Using internal magnetic fields to obtain pore size distributions of porous media

This article reviews an NMR method, Decay due to Diffusion in Internal Field (DDIF), to obtain detailed pore size information in the fluid‐filled porous media. This method detects molecular diffusion of the fluid in the pore space subject to the magnetic field inhomogeneity produced by the susceptibility contrast between the fluid and the solid materials. We discuss the use of stimulated echoes to create and detect the magnetization modulation, different encoding regimes (weak and strong), and the analysis of the multiexponential decay using Laplace inversion. We also discuss briefly the theoretical treatment of diffusion in porous media using the eigenmode formalism. We note that the application of this technique does not use applied field gradients.