First-order versus second-order interface localization transition of thin Ising films with competing walls

From extensive Monte Carlo simulations of a lattice gas model we present evidence that the order of the gas-liquid transition of a fluid confined between ``competing'' walls can change from second to first order when the thickness D of the thin film is varied. This situation typically arises when the wetting transition of the corresponding semi-infinite system is first order, and thus permits the study of a tricritical interface-localization-delocalization transition via control of the film thickness.