Thermal fluctuation forces and wetting layers in colloid-polymer mixtures: derivation of an interface potential

We discuss wetting layers in phase-separated colloid-polymer mixtures adsorbed at a vertical wall, observed in recent laser scanning confocal microscopy experiments. Matching of colloid and solvent dielectric properties renders van der Waals forces negligible and provides a system governed by short-range forces and thermal fluctuations on which the subtle predictions of renormalization group (RG) theory for wetting can be tested. The width w of the fluid-fluid ("liquid-gas") interface bounding the wetting layer scales with the square root of the wetting layer thickness l, in qualitative agreement with RG theory for short-range complete wetting in three dimensions. The measured wetting layer thickness l as a function of the height h above the horizontal plane of bulk phase separation is compared with two distinct theoretical predictions. A simple heuristic interface potential V(l), first proposed in a previous report, is now fully derived, and confronted here with the interface potential based on the linear RG theory. The heuristic approach does not capture fully the RG treatment. While fundamental differences exist between the two approaches, the resulting predictions for l(h) are almost identical. However, the theory does not follow the precise shape of the experimental curve of l(h).