Hard colloidal rods near a soft wall: wetting, drying, and symmetry breaking

– Within an Onsager-like density functional theory we explore the thermodynamic and structural properties of an isotropic and nematic fluid of hard needle-like colloids in contact with a hard substrate coated with a soft short-ranged attractive or repulsive layer. As a function of the range and the strength of the soft interactions we find wetting and drying transitions, a pre-drying line, and a symmetry-breaking transition from uniaxial to biaxial in the wetting and drying film. Whereas bulk liquid crystal phases of suspensions of colloidal hard rods have been essentially understood due to Onsager’s work in the 1940’s [1, 2] and simulations [3] and density functional theories [4–6] in the late 1980’s, their surface and interfacial properties are subject of ongoing study, not only experimentally but also theoretically and numerically. Good progress was made during the past decade in the theoretical study of planar free isotropic-nematic (IN) interfaces, e.g. it is known by now that the nematic director in the thermodynamically stable state is parallel to the interfacial plane [7–9], that complete wetting of the IN interface by another nematic phase occurs near the triple point in binary rod mixtures [10], and that the one-particle distributions in the IN interface are only weakly biaxial [11–13]. The effect of external substrates on suspensions of hard rods has also been studied. For a planar hard wall, for instance, evidence of complete wetting of the wall-isotropic (WI) interface by an intervening nematic film was provided by theory [14] and simulations [15]. Other studies were concerned with properties of a hard-rod fluid in contact with a “penetrable” wall, which restricts only the translational degrees of freedom of the rods [16]. It was shown that such a wall favors homeotropic anchoring of the nematic director, and that the WI interface exhibits complete wetting by the homeotropically aligned nematic phase upon approach of IN coexistence [8,17]. The common feature of all these studies is that the chosen wall potential does not allow to control the degree of surface nematic order. This is in contrast with Landau-de Gennes theory, which predict rich surface phase diagrams for liquid crystals [18–20]. A drawback of this formalism is, however, that the effects of particular surface-particle interactions are hidden in