Stress isotherms of porous thin materials: Theoretical predictions from a nonlocal density functional theory

Recent beam-bending (EB) experiments of microporous films with very small pores have shown that the fluid confined in these pores exhibits monotonic compressive stresses as the relative pressure is varied! from vacuum, to saturation (relative vapor pressure; p/p(0) = 1). The variation of the stress near saturation is found to be linear in ln(p) and given by the saturated liquid density rho(I) to within 20%. Capillary condensed fluids are traditionally described by the Laplace-Kelvin (LK) theory. LK theory correctly predicts the slope of the stress near saturation to be rho(I), but it also predicts that the stress should be zero at saturation and tensile between saturation and the capillary transition pressure;Hence, LK theory does not capture the monotonic compressive stress observed in BE experiments. This report describes the results of density functional theory calculations for a simple fluid confined to a slit pore network. We show how the presence of even a small amount of polydispersity in pore size leads to both a monotonic compressive stress as well as the observed LK slope.