Mesoporous materials are being widely used in the chemical industry in various environmentally friendly separation processes and as catalysts. Our research can be broadly described as an effort to understand the behavior of fluids confined in such materials. More specifically we try to understand the influence of state variables like temperature and pore variables like size, shape, connectivity and structural heterogeneity on both the dynamic and equilibrium behavior of confined fluids. The dynamic processes associated with the approach to equilibrium are largely unexplored. It is important to look into the dynamic behavior for two reasons. First, confined fluids experience enhanced metastabilities and large equilibration times in certain classes of mesoporous materials, and the approach to the metastable/stable equilibrium is of tremendous interest. Secondly, understanding the transport resistances in a microscopic scale will help better engineer heterogeneous catalysts and separation processes. Here we present some of our preliminary studies on dynamics of fluids in ideal pore geometries. The tool that we have used extensively to investigate the relaxation dynamics of fluids in pores is the dynamic mean field theory (DMFT) as developed by Monson[P. A. Monson, J. Chem. Phys., 128, 084701 (2008) ]. The theory is based on a lattice gas model of the system and can be viewed as a highly computationally efficient approximation to the dynamics averaged over an ensemble of Kawasaki dynamics Monte Carlo trajectories of the system. It provides a theory of the dynamics of the system consistent with the thermodynamics in mean field theory. The nucleation mechanisms associated with confined fluid phase transitions are emergent features in the calculations. We begin by describing the details of the theory and then present several applications of DMFT. First we present applications to three model pore networks (a) a network of slit pores with a single pore width; (b) a network of slit pores with two pore widths arranged in intersecting channels with a single pore width in each channel; (c) a network of slit pores with two pore widths forming an array of ink-bottles. The results illustrate the effects of pore connectivity upon the dynamics of vapor liquid phase transformations as well as on the mass transfer resistances to equilibration. We then present an application to a case where the solid-fluid interactions lead to partial wetting on a planar surface. The pore filling process in such systems features an asymmetric density distribution where a liquid droplet appears on one of the walls. We also present studies on systems where there is partial drying or drying associated with weakly attractive or repulsive interactions between the fluid and the pore walls. We describe the symmetries exhibited by the lattice model between pore filling for wetting states and pore emptying for drying states, for both the thermodynamics and dynamics. We then present an extension of DMFT to mixtures and present some examples that illustrate the utility of the approach. Finally we present an assessment the accuracy of the DMFT through comparisons with a higher order approximation based on the path probability method as well as Kawasaki dynamics.

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This dissertation focuses on the fundamental understanding of liquid dynamics confined in polymer membranes. Such knowledge guides the development of better polymer membranes for practical applications and contributes to the general understanding of confined liquid dynamics in various nanoporous materials. First, we investigate the membrane transport by experimental measurements on a PFSA membrane and computer modeling of the confined liquid molecules. We probe the nano-scale environment in the ionomer membrane by determining the activation energy of diffusion. We notice two structural features of the PFSA membrane that dominate membrane transport. At relatively high hydrations, the nano-scale phase-separation creates bulk-like water in the ionomer membrane and prompts fast transport of mobile species. At relatively low hydrations, the nanoconfinement of the polymer matrix leads to the ordering of confined water and prompts a high energy barrier for transport. We then delve deeper into the confinement effect by molecular modeling of various nanoconfining geometries, including carbon nanotubes, parallel graphene sheets, and parallel rigid rods. We notice retarded water dynamics under hydrophobic confinement regardless of the geometry. We further investigate the confined water by determining the residence time of water around water, which evaluates the timescale of associations between water molecules. We learn that a decreasing confinement size prompts longer associations among water molecules. Further, we propose that the prolonged associations are responsible for the retarded water dynamics under hydrophobic confinement. Next, we turn our attention to the effect of interactions between mobile species (mostly water molecules) and a confining surface. In ionomer membranes, interactions between mobile species and the ionic groups dominate the water-surface interactions. We start by looking at water-ion interactions in bulk solutions. Using solutions at varying concentrations, we notice a temperature-concentration superposition behavior from diffusion coefficients of water molecules and ions in the solutions in both experimental and computational results. Observation of this superposition behavior in bulk solutions is unprecedented. The temperature-concentration superposition parallels the well-known time-temperature superposition. We are able to extract the offset of reciprocal temperature, which fits well to a Williams-Landel-Ferry type equation. The temperature-concentration superposition points to the new perspective that the effect of ions on water dynamics can be similar to the effect of lowering temperature. We further investigate the effect of ions by modeling ions/charges onto confining geometries. Remarkably, we reveal that the presence of ions can break the ordered water structure induced by confinement. The hydrophobic confinement prompts the ordering of water molecules, which leads to slower diffusion and higher activation energy. The presence of ions/charges on the confining surface has multiple effects on the dynamics of confined water. First, the ions associate strongly with neighboring water molecules while breaking the hydrogen-bonding network between water molecules. Second, the disruption of the hydrogen-bonding network leads to decreased activation energy of diffusion and enhanced water mobility. At relatively high ion density, the water-ion interactions overcome the structure-breaking effect and lead to retarded water diffusion. Overall, the studies presented in this dissertation augment our understanding of water transport in nanostructures by revealing the rich behavior of liquid-water dynamics under both hydrophobic and ionic confinement. / Doctor of Philosophy / Polymer separations membranes contribute to important applications such as fuel cells and water desalination. Optimizing the separation ability of polymer membranes improves their practical performance. The transport property of a polymer membrane depends on its nanoscale and microscale structures. This dissertation focuses on the nanoscale structure-transport relations in ionic polymer membranes. We utilize nuclear magnetic resonance techniques and molecular dynamics simulations to probe the transport properties. We investigate the effects of membrane nanostructure and water-ion interactions on the dynamics of confined water. Such knowledge not only guides the development of high-performance membranes but also contributes to the fundamental understanding of liquid dynamics in nanoporous materials.

The use of induction fans for face ventilation in room and pillar mines has proved to be an efficient, flexible, and viable technique. In addition to their merits over conventional systems, induction fans enable remote controlled mine operations with low maintenance requirements. Theoretical investigations were conducted initially to verify the potential of free air jets in mine ventilation. A laboratory model using water as the fluid medium was designed to study the flow characteristics of a jet fan in a blind entry. The model was tested in a variety of brattice curtain and nozzle combinations to investigate the ventilating efficiency of jet fans. A jet fan was selected and tested in a full scale model and in a coal mine. Experiments were conducted to evaluate the laboratory flow models. Flow quantities and velocities in the entry were measured using state-of-the-art instrumentation to quantify various parameters. Air velocities near the face were found to be satisfactory to dilute contaminants from the face. A model for the axial velocity profile of the jet was suggested. Beyond 25m distance from the jet fan exit the jet tended to move away from the wall to the opposite wall. Carbon dioxide was used as a tracer gas to measure the effective ventilating air quantity near the face and re-circulation in various tests. The re-circulation involved in the system was found to be less than 40% in all the experiments. It was also found that the use of line curtains in combination with a jet a fan can eliminate any type of re-circulation. Numerical modeling of a jet fan in a typical coal mine heading was conducted to obtain details of the flow. The results of the simulation using computational fluid dynamics were similar to the flow patterns observed in the experiments. It was found that a jet fan can effectively ventilate an entry as deep as 40m. Fan positioning, airway geometry, airway surface properties, and mine layout severely affect its performance. It is therefore necessary to understand the flow mechanics of a jet fan in a mine heading before applying the technique for a particular situation. / Ph. D.