PHASE-TRANSITIONS IN A MODEL POROUS-MEDIUM

An exact solution is presented for Ising-like transitions in a decorat ed lattice model of a porous medium. The model is solved by decimation of the spins, leading to a space-filling lattice with renormalized pa rameters. The critical temperature is found to vary as I/InL, where L is the number of sites between intersections of the spin chains. Some of the critical exponents differ from those of the ordinary Ising prob lem. We have also studied the case of a single, infinitely long pore, using both exact and approximate methods. An exploration of finite-wid th effects reveals surprisingly small (quantitative) deviations from m ean-field theory.