Similarities and differences in the nonlinear flow behavior of simple and of molecular liquids

After the introduction of viscosity coefficients describing the non-Newtonian flow behavior and the normal pressure differences in a plane Couette symmetry, methods for the calculation of these coefficients are presented and some remarks on the connection with the linear viscoelasticity are made. The nonlinear flow behavior stems from the influence of the vorticity of the flow field and of the symmetric traceless shear rate tensor on the primary shear-rate-induced anisotropy of molecular distribution functions. In the case of molecular liquids the orientational distribution of the molecular axes (alignment) is the most relevant quantity; in simple liquids it is the pair-correlation function. Results are presented which can be inferred from an extended version of irreversible thermodynamics of molecular liquids; from a Fokker-Planck equation approach and for simple liquids, from a kinetic equation of Kirkwood-Smoluchowski type for the pair-correlation function. Finally, some modifications are discussed which lead to a nonanalytic shear-rate and frequency dependence as observed for the viscosity coefficients in the computer simulations.