Segregation in Dense Sheared Systems

Most of the chapters to this point have focused on the behavior of granular materials as a single phase. While polydispersity is often introduced in simulations to prevent crystallization, the polydispersity sufficiently narrows so that the dynamics and statistics are typically still treated as a single system, without concern of the response of the individual constituents. Most granular materials in industry and nature, however, are composed of multiple constituents-particles differing in size, shape, and material properties such as density and elastic response. When subjected to shear, jostling, or other excitation, the constituents tend to unmix by particle property. This manifests in a range of striking patterns in nature, from fingering in pyroclastic flows to gravel patches in riverbeds. It also confounds issues in particle processing industries, such as in the pharmaceutical industry when powders need to be well mixed on the scale of a pill diameter. Furthermore, the behavior of granular materials depends on a representative particle size and density, so segregation can affect the local and global behavior of a granular mixture.