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.