Diffusive diffraction measurements in porous media: Effect of structural disorder and internal magnetic field gradients

Pulsed Field Gradient NMR (PFG-NMR) method used to measure the self-diffusion coefficient of liquids can also be exploited to probe the local geometry of porous media. In most practical cases, the measured diffusion attenuation is generally Gaussian and can be interpreted in terms of an apparent diffusion coefficient. Using well chosen experimental conditions, a so called “diffusive diffraction” phenomenon can be observed in the diffusion curve with a specific shape and maxima location characteristic of the system local dimensions. In this paper we investigate this phenomenon by presenting new experimental results obtained on several porous model systems of packed sphere particles. Using different experimental approaches, the diffusion pattern could be finely observed and interpreted in the context of the pore hopping model formalism. Different calibrated systems of polystyrene and glass spheres with known mean diameter and polydispersity were used to investigate specifically the influence of structural heterogeneity and local internal gradients. Structural data obtained in that way were found in close agreement with laser diffraction granulometry measurement and Scanning Emission Microscopy.