A granular material is a conglomeration of discrete solid, macroscopic particles
characterized by a loss of energy whenever the particles interact. 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 and to asteroid belts of the solar system with
individual grains being asteroids. Some examples of granular materials are
snow, 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. The soldier/physicist Brigadier Ralph
Alger Bagnold was an early pioneer of the physics of granular matter and whose
book The Physics of Blown Sand and Desert Dunes remains an important
reference to this day. According to material scientist Patrick
Richard, "Granular materials are ubiquitous in nature and are the
second-most manipulated material in industry".
In some sense, granular materials do not constitute a single phase of matter but
have characteristics reminiscent of solids, liquids, or gases depending on
the average energy per grain. However in each of these states granular materials
also exhibit properties which are unique.
Granular materials also exhibit a wide range of pattern forming behaviors when
excited. As such granular materials under excitation can be thought of as an
example of a complex system. Granular solids
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. In general, stress in a granular solid is not distributed uniformly but
is conducted away along so-called force chains which are networks of grains
resting on one another. Between these chains are regions of low stress whose
grains are shielded for the effects of the grains above by vaulting and
arching. Granular gases
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. Unlike conventional gases, granular materials
will tend to cluster and clump due to the [dissipative] nature of the
collisions between grains. This clustering has some interesting
consequences. For example, if a partially partitioned box of granular
materials is vigorously shaken then grains will over time tend to collect in
one of the partitions rather than spread evenly into both partitions as would
happen in a conventional gas. This effect, known as the granular Maxwell's
Demon, does not violate any thermodynamics principles since energy
is constantly being lost from the system in the process.
Jamming transition Granular systems are known to exhibit
jamming and undergo a jamming transition which is thought of a thermodynamic
phase transition to a jammed state. Lubachevsky-Stillinger algorithm of
jamming allows one to produce simulated jammed granular configurations.
Pattern formation Excited granular matter is a rich
pattern-forming system. Some of the pattern-forming behaviours seen in
granular materials are: The un-mixing or segregation of unlike
grains under vibration and flow. An example of this is the so-called Brazil
nut effect where Brazil nuts rise to the top of a packet of mixed nuts when
shaken.The cause of this effect is that when shaken, granular materials move in
a circular pattern. some larger materials get stuck while going down the
circle and therefore stay on the top. The formation of surface patterns in
vibrated granular layers. These patterns include but are not limited to stripes,
squares and hexagons. These patterns are thought to be composed of fundamental
excitations of the surface known as oscillons.
The formation of sand ripples, dunes, and sandsheets
Some of the pattern-forming behaviours have been possible to reproduce in
computer simulations. There are two main computational approaches to such
simulations, time-stepped and event-driven, the former being the most
efficient for a higher density of the material and the motions of a lower
intensity, and the latter for a lower density of the material and the motions
of a higher intensity. = Acoustic effects=
Some beach sands, such as those of the aptly named Squeaky Beach, exhibit
squeaking when walked upon. Some desert dunes are known to exhibit booming
during avalanching or when their surface is otherwise disturbed. Granular
materials discharged from silos produce loud acoustic emissions in a process
known as silo honking. Granulation
Granulation is the act or process in which primary powder particles are made
to adhere to form larger, multiparticle entities called granules.
References External links
Fundamentals of Particle Technology – free book
Lu, Kevin; et al.. "Shear-weakening of the transitional regime for granular
flow". J. Fluid Mech. 587: 347–372. Bibcode:2007JFM...587..347L.
doi:10.1017/S0022112007007331. Mester, L., The new physical-mechanical
theory of granular materials. 2009, Homonnai, ISBN 978-963-8343-87-1
Pareschi, L., Russo, G., Toscani, G., Modelling and Numerics of Kinetic
Dissipative Systems, Nova Science Publishers, New York, 2006.
See also Fragile matter
Liquefaction Particulate
Paste Metal powder