Granular material

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.

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Examples of granular materials would include nuts, coal, sand, rice, coffee, corn flakes, fertilizer, and ball bearings. Powders are a special class of granular material due to their small particle size, which makes them more cohesive and more easily suspended in a gas. 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 Charles-Augustin de Coulomb, whose law of friction was originally stated for granular materials.

According to material scientist Patrick Richard, "Granular materials are ubiquitous in nature and are the second-most manipulated material in industry (the first one is water)".

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 anisotropic
• Granular materials exhibit avalanches.

References

• Duran, J., Reisinger A., Sands, Powders, and Grains: An Introduction to the Physics of Granular Materials. November 1999, Springer-Verlag New York, Inc., New York, ISBN 0-387-98656-1.
• Richard, P., Slow relaxation and compaction of granular systems. Nature Materials 4, 121–128 (2005) doi:10.1038/nmat1300
• Rodhes, M (editor),Principles of powder technology, John Wiley & Sons, 1997 ISBN 0-471-92422-9
• Fayed, M.E., Otten L. (editor), Handbook of powder science & technology, second edition, Chapman & Hall, ISBN 0-412-99621-9

See also

• Sandcastle
• Fragile matter
• Paste (rheology)
• Liquefaction