Nonempirical free volume viscosity model for alkane lubricants under severe pressures

Viscosities i and diffusion coefficients Ds of linear and branched alkanes at pressure 0 smaller than P smaller than 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 i(P,T) a parameter-free predictor for lubrication simulations.