COMPUTER-SIMULATIONS OF THE RHEOLOGICAL BEHAVIOR OF CONFINED FILMS

Shearing of mono- and bilayer monatomic films confined between planar solid surfaces is investigated by Monte Carlo simulations in the isost ress-isostrain ensemble, where temperature, number of film atoms, rela tive transverse alignment of the surfaces, and applied normal stress a re thermodynamic state variables. The surfaces consist of individual a toms that are identical with film atoms and are rigidly fixed in the f ace-centered cubic (fee) (100) configuration. The lattice constant l o f the walls is varied so that the walls are either commensurate with t he (solid) film at fixed nominal lattice constant l(f) (i.e. l/l(f)=1) , homogeneously compressed (l/l(f)<1), or stretched (l/l(f)>1). Rheolo gical 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 dire ctions, then the layers exhibit a high degree of fee order. As such or dered 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 s train increases, the response becomes highly nonlinear: T rises to a m aximum (yield point) and then decays monotonously to zero,where the ma ximum misalignment of the walls occurs. The dependence of T on the she ar strain up to states just beyond the yield point can be interpreted as the nonlinear response of an elastic solid to deformation. Orientat ional correlation functions indicate that the films are not necessaril y solid, even when the walls are in proper alignment. The results sugg est that the principle mechanism by which disordered nonsolid films ar e able to resist shearing is ''pinning'': the film atoms are trapped i n effective cages formed by their near neighbors and mutual attraction of the walls for the caged atoms pin them together.