Long term segregation dynamics of granular mixtures in a horizontal rotation drum

We study the coarsening dynamics of stripe patterns in the axial segregation of mixtures of granular materials. Experimental studies of this classical experiment are numerous; different conditions and geometries (cylindrical and rectangular cross sections of the drum, dry and wet environments) and many different types of granular mixtures have been studied in the past by various groups. Our experiments focus on a horizontally rotating cylindrical container lled to one half with a mixture of glass beads. The cylinder is lled up completely with water immersing the beads. We concentrate the investigations on bimodal distributions of glass spheres with different radii, which are initially randomly mixed. A slow rotation speed is chosen so that the particle motion is mainly restricted to a uidized layer at the top of the granular material. An immediate radial segregation of the large and small particles (within few resolutions) is followed by an axial segregation on a slower time scale (several hundred revolutions). Subsequently, a coarsening process leads to the destruction of individual stripes in the patterns and towards a complete axial demixing of the two particle species. In the experiment, we determine the long term coarsening dynamics of the stripes in dependence upon the rotation speed of the cylinder. Different regimes of the stripe dissolution can be observed in dependence upon the rotation frequency and the composition of the mixtures. In order to characterize the process of dissolution of stripes in the pattern, we study the evolution of regular preparations of stripes and employ tracer methods that allow us to compare the coarsening dynamics of the stripe pattern with the diffusion characteristics of single particles. An NMR tomography experiment is set up to provide a three-dimensional picture of segregation bands. From the NMR data, one obtains the three-dimensional distribution of the water lled cavities in the tube, which yields the positions of individual glass spheres as negative images. The existence and geometrical dimensions of an axial core formed by the smaller component inside the stripes of the larger beads is resolved. The process of stripe pattern coarsening is modelled by a simple exchange process for one of the particle species between neighbouring stripes