Critical wetting scaling

Fig. 32. Phase diagram of the surface plotted in terms of the scaled variables h jy and g/y. For g/y < —2 one observes critical wetting and for g/y > —2 one observes first-order wetting. In the latter regime, mean field theory predicts melaslable wet and non-wet regions limited by the two surface spinodal lines ft, and ft( respectively. Also two quenching experiments arc indicated where starting at a rescaled time r = 0 from a stable state in the non-wet region one suddenly brings the system by a change of fti into the metastahle wet or unstable non-wet region, respectively. From Schmidt and Binder (1987).

It is possible that this theory can be adapted to explain molten metal-water thermal explosions although many needed data are still unavailable. One might presume that, at the molten metal-wet surface interface, there is some chemical reaction. Possibly that of the metal plus water or metal plus surface to lead to localized formation of salt solutions. These may then superheat until homogeneous nucleation occurs. The local temperature and pressure would then be predicted to be far in excess of the critical point of pure water (220 bar, 647 K) and a sharp, local explosion could then result. Fragmentation or subsequent other superheat explosions would then lead to the full-scale event. 

These comments should not be interpreted to mean that measures of wettability are useless at predicting adhesion. They do seem clearly to indicate that contact angles and critical surface tensions reported for wood are not necessarily thermodynamic quantities or well-defined material parameters. Because most contact angles are dynamic values, they should be interpreted with caution and considered as relative measures of adhesion, for which the absolute scale is yet unknown. Further, we need to keep in mind that although wetting is necessary for adhesion, it may not be the limiting factor in many real situations. 

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