The thermal conductivity λ, shear viscosity η, and self-diffusion coefficient D of the Lennard-Jones fluid truncated and shifted at the cut-off radius r c = 2.5 σ (LJTS fluid) are determined for a wide range of thermodynamic states (T * = [0.6, 10.0] and ρThe simulations are carried out using a non-equilibrium molecular dynamics (NEMD) method that was introduced recently and in which two gradients are applied simultaneously. It is shown that the two-gradient method is well-suited for studies of liquid and supercritical states. Data for λ, η, and D for about 350 state points are reported. Two variants of the simulation method, which differ in the accuracy and efficiency, are explored and found to yield consistent data. Correlations for λ, η, and Dρ of the LJTS fluid are provided. The data and the correlations are compared to literature data of Lennard-Jones (LJ) type fluids and good agreement is observed. The truncation of the LJ potential causes a slight increase in D, while it has no significant effect on λ and η.

A method for the simultaneous determination of the thermal conductivity λ and the shear viscosity η 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 ∂T /∂y and a velocity gradient ∂v/∂y and the resulting heat flux and momentum flux are measured to determine λ and η. The influence of the magnitude of ∂T /∂y and ∂v/∂y on λ and η is investigated. The cross-effects are negligible, even for large gradients. The same holds for the influence of ∂T /∂y on λ. However, there is a significant influence of ∂v/∂y on η, i.e. shear-thinning. The two-gradient method is applied in different ways: for small ∂T /∂y temperature-averaged values of λ and η are obtained. As ∂T /∂y has no significant influence on the results, simulations with large ∂T /∂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 λ and η, 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.

Using molecular dynamics simulation, we study the effect of a lubricant on indentation and scratching of a Fe surface. By comparing a dry reference case with two lubricated contacts -differing in the adsorption strength of the lubricant -the effects of the lubricant can be identified. We find that after an initial phase, in which the lubricant is squeezed out of the contact zone, the contact between the indenter and the substrate is essentially dry. The number of lubricant molecules confined in the tip-substrate gap increases with the lubricant adsorption energy. Trapped lubricant broadens the tip area active in the scratching process -mainly on the flanks of the groove -compared to a dry reference case. This leads to a slight increase in chip height and volume, and also contributes to the scratching forces.

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