. The fundamental difference between the friction theory and other available theories is the fact that the viscosity of dense fluids which characterizes pure shear flow, is approached as a mechanical, rather than a traort property and it is not until the approaching of the dilute gas limit that the kinetic principles of the theory of gases become important. Thus, since this new theory is applicable to dense fluids, in most cases, it should be used together with a dilute gas model (the Chapman-Enskog theory) in order to cover the low-to high-pressure range According to the van der Waals theory of fluids, in the case of a fluid at rest, the total pressure ( ) is given by where and are the contribution to the pressure coming from the short- range repulsive and the long-range intermolecular forces respectively. In turn, the shear stress (i.e. dragging force) acting between the moving fluid layers can be written as the addition of a dilute gas collision term which becomes important at the ideal gas limit, and a residual friction term.