Effect of fluid-substrate attraction and pore geometry on fluid adsorption.

We employ grand canonical ensemble Monte Carlo simulations to investigate the impact of substrate curvature on the phase behavior of an adjacent fluid. The substrates consist of a periodic sequence of grooves in the x direction; the grooves are infinitely long in the y direction. The shape of the grooves is controlled by a parameter eta. For eta = 0 the substrates are planar. If eta = 1, the grooves are wedge shaped. If eta > 1 the grooves become concave and in the limit eta = infinity rectangular. The fluid-substrate potential representing a groove consists of two contributions, namely, that of the homogeneous substrate base corresponding to a semi-infinite solid and that of a finite piece of solid with nonplanar surfaces. Whereas the former contribution can be calculated analytically, the latter needs to be evaluated numerically. For very large values of eta, that is in (almost) rectangular grooves, we observe capillary condensation of that portion of fluid located inside the grooves. As eta decreases capillary condensation gives way to continuous filling. In all cases, a nearly planar film-gas interface eventually forms in the direction normal to the surface of the substrate base and outside the grooves if one increases the chemical potential sufficiently.