Dispersivity of Bidisperse Packings of Spheres and Evidence for Distinct Random Structures

The intrinsic longitudinal and transverse dispersivity of bidisperse random packings of spheres with size ratio 5∶1 was determined by pulsed field gradient nuclear magnetic resonance, in the dilute regime where small spheres occupy between 0% and 5% of the packings' volume. Small spheres plugging pores systematically raise the mechanical transverse and longitudinal dispersivity above that of reference packings of monodisperse spheres. NMR-derived porosities, widths of velocity distributions, and dispersivities reveal distinct states of structural disorder above and below a relative sphere concentration n/N=1, where n and N are the number densities of small and large spheres.