Nonempirical Free Volume Viscosity Model for Alkane Lubricants under Severe Pressures

Viscosities η and diffusion coefficients D_{s} 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.