Surface phase diagrams for wetting on heterogenous substrates

We propose a simplified description of fluid adsorption on heterogeneous mi cropatterned substrates. Using this approach, we are able to rederive resul ts obtained earlier using effective interfacial Hamiltonian methods and pre dict a number of new examples of surface phase behavior for both singly and periodically striped substrates. In particular, we show that, for a singly striped system, the manner in which the locus of surface unbending phase t ransitions approaches the prewetting line of the infinite pure system, in t he limit of large stripe widths, is nontrivial and sensitive to several cha racteristic length scales and competing free-energies. For periodic substra tes, we investigate finite-size deviations from Cassie's Law for the wettin g temperature of the heterogeneous system when the domain sizes are mesosco pic. (C) 2001 American Institute of Physics.