Statistical mechanics of Cassie's law

The wetting of chemically heterogeneous surfaces is typically discussed in terms of Cassie's law (1948, Discuss. Faraday Sec., 3, 11), which defines t he cosine of the contact angle in terms of an average over the different ch emical regions, weighted by their relative surface coverage. This paper der ives an exact formulation of Cassie's law, in the context of the wetting of chemically patterned substrates. Deviations from Cassie's law arise from q uasi-two-dimensional fluid-mediated interactions, analogous to solvation fo rces.