Bulk viscosity of gaseous argon from molecular dynamics simulations

Bulk viscosity of simple liquids

Anomalous intrinsic properties of bulk viscosity at normal states of simple liquids were revealed by molecular dynamics simulations, compared with shear viscosity and thermal conductivity. To this end, we analyzed the partial contributions of the time correlation function in the Green-Kubo formula not only from pair and three-body interactions but also from attractive and repulsive parts. The most distinct feature of bulk viscosity from the other transport properties is sensitivity to the attractive interaction. What is even more interesting is that the effect of attractive interactions is greatly reduced by the introduction of the three-body repulsion, even though the strength of the three-body repulsion itself is very weak and has little effect directly on all the transport coefficients. The mechanism behind this is elucidated in detail through the change in the pair distribution function brought about by the introduction of the three-body interaction. We conclude that the origin of the anomalous properties is that the transport process corresponding to bulk viscosity is a scalar process that does not involve directional flow, unlike other transport processes.

Influence of repulsion on entropy scaling and density scaling of monatomic fluids

Entropy scaling is applied to the shear viscosity, self-diffusion coefficient, and thermal conductivity of simple monatomic fluids. An extensive molecular dynamics simulation series is performed to obtain these transport properties and the residual entropy of three potential model classes with variable repulsive exponents: n, 6 Mie (n = 9, 12, 15, and 18), Buckingham's exponential-six (α = 12, 14, 18, and 30), and Tang-Toennies (αT = 4.051, 4.275, and 4.600). A wide range of liquid and supercritical gas- and liquid-like states is covered with a total of 1120 state points. Comparisons to equations of state, literature data, and transport property correlations are made. Although the absolute transport property values within a given potential model class may strongly depend on the repulsive exponent, it is found that the repulsive steepness plays a negligible role when entropy scaling is applied. Hence, the plus-scaled transport properties of n, 6 Mie, exponential-six, and Tang-Toennies fluids lie basically on one master curve, which closely corresponds with entropy scaling correlations for the Lennard-Jones fluid. This trend is confirmed by literature data of n, 6 Mie, and exponential-six fluids. Furthermore, entropy scaling holds for state points where the Pearson correlation coefficient R is well below 0.9. The condition R > 0.9 for strongly correlating liquids is thus not necessary for the successful application of entropy scaling, pointing out that isomorph theory may be a part of a more general framework that is behind the success of entropy scaling. Density scaling reveals a strong influence of the repulsive exponent on this particular approach.