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H. Ted Davis, Fellow

Fluid Physics for the Technologies

This research has grown from a program that dealt with enhanced oil recovery processes. Although special emphasis is placed on microstructural fluids, the scope of the project has extended into fundamental investigations of adsorption and catalysis in zeolite crystal structures. The composite of this work is directed towards process technologies potentially beneficial for United States industrial growth. These include petroleum recover, petrochemical and petroleum catalysis processes, surfactancy, drug delivery with fluid microstructures, magnetic memory, and the like.

The first part of this project deals with molecular theory simulations of simple, complex, and polymer-like fluids and microstructure and transport at interfaces and in microporous media. Recent progress on molecular theory of structure, transport and flow of fluids in micropores requires molecular dynamics (MD) simulations of flows in micropores. This group's molecular theory of Couette, Poiseuille, and squeezing flows indicate the need to rethink the concept of effective viscosity in micropores. Molecular dynamics is planned for fluids in the triply periodic porous media in an extension of this work. Further investigation is determining mechanisms of surfactancy and the formulation of association colloids.

Further research deals with application of first principles molecular dynamics (FP-MD) algorithms to the study of zeolites and related aluminosilicates. Zeolites have open crystalline structures with large empty cages and channels formed by elementary polyhedral units. Target applications are sodalite and zeolite A; however, in order to gain insight into the electronic structure and bond properties of various different sites in these materials, investigation was begun on Al2O3 systems of great importance to high-pressure research and geophysics. The work centers around first principles studies of zeolites utilizing a state-of-the-art first principles MD with VCS developed by Professor Renata Wentzcovitch of the Chemical Engineering and Materials Science Department at the University of Minnesota. It solves the electronic structure of the system at every time step of the dynamics self-consistently in order to give an accurate representation of the interatomic bonding. This version of MD goes beyond previous implementations by introducing strains in the simulations and removing essentially all structure constraints. This particular feature makes it ideally suited to investigate the behavior of unisotropic materials at a wide variety of temperatures and pressures.

These researchers are now calculating charge densities and the electronic band structure of several zeolites by direct minimization of crystal structures. The electronic charge density with zeolite pores is required in order to facilitate semi-classical MD and monte-carlo simulations of transport and adsorption properties. Further FP-MD studies, with the inclusion of simple adsorbates, quantify the acid/base characteristics of adsorption sites and provide qualitative insight into the adsorption process. These studies are important to fundamental research in catalysis and separations technology utilizing zeolites.

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The ability to model pore-scale fluid flow and species transport is critical to the study of several related research problems, including capillary electrophoresis, nuclear magnetic resonance signal attenuation, and scaling of hydrodynamic dispersion in random media. These research problems share a common interest in the simulation of flow, diffusion, and surface reactions in the pore geometry of realistic random media.

The last part of work being done in this project is on MD studies of the rate of evaporation of water from molecular clusters. A first-order phase transition from a supersaturated vapor to a liquid proceeds by nucleation, growth, and aging stages. The nucleation step (i.e., the formation of a critical cluster on which condensational growth can occur) has remained elusive for direct experimentation. A number of nucleation theories have been advanced. The original "classical" theory has remained one of the most successful to quantitatively predict the onset conditions and nucleation rates. The thermodynamic based theory, it uses macroscopic properties to predict molecular phenomena, further assumes an unstable equilibrium between clusters and the surrounding vapor equalling evaporation and condensation rates for all cluster sizes below critical. With MD simulation, these researchers are verifying the assumptions underlying the different theories. The model system consists of 30 to 100 molecule cluster of water molecules surrounded by a vapor phase and Lennard Jones (LJ) molecules in the role of carrier gas, that, analogous to the experimental conditions, act as a thermostat for latent heat of evaporation or condensation of water molecules. The Nose thermostat is employed for the LJ phase; the water sub-system is run microcanonically. This system is contained by soft repulsion walls, positioned at a distance large compared to the cluster size. The tendency of the cluster to accept or loose a molecule is then investigated as a function of number of constituebt molecules. This work shed light on the critical cluster size, indirectly an experimentally accessible quantity through the Kashchiev theorm.

This information is available in alternative formats upon request by individuals with disabilities. Please send email to alt-format@msi.umn.edu or call 612-624-0528.

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