  The molecular mechanism of slip at the interface between polymer melts and
weakly attractive smooth surfaces is investigated using molecular dynamics
simulations. In agreement with our previous studies on slip flow of
shear-thinning fluids, it is shown that the slip length passes through a local
minimum at low shear rates and then increases rapidly at higher shear rates. We
found that at sufficiently high shear rates, the slip flow over atomically flat
crystalline surfaces is anisotropic. It is demonstrated numerically that the
friction coefficient at the liquid-solid interface (the ratio of viscosity and
slip length) undergoes a transition from a constant value to the power-law
decay as a function of the slip velocity. The characteristic velocity of the
transition correlates well with the diffusion velocity of fluid monomers in the
first fluid layer near the solid wall at equilibrium. We also show that in the
linear regime, the friction coefficient is well described by a function of a
single variable, which is a product of the magnitude of surface-induced peak in
the structure factor and the contact density of the adjacent fluid layer. The
universal relationship between the friction coefficient and induced fluid
structure holds for a number of material parameters of the interface: fluid
density, chain length, wall-fluid interaction energy, wall density, lattice
type and orientation, thermal or solid walls.
