Rheology and local structure of thin films confined between thermally corrugated walls

The rheological behavior of a monolayer film of spherically symmetric molecules confined between two solid surfaces (i.e., walls) is investigated in isostress-isostrain ensemble Monte Carlo simulations. The walls consist of individual atoms interacting with film molecules via the Lennard-Jones potential. By employing the Einstein model, wall atoms are also subject to a harmonic binding potential and may depart from their equilibrium lattice sites so that the walls are thermally corrugated. The film can be exposed to a shear strain by moving the walls relative to each other in transverse directions. Molecular expressions for the shear stress are derived which differ from the ones previously obtained for thermally decoupled walls (i.e., in which wall atoms are rigidly fixed in their equilibrium lattice positions). Unlike the film's local structure the shear stress depends sensitively on the degree of thermal corrugation of the walls. The larger it is the more plastic is the response of the film to an applied shear strain.