Molecular simulation and continuum mechanics investigation of viscoelasticproperties of fluids confined to molecularly thin films

A combination of molecular dynamics simulations of oscillatory shear flow a nd continuum mechanics is used to investigate viscoelastic properties of ma terials confined to molecularly thin films. The atoms of the simple liquid interact via a repulsive Lennard-Jones potential. The chain molecules are m odeled as strings of similar spheres connected via finite extensible nonlin ear elastic springs. The fluid is confined between two surfaces composed of identical spheres that are moved to simulate oscillatory flow. In order to mimic experiments, the temperature is controlled by coupling the wall atom s to a heat bath, and the viscoelastic properties are obtained via an analy sis using continuum mechanics. Both simple and polymeric fluids exhibit lin ear viscoelastic behavior under typical simulation conditions, although ine rtial effects play an important role in determining the flow behavior. Simp le fluids display a smooth transition from liquidlike to solidlike behavior when confined to molecularly thin films, whereas linear chain polymers and gels display predominantly elastic shear response at all frequencies inves tigated. These results are in qualitative agreement with the surface forces apparatus experiments on similar systems. (C) 2001 American Institute of P hysics.