Interfaces, wetting, and capillary nematization of a hard-rod fluid: Theory for the Zwanzig model

We investigate interfacial and capillary phenomena in a simple model for a fluid of hard rods, viz. the Zwanzig model, in which the orientations of re ctangular blocks are restricted to three orthogonal directions. The theory, which is based on an Onsager-like free energy functional, predicts local b iaxial ordering at the "free" interface between the coexisting isotropic an d nematic phases. For an isotropic bulk fluid in contact with a single plan ar hard wall, we find a continuous surface phase transition from uniaxial t o biaxial local symmetry, followed by complete wetting of the wall-isotropi c fluid interface by a nematic film with director parallel to the wall, as the reservoir density approaches its value at bulk coexistence. For a fluid confined by two parallel hard walls we determine a first-order capillary n ematization transition at large wall separation, which terminates in a capi llary critical point when the wall separation is about twice the length of the rods. This transition is the analog of the capillary condensation obser ved for simple fluids confined by attractive walls but is purely entropy dr iven here. (C) 2000 American Institute of Physics. [S0021-9606(00)51535-9].