It has previously been shown that the orientation of similar GB particles at a nematic-vapour interface can be easily switched between face-on for κ/κ < 1 to side-on κ/κ > 1, 37 whereas only a very slight preference for the face-on orientation was observed if κ/κ < 1 for the isotropic-bulk systems studied here (Fig. ). The greater propensity for a nematic bulk fluid to give the face-on orientation at the vapour interface compared with an isotropic bulk fluid can be understood qualitatively in terms of differences between the interfacial energy of the face-on relative to the side-on orientation in the two cases. For the isotropic fluid, this energy is described by eqn (7), whereas for the nematic fluid, interactions with the adjacent anisotropic fluid layer must also be taken into account. Assuming that, compared with the isotropic fluid, each fluid particle in the interfacial layer of the nematic fluid interacts with an additional particle of the same orientation in the next fluid layer, the face-on orientation is predicted to be more energetically stabilised relative to the side-on orientation in the nematic fluid than in the isotropic fluid for the systems studied here with κ < 1 and in ref 37, for which the face-face interactions were always stronger than the side-side ones (κ < 1).