The crucial role of chemical detail for slip-boundary conditions: molecular dynamics simulations of linear oligomers between sliding aluminum surfaces

We study the slip-boundary conditions of short, linear paraffins and olefins confined between two sliding aluminum surfaces with molecular dynamics. Our simulations are based on a recently developed force field for the interaction between organic molecules and bulk aluminum. The lubricant molecules investigated all consist of six monomers but differ in the existence or location of merely one double bond. It turns out that this small change in the chemistry of the lubricant molecules can alter slip lengths quite dramatically, and is not strongly correlated with surface energies and bulk viscosity of the lubricant. For example, α and β-hexene have similar large negative slip length of Λ ≈ −8 A, even though α-hexene adheres twice as strongly to the surface as β-hexene. Eliminating the double bond in β-hexene reduces the surface energy by another factor of two, but increases Λ from −8 to 120 A. These results and those of additional simulations based on unrealistic, albeit occasionally used model potentials, make us conclude that surface energies and/or molecular geometries alone are not reliable indicators for slip-boundary conditions. Instead, it is necessary to consider the full chemical detail. As a more encouraging result, we find that the bulk viscosity appears to describe the dissipation within the sheared fluid close to the wall quite well, despite significant ordering near the boundaries. Moreover, all our systems show a relatively weak dependence of the slip length on the normal pressure.