FLUIDS IN MICROPORES .4. THE BEHAVIOR OF MOLECULARLY THIN CONFINED FILMS IN THE GRAND ISOSTRESS ENSEMBLE

The behavior of molecularly thin prototypical confined films [Lennard- Jones (12,6) fluid constrained between two plane-parallel walls consis ting of like atoms fixed in the fee (100) configuration] is studied by Monte Carlo in a new (grand isostress) ensemble whose parameters are the thermodynamic state variables [temperature T, chemical potential m u, and normal stress (load) applied to the walls T-zz] controlled in t he surface forces apparatus used to study lubrication experimentally o n a molecular scale. Additional parameters of the ensemble not general ly controlled in this experiment are the film-wall interfacial area A and the crystallographic alignment (registry, or shear strain OL) Of t he walls. A multiplicity of phases is found to comport with a given ch oice of the parameters. The thermodynamically stable one minimizes the grand isostress potential (free energy). By means of thermodynamic in tegration the stable phase of the film is determined as a function of ct at fixed T, mu, T-zz, and A. Solid films comprising integral number s of layers of atoms parallel with the walls are stable when the walls are appropriately aligned. When such films are sheared (alpha is vari ed), they undergo drainage (imbibition) transitions at critical strain s, whereupon the him loses (gains) whole layers. Depending on the cond itions, the solid film may melt just prior to the transition.