Nonempirical free volume viscosity model for alkane lubricants under severe pressures

Viscosities 𝜂 and diffusion coefficients 𝐷𝑠 of linear and branched alkanes at pressure 0 <𝑃 <0.7  GPa and temperature 𝑇=500-600  K are calculated from molecular dynamics simulations. Combining Stokes-Einstein, free volume, and random walk concepts results in an accurate viscosity model for the considered 𝑃 and 𝑇. All model parameters (hydrodynamic radius, random walk step size, and step frequency) are extracted from equilibrium molecular dynamics via microscopic ensemble averages rendering 𝜂⁡(𝑃,𝑇) a parameter-free predictor for lubrication simulations.