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. A common thread in all of our research is to develop and use the most accurate and realistic models for the interactions between molecules and to predict properties that can be compared directly with experiment, or to guide the development of new experiments.

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

In the past year, we performed the first atomistically detailed molecular dynamics simulation of micellization. We simulated the formation of water-containing reversed micelles in supercritical carbon dioxide. Remarkably, the reversed micelles form over a 1-2 nanosecond time scale.

We performed simulations of lubricants (particularly dodecane) confined to nanoscale (3-4 nm) gaps between mica-like surfaces to attempt to understand the extraordinarily large changes in the relaxation times associated with such confinement. We have made progress in demonstrating 1-2 order of magnitude increases. We continue this work in close collaboration with the experimental group of Steve Granick at the University of Illinois.

We performed molecular dynamics simulations of strontium chloride in supercritical water and made the first successful duplication of experimental EXAFS (extended X-ray absorption fine structure) measurements indicating changes in the solvation shell in this mixture.

We performed molecular dynamics simulations of "short" polyethylene (aC₁₀₀ alkane) and particularly simulated the start-up of homogeneous shear in this system. This has led to new insights into the Doi-Edwards theory for the rheology of polymers. In fact, we have shown that the Doi-Edwards reptation theory explains the behavior of this polymer well, despite the fact that the molecules in a C₁₀₀ melt are, in principle, too short to exhibit reptation dynamics.

Snapshots of simulation showing formation of reversed-micelle-like aggregates in water/surfactant/carbon dioxide mixtures over a period of 1 nanosecond. The color scheme of the various species is as follows: light blue for perfluoroalkane tail, dark blue for alkane tail, yellow for sulfur, red for oxygen, grey for sodium ion, and white for hydrogen. Carbon dioxide molecules are not shown for clarity. The number of atomic units in the simulation is 42,618.

Significance

This research will lead to better understanding of the basis for the viscous properties of lubricant, 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

S. Salaniwal, S. T. Cui, P. T. Cummings, and H. D. Cochran, "Self-assembly of reverse micelles in water/surfactant/carbon dioxide systems by molecular simulation," Langmuir 15, 5188 (1999).

S. T. Cui, P. T. Cummings, and H. D. Cochran, "Molecular dynamics simulation of the rheological properties of a model alkane fluid under confinement," J. Chem. Phys. 111, 1273 (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 (in press).