Granular Segregation Studies at New Mexico Resonance

MRI can non-invasively measure flow in opaque materials as well as being able to measure dynamic variables (velocity, diffusion). Due to these advantages MRI is an invaluable tool for studing the internal structure of granular matter, granular flow, and the segregation of granular materials.

Density Segregation Studies

We find that for smooth spherical particles of the same size with different densities radial segregation developes quickly for density ratios up to 4.7. This has been observed by a number of other researchers . However we find no evidence of surface axial (banding) segregation in any system tested. We conclude, therefore, that density differences may effect the axial segregation phenomenon but cannot be its sole cause.

Axial Banding by Size Alone

The causes of axial banding segregation are still unknow. Using MRI we have found that one factor that differs between a banding and nonbanding biparticulate system is the velocity depth profile along the cylinder axis (right). Particles experience a different shear rate dependant on particle size ratio and particle composition within a slice along the axis. Furthermore, particles differeing in size alone such as 1mm and 3mm glass beads do form a banded state (left).

Subsurface Radial Segregation Studies Using MRI

Radial segregation of the core of small particles was imaged using MRI. Only the smaller core particles are visible in these images. The core behaves much the same as the overall flow with its own flowing layer depth, and a surface which follows closely in shape the free surface. In addition, the core becomes more spread out as the rotation rate is increased. This dispersion, which allows the smaller particles greater freedom of movement may be one reason that a higher rotation rate encourages axial banding to develop.

1D MRI Studies of Band Formation

When an initially well-mixed system is rotated in the horizontal drum mixer the growth of bands can be seen using 1D MRI imaging - this type of imaging allows images to be taken rapidly, at 1Hz. Only the smaller particles are visible in the images (left). Furthermore the percentage of small particles in the system influences whether or not a surface band can be seen. When the percentage of small particles is reduced to less than 35% no surface band is seen. However in all cases, down to 10% small particles, a bulge of small particles forms at the center of the cylinder which can be seen in the MRI images (right).

The first Images of the Full 3D Subsurface Structure of Axial Segregation Were Acquired at New Mexico Resonance!

These images cast doubt on the idea that the dynamic angle of repose is responsible for the axial segregation phenomenon. The radial core first forms undulations which merge and grow sometimes emerging at the surface but not always. This indicates that a subsurface mechanism is at least partially responsible for banding segregation.

Sanfratello, L. and Fukushima, E. Experimental Studies of Density Segregation in the 3D Rotating Cylinder and the Absence of Banding (2009) Granular Matter 11(2), pp. 73-78.

Sanfratello, L. NMR studies of flowing and segregating granular materials. PhD Thesis, University of New Mexico (2007).

Hill, K.M., Caprihan, A., Kakalios, J. Axial segregation of granular media rotated in a drum mixer: Pattern evolution (1997) Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, 56 (4), pp. 4386-4393.

Hill, K.M., Caprihan, A., Kakalios, J. Bulk segregation in rotated granular material measured by magnetic resonance imaging (1997) Physical Review Letters, 78 (1), pp. 50-53.

Nakagawa, M., Altobelli, S.A., Caprihan, A., Fukushima, E. NMRI study: Axial migration of radially segregated core of granular mixtures in a horizontal rotating cylinder (1997) Chemical Engineering Science, 52 (23), pp. 4423-4428.

Hill, K.M., Kakalios, J., Caprihan, A. Pattern evolution of granular media rotated in a drum mixer (1997) Materials Research Society Symposium - Proceedings, 463, pp. 227-232.

Nakagawa, Masami, Waggoner, Allen R., Fukushima, Eiichi. NMRI measurements of flow of granular mixtures (1996) NASA Conference Publication, (3338), pp. 251-256.

Nakagawa, Masami, Altobelli, Stephen A., Caprihan, Arvind, Fukushima, Eiichi. Segregation phenomena in a horizontal rotating cylinder (1995) Proceedings of Engineering Mechanics, 2, pp. 1272-1275.

Nakagawa, Masami. Axial segregation of granular flows in a horizontal rotating cylinder (1994) Chemical Engineering Science, 49 (15), pp. 2540-2544.

Last Updated: 1:02 PM 03/13/2009