Statistical mechanics of Cassie's law

The wetting of chemically heterogeneous surfaces is typically discussed in terms of Cassie's law (1948, Discuss. Faraday Soc., 3, 11), which defines the cosine of the contact angle in terms of an average over the different chemical regions, weighted by their relative surface coverage. This paper derives an exact formulation of Cassie's law, in the context of the wetting of chemically patterned substrates. Deviations from Cassie's law arise from quasi-two-dimensional fluid-mediated interactions, analogous to solvation forces.