Granular materials segregate. Small differences in either size or density
lead to flow-induced segregation, a complex phenomenon without parallel in
fluids. Modeling of mixing and segregation processes requires the confluence of several
tools, including continuum and discrete descriptions (particle dynamics, Monte
Carlo simulations, cellular automata computations) and, often, considerable geometrical
insight. None of these viewpoints, however, is wholly satisfactory by itself.
Moreover, continuum and discrete descriptions of granular flows are regime dependent,
and this fact may require adopting different subviewpoints. This review organizes
a body of knowledge that forms—albeit imperfectly—the beginnings of an
expandable continuum framework for the description of mixing and segregation of
granular materials. We focus primarily on noncohesive particles, possibly differing in
size, density, shape, etc. We present segregation mechanisms and models for size and
density segregation and introduce chaotic advection, which appears in noncircular
tumblers. Chaotic advection interacts in nontrivial ways with segregation in granular
materials and leads to unique equilibrium structures that serve as a prototype for
systems displaying organization in the midst of disorder.