The adhesion of the water to the solid surfaces depends formally on the refractive index of the solid surface. If it is larger than 1.333 ( n>1.333 for most of the solids), the dispersion attraction between the water and the solid surface (adhesion) is larger than the dispersion attraction between the water molecules (cohesion). So, if n(solid) > 1.333 there is adhesion of the water to the solid surface. The surface can be wetted well. How about the hydrophobized surface ? For example if we hydrophobize clean glass surface with Hexamethyldisilazane, we can easily achieve contact angle larger than 85 deg. Well, the glass ( n = 1.52) continues to have the same refractive index, but there is significantly weaker adhesion to the glass. So it appears that the refractive index of the glass is no so important for the adhesion. If this topic is interesting for anyone we can have discussion.
Dear Colleague, the surface tension of solids in the simplest case is divided into dispersive and dipolar components, which can be estimated e.g. by measuring the water and diiodomethane contact angles. The dispersive part may be correlated with the optical refractive index, the dipolar part cannot. Moreover, the refractive index is a bulk property, while the surface tension is determined by the topmost layer. Even a monomolecular coverage can change profoundly the wettability, which cannot be detected by refractive index measurements.
The presence of correlation between two variables is NOT enough to decide whether there is a physical relationship between them or not. It is well known that "correlation does not imply causation".
Based on that, and keeping in mind the physical meaning of surface tension and the refractive index, it is opvious that there is no cause-and-effect relationship between them.
Thus, there is no contradictions when the surface tension is largely changed, via a surface treatment, while the refractive index is nearly unchanged. In such case the surface treatment spoils the correlation but not the physics.
Dear colleagues, Thank you for discussing this important topic. In fact, the surface tension appears due to the difference in the standard chemical potential in bulk and this one on the surface. The refractive index is roughly related with the electric susceptibility of the dielectric, and consistently to the polarizability. What is interesting is that even at 90 deg < CA < 110 deg the water droplet cannot roll over the surface but it behaves as glued to the surface. This is because of the adhesion driven by the refractive index difference between the water ( n=1.333) and glass (n= 1.52). In other words adhesion exists despite of the surface treatment. The point is that we can increase or decrease this adhesion by surface treatment. For this reason the adhesion should have surface and bulk contribution. So, it appears another important question: Can we manipulate the van der Waals repulsion between a bubble and solid surface by hydrophilizing the surface ? I will appreciate if you can add some discussion on this issue.