Dynamics of size segregation and mixing of granular materials in a 3D-blender by NMR imaging investigation

Magnetic Resonance Imaging (MRI) was used to characterize the kinematics of mixing and size segregation of dry binary mixtures (diameters dmin and dmax) in a Turbula® shaker-mixer. It was found that the filling level F of the cylindrical container should not exceed 80%; otherwise, a dead zone appears in the centre of the cell. When F=66% and the two different species (dmin≠dmax) are in equivalent proportion, segregation is observed when R=dmax/dmin is greater than 1.1. Furthermore, the slower the rotation, the larger the segregation. Moreover, it is demonstrated that this blender is very efficient for dry materials within the three following conditions: (i) the rotation speed is fast enough, (ii) one tries to mix a little amount of small particles in a sea of large ones, and (iii) the concentration of the smaller particles does not exceeds 10%. Otherwise, large particles segregate quite fast towards the container walls. A segregation index S, based on density fluctuation, has been defined. When studied as a function of the number of rotations, S allows to define a characteristic time that is much shorter (1.4 rotations) for segregation than for mixing (10.7 rotations). However, it is demonstrated that this index S is not sufficient to study the real segregation mechanism and the flow pattern. It is also shown that segregation in Turbula® results from both surface and bulk segregation mechanisms. The surface effect is related to shear percolation during flow close to the free surface and it is observed whatever the rotation speed is. On the other hand, the bulk effect disappears when sample rotation is large enough since “it averages the gravity force at zero”. At last, it is proved that a low concentration system can be understood via a self-consistent approach with a single small particle in a “sea” of large ones.