Universal Shape of the Fluid Density Profiles Near a Solid Boundary: LJ Vapor Near Weakly Attractive and Hard Walls

The universality class of the fluid critical behavior near surfaces with vanishing fluid-wall attraction is studied by the analysis of the density profiles of the saturated Lennard-Jones (LJ) vapor near hard and weakly attractive walls. The depletion of the fluid density ρ(z) toward such surfaces can be described in a universal way by the exponential law ρ(z) = ρ0 (1 − 0.5 exp(−z/ξ)) at z/ξ > 1, where z is the distance to the surface, ξ is the correlation length and ρ0 is the density of the saturated bulk vapor. The surface critical behavior of fluids exhibiting density depletion at the critical point agrees with the universality class of the ordinary transition in Ising magnet at zero or vanishing surface field. In the narrow pore with hard walls, the perturbation of a vapor by a surface is enhanced, as the density of the saturated vapor in the pore approaches the density of the bulk vapor spinodal and the correlation length of density fluctuations increases.