such as conductors and semiconductors. Controlling the heat flux and the temperatures becomes thus crucial. On the other hand, it is known that phase-change or multiphase cooling systems present the highest cooling capacity. 1-3 For instance, with the phase-change, electronic cooling is often orders of magnitude more efficient than monophase systems. Correspondingly, the three-phase zone is of primary importance as most of the heat is dissipated in the contact line region 4,5 where phase-change is concentrated. In this zone, the liquid-gas interface is highly curved at the microscale region, and both topography and wettability characteristics are at least as important as the solid thermal conductivity. 6 When a solid surface in contact with a cooling liquid is heated, before the liquid boils, a so-called superheat, ΔT, appears where ΔT = T w - T sat , where T w is the temperature of the wall and T sat is the liquid saturation temperature. Thus, it is necessary to reach a surface temperature higher than the equilibrium saturation temperature to activate the boiling phenomenon. For safety and energy-saving reasons, the decrease of the superheat is important in many applications. A simple way to achieve that is to coat the solid surface by some hydrophobic layer. This has been studied before. 7-9 Controlling the location of boiling by microscopic treatment down to the nanoscale is still a challenge. In fact, in microdevices, the surface roughness should be kept as low as possible, because its scale may be near to the refrigerant channel size, to avoid important side-effects such as very high pressure drops. Understanding the complexity of the nucleate pool boiling is still a challenge as all surfaces features such as detailed topography and wettability down to nanometric scales are relevant and influence the heat exchange. 10,11 Until recently, surface wettability modification to study boiling has always involved cavities 12-16 as nucleation sites. Several papers have shown that the use of nanoparticles could enhance the heat transfer 16-19 with uncontrolled cavities resulting from the nanocoating method. Thomas et al. 20 and Balss et al. 21 showed that on smooth uniform surfaces, low wettability decreased the onset temperature for nucleate boiling in fast transient events. Zhang and co-workers 22 investigated the stability of air nanobubbles at the interface between a hydrophobic solid surface and water at ambient temperature. Hibiki and Ishii 23 showed that the number of nucleation sites is a function of the wettability of the surface, while Agrawal and co-workers 24 proved on isothermal systems that nanobubbles are positioned on hydrophobic patterns. The nucleation theory shows that the free energy to create a vapor nucleus of critical Received: September 16, 2011 Revised: December 2, 2011 Published: December 13, 2011 Article pubs.acs.org/Langmuir © 2011 American Chemical Society 1618 dx.doi.org/10.1021/la203636a | Langmuir 2012, 28, 1618-1624