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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). The constituents that compose granular material must be large enough such that they are not subject to thermal motion fluctuations. Thus, the lower size limit for grains in granular material is about 1 µm. On the upper size limit, the physics of granular materials may be applied to ice floes where the individual grains are icebergs.
Examples of granular materials would include nuts, coal, sand, rice, coffee, corn flakes, fertilizer, 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.
In some sense, granular materials do not constitute a single phase of matter but 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. Depending on the average energy of the individual grains they may exhibit the properties of solids, liquids, or gases. When the average energy of the individual grains is low and the grains are fairly stationary relative to each other, the granular material acts like a solid. When the granular matter is driven and energy is fed into the system (such as by shaking) such that the grains are not in constant contact with each other, the granular material is said to fluidize and enter a liquid-like state. If the granular material is driven harder such that contacts between the grains become highly infrequent, the material enters a gaseous state. Correspondingly, one can define a granular temperature equal to the root mean square of grain velocity fluctuations that is analogous to thermodynamic temperature.
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.
References
Duran, J., Sands, Powders, and Grains: An Introduction to the Physics of Granular Materials. 2000, Springer-Verlag New York, Inc., New York.
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