Singlet-assisted diffusion-NMR (SAD-NMR): redefining the limits when measuring tortuosity in porous media.

Long-lived singlet order is exploited in diffusion NMR experiments to successfully measure the tortuosity of randomly packed spheres with diameters ranging from 500 to 1000 μm. The pore spaces in such packings have characteristic length scales well beyond the length scale limit set by spin relaxation in conventional NMR-diffusion experiments. Diffusion times of up to 240 s were used to obtain the restricted diffusion coefficient as a function of diffusion time in the long-time diffusion regime. Experimental results were validated with numerical simulations and data from X-ray micro-computed tomography.