Magnetic resonance relaxation in porous media interpreted by Brownstein-Tarr theory
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Date
2025-10
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University of New Brunswick
Abstract
Magnetic resonance relaxation measurements provide a powerful, nondestructive, approach to study of the pore structure and fluid dynamics in porous materials. Such measurements yield valuable information on diffusivity, fluid-solid surface interactions, and the pore size distribution. However, existing theoretical interpretations of relaxation remain incomplete and sometimes contradictory, particularly in complex porous systems. A key theoretical framework for magnetic resonance relaxation is Brownstein-Tarr theory, which, despite its wide use, is not yet fully appreciated.
This thesis enhances the understanding of Brownstein-Tarr theory by addressing its limitations and extending its applicability. Through experimental observations and newly developed methodologies, grounded in Brownstein-Tarr theory, we provide insights into previously overlooked relaxation behaviours, particularly in cases where molecular diffusion plays a significant role.
A novel method was developed for determining pore size and surface relaxivity in porous paramagnetic materials through variable temperature measurements. The temperature-dependent variation of the T2 relaxation time encodes information about pore size and surface relaxivity. Our approach, which utilizes basic magnetic resonance relaxation measurements, with a simple non-linear fitting process, yields reliable estimates of the pore size and surface relaxivity.
While the fast-exchange regime of Brownstein-Tarr theory is widely used for pore size distribution estimation, not all porous materials conform to this assumption. We present a novel method for determining the pore size distribution, specifically focusing on samples in the intermediate regime of Brownstein-Tarr theory, where diffusion effects significantly influence relaxation behaviour.
We further explore the temperature dependence of relaxation behaviour in a range of porous materials. Based on Brownstein-Tarr theory, the relaxation behaviour changes with the relaxation-diffusion regime. We explain the observed temperature-dependent relaxation behaviour in different samples based on the relaxation-diffusion regime and sample properties.
Pore size governs relaxation across all relaxation-diffusion regimes. To isolate the effect of pore size, we conducted magnetic resonance measurements on glass bead packs with a range of pore sizes but with identical glass material. The results demonstrate that both relaxation times and their temperature dependence vary systematically with pore size in paramagnetic and diamagnetic glass bead systems.