Fluids in porous media: The case of neutral walls

The bulk phase behavior of a fluid is typically altered when the fluid is brought into confinement by the walls of a random porous medium. Inside the porous medium, phase transition points are shifted, or may disappear altogether. A crucial determinant is how the walls interact with the fluid particles. In this work, we consider the situation whereby the walls are neutral with respect to the liquid and vapor phase. In order to realize the condition of strict neutrality, we use a symmetric binary mixture inside a porous medium that interacts identically with both of the mixture species. Monte Carlo simulations are then used to obtain the phase behavior. Our main finding is that, in the presence of the porous medium, a liquid-vapor type transition still occurs, but with critical exponents that deviate from bulk Ising values. In addition, we observe clear violations of self-averaging. These findings provide further evidence that random confinement by neutral walls induces critical behavior of the random Ising model (i.e. Ising models with dilution type disorder, where the disorder couples to the energy).