Granular material
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A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact (the most common example would be friction when grains collide).
Examples of granular materials would include nuts, coal, sand, rice, coffee, snow, ball bearings, and all powders. Granular materials are commercially important in applications as diverse as pharmaceutical industry, agriculture, and energy production. Research into granular materials is thus directly applicable and goes back at least to Coulomb, whose law of friction was originally stated for granular materials.
Granular materials have flow characteristics that roughly resemble those of ordinary Newtonian fluids. However, granular materials dissipate energy quickly, so techniques of statistical mechanics that assume conservation of energy are of limited use.
Bulk flow characteristics of granular materials do differ from those of homogeneous fluids in several important ways:
- Shearing or shaking a granular material may result in its becoming inhomogeneous in space and time (see Brazil nut effect).
- Granular materials tend to clog when forced through a constriction (as in a salt cellar)
- A compacted granular material must expand (or dilate) before it can deform
- Turbulence is almost impossible to achieve in granular materials
- Granular materials can support (small) shear stresses indefinitely
- Granular materials are often inhomogeneous and nonisotropic
- Granular materials exhibit avalanches.
