Density Dependence of the Viscosity of Some Noble Gases

A theoretical model of the viscosity of gases at subcritical densities is presented. Up to now a suitable description of the viscosity of gases at these densities is missing, except for very rarefied gases. In the introduction a short description is given of what is already known about the viscosity of fluids, as far as needed to understand the following sections. Then a new theoretical model is proposed, based on conclusions drawn from the comparison of computer simulations and experimental data taken over a large density range. The value of the volume of close packing resulting from this comparison shows that at the critical density the distance between the molecules is nearly equal to the effective range of the intermolecular potential. Consequently, at subcritical densities the gas consists of a collection of clusters of molecules. The momentum is then transported by intracluster and intercluster transport. The theoretical model describes the gradual transition from intercluster transport to intracluster transport as a function of the density. The application of this model to some noble gas data shows that the viscosity is described within the experimental accuracy. From this application could be concluded that already at the lowest densities, clusters are usually present.