Magnetic resonance relaxation in porous media

Porous media represent a diverse class of materials in which microstructural features govern key physical, chemical, and biological properties. Magnetic resonance relaxation measurements provide a powerful, nondestructive, approach to study of the pore structure and fluid dynamics in porous media. Such measurements yield valuable information on diffusivity, fluid-solid surface interactions, pore size distributions, and surface relaxivity. However, existing theoretical interpretations of relaxation remain incomplete and sometimes contradictory, particularly in complex porous systems. Brownstein-Tarr (BT) theory is a key framework for understanding magnetic resonance relaxation in porous media. Despite its wide use, the theory is not fully appreciated. Often, fast exchange behaviour is assumed, although it is only one regime of behaviour in Brownstein-Tarr theory. This review provides a brief outline of Brownstein-Tarr theory and its applications in porous media. Interpretation of magnetic resonance (MR) measurements with Brownstein-Tarr theory relies on identifying the specific relaxation-diffusion regimes. Diffusivity plays different roles across distinct regimes, particularly in response to temperature change. However, the influence of diffusivity on observed relaxation lifetimes has rarely been considered. More recent studies that emphasize the effect of diffusivity on relaxation have advanced our understanding of Brownstein-Tarr theory and relaxation behaviour in porous media.