Computer Simulations of the Rheological Behavior of Confined Films

Abstract

Shearing of mono- and bilayer monatomic films confined between planar solid surfaces is investigated by Monte Carlo simulations in the isostress-isostrain ensemble, where temperature, number of film atoms, relative transverse alignment of the surfaces, and applied normal stress are thermodynamic state variables. The surfaces consist of individual atoms that are identical with film atoms and are rigidly fixed in the face-centered cubic (fcc) (100) configuration. The lattice constant ℓ of the walls is varied so that the walls are either commensurate with the (solid) film at fixed nominal lattice constant ℓ _f (i.e. ℓ _f = 1), homogeneously compressed (ℓ _f > 1), or stretched ℓ _f > 1). Rheological properties as shear stress T_zx and modulus c ₄₄ are correlated with molecular structure of the layers, as reflected in orientational correlations. If the surfaces are properly aligned in transverse directions, then the layers exhibit a high degree of fcc order. As such ordered films are subjected to a shear strain (by reversibly moving the surfaces out of alignment), they respond initially as an elastic solid: at small strains T_zx depends linearly on the strain. As the shear strain increases, the response becomes highly nonlinear: T_zx rises to a maximum (yield point) and then decays monotonously to zero, where the maximum misalignment of the walls occurs. The dependence of T_zx on the shear strain up to states just beyond the yield point can be interpreted as the nonlinear response of an elastic solid to deformation. Orientational correlation functions indicate that the films are not necessarily solid, even when the walls are in proper alignment. The results suggest that the principle mechanism by which disordered nonsolid films are able to resist shearing is “pinning”: the film atoms are trapped in effective cages formed by their near neighbors and mutual attraction of the walls for the caged atoms pin them together.

Key Words:

• Rheology
• confined films
• Isostress-Isostrain ensemble