We report shear viscosity results obtained using extensive equilibrium molecular dynamics simulations. By direct numerical comparison, we show the equivalence of the Green-Kubo and Einstein approach to the calculation of viscosity in both the atomic and molecular representations. Comparing the results for two models of linear alkanes, we discuss the molecular factors determining their low-temperature liquid-state transport properties. In the mass range considered here (⩽C16), large corrections to Rouse-dynamics scaling are observed, as expected. We indicate, however, how the scaling relation between rotational-diffusion-time and shear-viscosity still provides a semi-quantitative way of estimating the latter, using simulations which are at least one order of magnitude shorter than required for direct determination of viscosity for n-alkanes.

We have extended a recently introduced united atom model of n-alkanes to investigate the liquid-state dynamics of squalane, a molecule with six, symmetrically placed, methyl side groups. We compare our results with experimental measurements of diffusion in the same system and with experimental and simulation results on n-decane and n-tetracosane. The model reproduces the significantly different temperature dependence of diffusion in squalane and n-alkanes of similar mass. The results of a detailed comparative study of the intrachain local dynamics in the different molecules makes apparent its correlation with global single-chain relaxation processes. For linear alkanes, we also make a comparison between united atom and asymmetric united atom models.

The crossover to Rouse-like behavior for the self-diffusion constant D, the viscosity η, and the equilibrium structural statistics of n-alkanes (6 ≤ n ≤ 66) is studied numerically. For small n the chains are non-Gaussian and the meanwhere a depends on the interaction model. At constant density, the Rouse model is used to extract the monomeric friction coefficient ζ and the viscosity η independently from the diffusion constant D and the longest relaxation time τ R . ζ D extracted from D is nearly independent of chain length while ζ τ obtained from τ R is much larger than ζ D for small n. The viscosity measured in a non-equilibrium molecular dynamics simulation is closely approximated by the value of η determined from τ R while η inferred from D is smaller for small n. For n > ∼ 60, the two estimates for both ζ and η agree as predicted from the Rouse model. D calculated from three interaction models is studied for increasing n and compared to experimental data.

We study the dynamics of a system with a nonconserved complex order parameter, following a deep quench. In contrast to the case when the order parameter has a discrete symmetry, we observe an effective value of the dynamical exponent at early times of /=0. 375+0.005. In this regime, finite-size scaling of the scattering function is observed. At later times, we observe a crossover to /=0. 5+0.02. Our results indicate that any renormalization of the kinetic coefficient must be small.

Nonlinearity and slip behavior of n-hexadecane in large amplitude oscillatory shear flow via nonequilibrium molecular dynamic simulation A comparison of simple rheological models and simulation data of n-hexadecane under shear and elongational flowsWe examine the shear flow of hexadecane confined between plates with separation of 1-10 nm using molecular dynamics simulations. We also performed non-equilibrium molecular dynamics ͑NEMD͒ simulations of bulk hexadecane to compare with the simulations in the confined geometry. The stiffness of hexadecane and its high melting temperature result in a tendency to crystallize at room temperature or large load. We find that when confined between hydrocarbon walls, shearing hexadecane exhibits a velocity profile with substantial slip at the wall and essentially constant velocity over most of the interior space between the walls. As the strength of the wall-fluid interaction increases the amount of slip decreases, but slip always occurs at the boundary for the range of parameters studied. The results are compared with recent surface force apparatus experiments on hexadecane and with similar simulations of model bead-spring fluids. © 1997 American Institute of Physics. ͓S0021-9606͑97͒50517-4͔