Model derived from hard sphere theory
Estimating the viscosity of liquids under pressure

At present, many correlations of the transport properties of dense fluids are based on the hard sphere model. One of the reasons for this success is that this theoretical model can be used to calculate transport property values for densities ranging from dilute gas to solidification. In addition, it is a physically realistic molecular model, since the Van der Waals model, which has been successfully applied to the equilibrium properties of dense fluids, is equivalent to the hard sphere model for transport properties.

According to Van der Waals, the fluid is made up of a set of molecules with a repulsive hard-core interaction energy and a weak, very long-range attractive interaction energy. This has important implications for transport properties, as the attractive interactions lead to a virtually uniform attractive energy surface throughout the fluid, and the molecules...