Chemical structure effects on the equilibrium and under shear properties of thin films in confined geometries: A molecular dynamics simulation study

We have made a comparative study of confined thin fluid films, composed of either n-decane or 4-propyl-heptane. The films are studied in equilibrium and under shear using molecular dynamics (MD) simulations. The films composed of linear chains present density profiles of methylene subunits with higher degree of layering than those composed of branched molecules. There are no significant differences in the diffusion coefficients of the two molecules studied in bulk, or in confined geometries. The diffusion coefficients for the confined films are strongly dependent on the strength of the frictional forces exerted by the wall, rather than on the density of the films. They also indicate that the confined films remain in a fluidlike state in all the simulations. The bulk values of the diffusion coefficient of n-decane are in excellent agreement with the experimental data. When the confining walls move in opposite directions, the fluid films develop shear flow with a very weak shear rate. Superimposed to the shear flow, the films seem to perform an oscillatory motion, where they alternately flow following the direction of motion of either wall. The steady state values of the shear stress increase linearly with the pressure normal to the confining walls, as also found experimentally. The films composed of linear chains exhibit higher resistance to the displacement of the walls than those composed of branched chains. This is because the films composed of linear chains have higher density of methylene subunits in the region of the pore where the fluid molecules exert frictional forces on the walls.