Shear viscosity of the Lennard-Jones chain fluid in its gaseous, supercritical, and liquid states

Abstract

Extensive nonequilibrium molecular dynamics (NEMD) simulations have been carried out in order to estimate the Newtonian shear viscosity of a fluid composed of short chains (up to 16 segments) of jointed spheres [Lennard-Jones chain (LJC)] over a large range of thermodynamic conditions. Using the NEMD results, it is shown that the zero-density contribution decreases with the chain length for a given temperature and is simply proportional to N-1/2 , where N is the number of spheres composing the chain. In addition, it has been noticed that the residual shear viscosity is proportional to the chain length. Then, using these results, a relation is proposed to correlate the shear viscosity of the LJC fluid using the LJ fluid (the monomer) as a reference. It is shown that this correlation is able to provide an excellent estimation of the LJC fluid viscosity compared to NEMD results for N<or=16 over the domain 0<or=rho <or=1.1 and 0.7<or=T <or=6 . Finally, it is shown that the LJC model is unambiguously more efficient than a simple LJ approximation when applied to estimate the shear viscosity of n -butane, if only the sphere or segment diameter is used as an adjustable parameter in both models.