Simultaneous determination of thermal conductivity and shear viscosity using two-gradient non-equilibrium molecular dynamics simulations

摘要

A method for the simultaneous determination of the thermal conductivity lambda and the shear viscosity eta of fluids by non-equilibrium molecular dynamics simulations is presented and tested using the Lennard-Jones truncated and shifted fluid as example. The fluid is studied under the simultaneous influence of a temperature gradient partial derivative T/partial derivative y and a velocity gradient partial derivative v/partial derivative y and the resulting heat flux and momentum flux are measured to determine lambda and eta. The influence of the magnitude of partial derivative T/partial derivative y and partial derivative v/partial derivative y on lambda and eta is investigated. The cross-effects are negligible, even for large gradients. The same holds for the influence of partial derivative T/partial derivative y on lambda. However, there is a significant influence of partial derivative v/partial derivative y on eta, i.e. shear-thinning. The two-gradient method is applied in different ways: for small partial derivative T/partial derivative y temperature-averaged values of lambda and eta are obtained. As partial derivative T/partial derivative y has no significant influence on the results, simulations with large partial derivative T/partial derivative y are evaluated using the local-equilibrium assumption, such that values are obtained at different temperatures in a single simulation. In addition to the results for lambda and eta, also results for the self-diffusion coefficient D are determined from evaluating the mean squared displacement. The new two-gradient method is robust, efficient and yields accurate results.