Everything about Granular Matter totally explained
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're 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, 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 don't 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's 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 and anisotropic
- Granular materials exhibit avalanches.
Further Information
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