Magnetic resonance imaging measurements evidence weak dispersion in homogeneous porous media

Abstract : We measure the dispersion coefficient through homogeneous bead or sand packings at different flow rates from direct MRI visualizations of the transport characteristics of a pulse of paramagnetic nanoparticles. Through 2D imaging we observe homogeneous dispersion inside the sample, but we show that entrance effects may induce significant radial heterogeneities, which would affect the interpretation of breakthrough curve. Another MRI approach then provides quantitative measurements of the evolution in time of the longitudinal particle distribution in the sample. These data can be analyzed to deduce the coefficient of dispersion independently of entrance effects. The values obtained for this " effective " dispersion coefficient are almost ten times lower than the commonly admitted values.