Effect of extreme wetting scenarios on pool boiling conditions in a quiescent medium

This study focuses on the detailed description of the heat transfer and bubble dynamics processes occurring for the boiling of water on surfaces with extreme wetting regimes, namely hydrophilicity and superhydrophobicity. The wettability is changed by modifying the surface chemistry and without significant variations in the mean surface roughness. Under these conditions and for the range studied here the effect of the extreme wetting regimes was dominant, thus the influence of surface topography was not addressed. A particular trend is observed for the boiling curve obtained with the superhydrophobic surfaces, as the heat flux increases almost linearly with the superheat, although with a much lower slope than the hydrophilic surfaces. This occurs due to the formation of a large stable vapour film over the entire surface just at around 1 K superheat, because of the almost immediate coalescence of the bubbles generated on the surface. This behaviour agrees with the so-called “quasi-Leidenfrost” regime recently reported in the literature and with a theoretical prediction of the heat flux that is presented in this study. Furthermore, a comprehensive analysis of bubble dynamics, useful for comparison with numerical simulations is given. Such analysis is based on the temporal evolution of the bubble diameter together with bubble contact angle and with the velocity of the contact line. The results suggest that the existing models and correlations can predict the trends of the bubble growth using a modified contact angle value, called the bubble contact angle (or its supplemental value), for the hydrophilic surfaces, even if they cannot accurately predict bubble sizes. Approximating the modified contact angle with the quasi-static contact angle, obtained during surface characterization is practical for a qualitative evaluation, but the results obtained here do not support for its use when estimating the bubble departure diameter. On superhydrophobic surfaces, the effect of the vapour film must be considered, since although this is not the starting point of the boiling process, it represents the actual working conditions when using this kind of surfaces.