Capillary condensation between mesocopically rough surfaces

Grand canonical ensemble Monte Carlo (GCEMC) simulations are employed to study a Lennard–Jones (12, 6) fluid confined between furrowed walls. The walls are constructed by gouging triangular grooves in planar substrates that are structureless on the molecular scale. The furrows are infinitely long in one transverse direction (y) and characterized by a period (sx) and amplitude (D) of corrugation. For a fixed separation sz=10σ (σ is the ‘diameter’ of a fluid molecule) between the substrates the phase behavior of the confined fluid is investigated as a function of sx, D, and temperature T. It depends crucially on substrate corrugation. For example, capillary condensation (i.e. confinement-induced condensation of gas) is observed for sx<30σ as μ increases under isothermal conditions. In the limit D=0 (planar substrate) capillary condensation is a discontinuous phase transition. For a nonplanar substrate (D=5σ) it occurs as a two-stage process where first liquid continuously fills the furrows until a nearly planar gas–liquid interface has formed separating liquid in the furrows from gas in the ‘inner’ subvolume of the system. In the second stage the gas condenses spontaneously. For sx≥30σ an initial gas condenses partially thereby forming fluid bridges. Bridges are characterized by high(er)-density fluid stabilized by the ‘tips’ of the opposite furrows. The fluid bridges enclose gas-filled ‘holes’ which shrink in size if μ increases further. Eventually, the ‘holes’ vanish spontaneously in favor of a liquid phase.