Comparison of Nonequilibrium Molecular Dynamics Simulations with Experimental Measurements of Viscosity

A better understanding of the viscous properties of lubricants can lead to the design of improved lubricants in automobile engines, resulting in better energy efficiency. Nonequilibrium molecular dynamics (NEMD) is a particularly useful technique for studying rheological properties, since the key algorithm is a direct implementation of the experimental method for measuring viscosity. While NEMD has been around for 30 years, and for a decade has been used to predict viscosity in systems ranging from carbon dioxide and its mixtures to short polymers, it was not until this year that, for the first time, a fully detailed simulation was compared with experimental measurements of a real fluid.

	Figure 2   Plot of reduced viscosity versus reduced strain rate for the experimental and simulation data.

Bair et al. compared a NEMD simulation of squalane (a low-molecular-weight fluid) with experimental measurements in both the linear (Newtonian) and nonlinear (non-Newtonian) regimes—the first comparison of the nonlinear rheology predicted by NEMD with experiment. Although it is not possible with today’s computers to perform NEMD simulations on the same state conditions as the experiments, and thus the experiments and simulations cannot be directly compared, the results can be put in comparable form using the standard rheological analysis technique of temperature-time superposition. Temperature-time superposition is a theoretically and experimentally well-established technique in polymer rheology for collapsing experimental data for a given polymer at different temperatures, densities, and strain rates onto a single curve characteristic of that polymer.

When Bair et al. used this technique and placed all of the simulation and experimental results for squalane on a single plot (Figure 2), their remarkable finding was that all of the data followed the same master curve, leading to the conclusion that the behavior of squalane predicted by NEMD in the non-Newtonian region is in good agreement with experiment.

INVESTIGATORS
P. T. Cummings and C. McCabe, University of Tennessee, Knoxville, and Oak Ridge National Laboratory; S. Bair, Georgia Institute of Technology.

PUBLICATION
S. Bair, C. McCabe, and P. T. Cummings, “Comparison of nonequilibrium molecular dynamics with experimental measurements in the nonlinear shear-thinning regime,” Phys. Rev. Lett. 88, 058302 (2002).