Entropic wetting and the fluid-fluid interface of a model colloid-polymer mixture

A recent density functional theory is used to investigate the free interface between demixed fluid phases in a model colloid-polymer mixture. Both the colloid and polymer density profiles oscillate on the colloid-rich side of the interface, provided the polymer reservoir packing fraction eta(p)(r) is sufficiently high. Results for the surface tension are in reasonable agreement with experiment. When the mixture is adsorbed against a hard wall, entropic depletion effects give rise to a wetting transition whereby the colloid-rich phase wets completely. Prior to complete wetting we find three layering transitions, the first of which extends far into the single-phase region. This pattern of surface phase transitions is very different from that observed for simple one-component fluids at planar substrates.