Change in conformation of low (1400) molecular weight polyethylene under shear. The graph shows the phenomenon of stress (or shear) overshoot: when shear is first applied to a polymer, the shear viscosity h initially overshoots its steady state value. The vertical axis is the instantaneous shear viscosity divided by the steady state value, and so it asymptotes to unity in all cases. The horizontal axis is the total shear — the product of the shear rate and the time since shearing began.

Peter Cummings, University of Tennessee

Research Objectives
Our research is aimed at elucidating the molecular basis for the properties of complex materials and liquid systems, such as lubricants, self-assembling micellar systems, polymers, and high-temperature aqueous solutions.

Computational Approach
We use parallel molecular dynamics codes, developed within our group, running on the NERSC T3E. We use a variety of parallelization strategies, including domain decomposition and data parallel (or replicated data). We have developed our own visualization tool, MDVIZ, which is PVM-based and can be used for remote visualization and steering of ongoing simulations.

Accomplishments
We performed equilibrium and non-equilibrium molecular dynamics simulations of a short polyethylene melt to study the steady-state and transient rheological response of the system to the onset of shear. We computed the diffusion coefficient of the melt under shear — a first for a complex molecular fluid. We developed a novel understanding of the properties of these systems in the non-linear regime based on the measured diffusion coefficients.

We simulated the rheological properties of several lube basestock fluids, and showed that molecular simulation can be used to predict properties such as the viscosity index as well as the pressure-viscosity coefficient at GPa pressures. These properties are used to characterize lubricant performance. The ability to predict these properties via simulation is leading to the molecular design of lubricants. We used molecular simulation of the rheological properties of perfluorobutane to demonstrate that one popular correlation of experimental viscosity data is incorrect.

We performed simulations of dodecane confined to a nanoscale gap between two mica surfaces to identify when solidification of the dodecane could be expected. Our findings are the first to be in full agreement with experiment. In addition, extensive calculations were performed on the self-assembly of reversed micelles in supercritical carbon dioxide. These are the first atomistically detailed simulations to exhibit reversed micellization.

Significance
This research will lead to better understanding of the basis for the viscous properties of lubricants, leading to the design of improved lubricants in automobile engines, which will, in turn, result in better energy efficiency. We also have significant efforts under way studying the effect of nanoscale confinement on the rheology of lubricants, which has relevance to hard disk drive lubrication. Another focus of our research is aimed at finding new candidates for replacing organic solvents in chemical processes with more environmentally benign alternatives, such as supercritical carbon dioxide. Finally, we perform simulations of supercritical water and aqueous solutions which have relevance to high temperature supercritical water oxidation.

Publications
T. Driesner and P. T. Cummings, "Molecular simulation of the temperature- and density-dependence of ionic hydration in aqueous SrCl₂ solutions using rigid and flexible water models,"
J. Chem. Phys. 111, 5141 (1999).

J. D. Moore, S. T. Cui, P. T. Cummings, and H. D. Cochran, “The transient rheology of a polyethylene melt under shear,” Phys. Rev. E 60, 6956 (1999).

J. D. Moore, S. T. Cui, H. D. Cochran, and P. T. Cummings, "Molecular dynamics study of a short-chain polyethylene melt," J. Non-Newt. Fluid Mech. 93, 83 (2000).