The orientation at the solid interface was found always to be predominantly face-on, while a predominantly side-on orientation was exclusively found at the vapour interface (Fig. ). In both cases, the degree of alignment was to depend systematically on the volume fraction of the bulk fluid and the shape anisotropy of the fluid particles, with higher densities and shape anisotropies giving a greater degree of alignment. It has previously been predicted that a variety of hard anisotropic particles align with their long axis parallel to a solid substrate, as this alignment maximises the free volume and hence is the entropically favoured orientation. The opposite argument can be applied at the vapour interface, where the particles are able to extend over the interface, increasing the free volume when their long axis is perpendicular to the interface. Studies of hard ellipsoids at penetrable walls also show this behaviour. Thus, for oblate particles, a face-on orientation at the solid interface and a side-on orientation at the vapour interfaces are predicted to be entropically favoured, which is qualitatively consistent with the results presented here.