where ŪF ( ŪS ) is the interaction energy of a particle in the face-on (side-on) orientation with its nearest neighbours in the interfacial layer and with the substrate. (Note that the nearest-neighbour fluid interactions are halved in these equations because each such interaction contributes to the energy of two interfacial fluid particles.) Ūi j is the interaction energy in the face-on/face-face orientation (for i = F) or side-on/side-side orientation (for i = S) with the substrate (for j = s) or another fluid particle (for j = f).