Non-equilibrium molecular dynamics simulations of structured molecules

Abstract

Corresponding-states theories fail to predict the large difference observed between n-butane and isobutane viscosities at similar reduced conditions. To investigate the molecular cause of these structural effects upon viscosity, nonequilibrium molecular dynamics simulations of Lennard-Jones site-site models representing n-butane and isobutane are performed over much of the density range for which experimental data are available. Simulated viscosities at zero shear agree very well with experimental data over the entire density range. Site size, non-equilibrium molecular alignment and molecular geometry are the primary factors causing both the similarities and differences between the isomers' viscosity and rheology.