Molecular-Based Simulation of Complex Fluids
P. T. 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
One of our key accomplishments was the first molecular-simulation-based prediction of the viscosity index -- a key measure of lubricant performance -- of typical lubricant base-stocks (in this case, alkane liquids in which each molecule contains 30 carbons). We have examined the impact of molecular architecture (linear vs branched) on the viscosity index and rheology of these lubricants.
Another key accomplishment is the development of intermolecular potentials for perfluoralkane systems that can be used in our study of reversed micelles in supercritical carbon dioxide.
Significance
This research will lead to better understanding of such issues as what makes one automotive lubricant better than another. This will lead to improved lubricants in automobile engines, which will, in turn, result in better energy efficiency. Our research also is aimed at finding new candidates for replacing organic solvents in chemical processes with more environmentally benign alternatives.
Publications
J. D. Moore, S. T. Cui, P. T. Cummings, and H. D. Cochran, "Lubricant characterization by molecular simulation," A.I.Ch.E. Journal 43, 3260-3263 (1997).
The image shows tetracosane, C24H50, confined between two walls 3.9 nm apart, undergoing a high degree of shearing (i.e., top wall moving to the right and bottom wall moving to the left) as is experienced in critical lubrication applications. The temperature and density of the tetracosane are at typical values for lubrication applications. The walls have butane molecules (C4H10) tethered to them.
Only the carbon atoms in each molecule are shown. The tethered butanes are colored in red, with the attached carbon shown in pink. The tetracosane molecules are in various colors (such as yellow, blue, green, and purple) so that they can be easily distinguished. All end groups, however, are colored black.
Note that the molecules have formed an ordered structure, with molecules lying in layers parallel and perpendicular to the plane of the image. The ordered nature of the tetracosane in this situation gives rise to a very large effective viscosity, in agreement with experiments on alkane liquids confined to nanoscale gaps.
S. T. Cui, P. T. Cummings, H. D. Cochran, J. D. Moore, and S. A. Gupta, "Nonequilibrium molecular dynamics simulation of rheology of linear and branched alkanes," Int. J. Thermophys. 19, 449-459 (1998).
S. T. Cui, H. D. Cochran, and P. T. Cummings, "Vapor-liquid phase coexistence of alkane-CO2 and perfluoroalkane-CO2 mixtures," J. Phys. Chem. (submitted, 1998).