Granular materials occur in many natural and industrial settings including avalanches and the pharmaceutical and agricultural industries. Any macroscopic collection of particles of sufficient size can be considered a granular material. While there is no strict lower limit on the size, the particles must be large enough so that they are not subject to thermal motion. Systems where this is clearly the case include grains of rice and sand, nuts, lentils, coal and ball-bearings. These should be contrasted with fluids, such as water, that are composed of much smaller particles - molecules - that are constantly moving. Another important distinction between fluids and granular systems is that in the latter, the particle-particle interactions are dissipative: when two grains of sand collide some of the kinetic energy is converted into heat. Thus if sand is poured onto a flat surface, for example, all motion rapidly ceases and the material remains in the form of a heap. If some source of energy is present, however, the granular system will display a dynamic behavior and may resemble a solid or a fluid depending on the rate of energy input.
Some examples of phenomena that occur in granular systems are:
One way to supply energy to a granular system is to shear it. This
video shows a model granular fluid
composed of inelastic dumbbells under the influence of boundary-driven
shear. We are interested in how the grains align with the flow, as this
strongly influences the properties of the system.