Lateral critical Casimir force in two-dimensional inhomogeneous Ising strip. Exact results.

We consider two-dimensional Ising strip bounded by two planar, inhomogeneous walls. The inhomogeneity of each wall is modeled by a magnetic field acting on surface spins. It is equal to +h1 except for a group of N1 neighboring surface spins where it is equal to -h1. The inhomogeneities of the upper and lower wall are shifted with respect to each other by a lateral distance L. Using exact diagonalization of the transfer matrix, we study both the lateral and normal critical Casimir forces as well as magnetization profiles for different temperature regimes: below the wetting temperature, between the wetting and the critical temperature, and above the critical temperature. The lateral critical Casimir force acts in the direction opposite to the shift L, and the excess normal force is always attractive. Upon increasing the shift L we observe, depending on the temperature regime, three different scenarios of breaking of the capillary bridge of negative magnetization connecting the inhomogeneities of the walls across the strip. As long as there exists a capillary bridge in the system, the magnitude of the excess total critical Casimir force is almost constant, with its direction depending on L. By investigating the bridge morphologies we have found a relation between the point at which the bridge breaks and the inflection point of the force. We provide a simple argument that some of the properties reported here should also hold for different models of the strip with the same type of inhomogeneity.