Character of the Phase Transition in Thin Ising Films with Competing Walls

By extensive Monte Carlo simulations of a lattice gas model we have studied the controversial nature of the gas-liquid transition of a fluid confined between two parallel plates that exert competing surface fields. We find that the transition is shifted to a temperature just below the wetting transition of a semi-infinite fluid but belongs to the two-dimensional Ising universality class. In between this new type of critical point and bulk criticality, a response function $x_{\mathrm{nn}}^{max}$ varying exponentially with $D$ is observed, $\frac{2 \ln {\chi }_{\mathrm{nn}}^{max}}{D}={\ell }^{-1\mathrm{}}$, where $\ell$ is a new length characterizing interfaces.