For fluid-fluid interactions, the interaction energy in either the face-face or side-side orientation was taken as the minimum of the GB pair potential in the specified ŪFs and ŪSs were calculated as the minimum in the interaction between a single GB ellipsoid and the substrate in the face-face and side-side orientations, respectively. As a particle-based substrate was used in this work rather than being perfectly flat, the energy minimum was averaged over 100 random positions on the substrate. Equation (7) also applies for the energy difference per fluid particle at the vapour interface, but with the fluid-substrate interactions set to zero, i.e. ŪFs = ŪSs = 0.