Evaluation of the activation energy of viscous flow in the dense gas-like model

We present a method to evaluate the activation energy of viscous flow in the dense gas-like model that adopts the concepts of both the kinetic theory and the rate theory. The activation energy is assumed to be the sum of the fraction 1/3ze of the excess energy and the fraction 1/3zp of the excess pressure of a liquid. The first fraction is motivated by an early work of the rate theory, and the second one is an analogy of that. This is made under speculation that the contribution from the valence electrons to the shear viscosity may be partly ignored for liquid metals. The model involves three parameters: one corrects the diameter of a molecule, and the other two, ze and zp, in the respective fractions measure deviation from the isotropic picture for forces acting on the molecules in a real liquid. Applying the present model for the entire fluid region of Ar, it is found that the term of the excess energy determines the main feature of the shear viscosity in the liquid region, while the term of the excess pressure plays a crucial role in the description of the feature of the shear viscosity rapidly increasing in the supercritical high-density region. The results are compared with the experimental data as well as those obtained from a recent work of elaborate molecular dynamics evaluation. The present model is also applied to liquids Cu and Ag, and the values for the parameters obtained in the model are compared with those for Ar.