Wetting properties of a hard-spherocylinder fluid on a substrate

A density-functional theory is used to analyze the wetting properties of a fluid made up of hard spherocylinders of a length-to-breadth ratio L/D=5 on a model substrate. The substrate imposes an exclusion boundary condition over the molecular centres of mass, while at the same time favoring a definite molecular orientation, either parallel or perpendicular to the substrate, in a region next to the substrate. The wetting properties of this system are seen to depend on the strength with which the substrate orients the molecules: as the latter is increased, wetting by nematic phase is followed by a region of partial wetting which then leads to reentrant wetting by nematic. The two wetting transitions correspond to wetting films with nematic director perpendicular and parallel to the substrate, respectively. Also, in the region of partial wetting, an anchoring transition occurs in the substrate-nematic interface between two different director configurations (parallel and perpendicular to the substrate). Finally, a metastable wetting transition by isotropic is also obtained. This model considerably enriches the wetting phenomenology of the hard-spherocylinder fluid on substrates, of which only the pure hard wall, with no surface control parameter available, has been considered so far.