RHEOLOGICAL PROPERTIES OF CONFINED THIN-FILMS

Shearing of monolayer and bilayer monatomic films confined between pla nar solid surfaces (walls) is simulated by a Monte Carlo technique in the isostress-isostrain ensemble, where temperature, number of film at oms, and applied normal stress are state variables. The walls consist of individual atoms that are identical with the film atoms and are fix ed in the fee (face centered cubic) (100) configuration. The lattice c onstant l of the walls is varied so that the walls are either commensu rate with the (solid) film at fixed nominal lattice constant l(f) (i.e ., l/l(f)=1) or homogeneously compressed (l/l(f)<1) or stretched (l/l( f)>1). Such theological properties as shear stress T-zx and modulus ar e correlated with molecular structure of the layers, as reflected in t ranslational and orientational correlation functions. If the walls are properly aligned in transverse directions, then the layers exhibit a high degree of fee order. As such ordered films are subjected to a she ar strain (by reversibly moving the walls out of alignment), they resp ond initially as an elastic solid: at small strains, T-zx depends line arly on the strain. As the shear strain increases, the response become s highly nonlinear: T-zx rises to a maximum(yield point) and then deca ys monotonically to zero, where the maximum misalignment of the walls occurs. The correlation functions indicate that the films are not nece ssarily solid, even when the walls are in proper alignment. The result s suggest that the principal mechanism by which disordered nonsolid fi lms are able to resist shearing is ''pinning'': the film atoms are tra pped in effective cages formed by their near neighbors and the mutual attraction of the walls for the caged atoms pins them together.