A granular material is a conglomeration of discrete solid Solid is one of the major states of matter. It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does. The atoms in a solid are tightly bound to each other,, macroscopic particles characterized by a loss of energy whenever the particles interact (the most common example would be friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, or material elements sliding against each other. It may be thought of as the opposite of "slipperiness" when grains collide)^[1]. 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 A micrometre is one millionth of a metre, or equivalently one thousandth of a millimetre or one thousand nanometres. It can also be written in scientific notation as 1×10−6 m, meaning 1⁄1000000 m. On the upper size limit, the physics of granular materials may be applied to ice floes where the individual grains are icebergs An iceberg is a large piece of ice from freshwater that has broken off from a snow-formed glacier or ice shelf and is floating in open water. It may subsequently become frozen into pack ice. Alternatively, it may come to rest on the seabed in shallower water, causing ice scour or becoming an ice island and to asteroid belts The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt region is also termed the main belt to distinguish it from other concentrations of minor planets within the Solar System, of the solar system The Solar System[a] consists of the Sun and those celestial objects bound to it by gravity, all of which were formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. Of the many objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets[e] whose orbits are almost circular and with individual grains being asteroids Asteroids, sometimes called minor planets or planetoids, are small Solar System bodies in orbit around the Sun, especially in the inner Solar System; they are smaller than planets but larger than meteoroids. The term "asteroid" has historically been applied primarily to minor planets of the inner Solar System, as the outer Solar System.

Some examples of granular materials are nuts Nut is a hard shelled fruit of some plants that has an indehiscent seed. While a wide variety of dried seeds and fruits are called nuts in English, only a certain number of them are considered by biologists to be true nuts. Nuts are an important source of nutrients for both humans and wildlife, coal Coal is a readily combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds. The harder forms, such as anthracite coal, can be regarded as metamorphic rock because of later exposure to elevated temperature and pressure. Coal is composed primarily of carbon along with variable, sand Sand is a naturally occurring granular material composed of finely divided rock and mineral particles. The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal settings is silica , usually in the form of quartz, rice Rice is the seed of the monocot plant Oryza sativa. As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East and South Asia, the Middle East, Latin America, and the West Indies. It is the grain with the second-highest worldwide production, after maize, coffee Coffee is a brewed drink prepared from roasted seeds, commonly called coffee beans, of the coffee plant. They are seeds of coffee cherries that grow on trees in over 70 countries. Green unroasted coffee is one of the most traded agricultural commodities in the world. Due to its caffeine content, coffee can have a stimulating effect in humans, corn flakes Corn flakes are a popular breakfast cereal originally manufactured by Kellogg's through the treatment of corn . A patent for the product was filed on May 31, 1895, and issued on April 14, 1896, under the name Granose, fertilizer Fertilizers are soil amendments applied to promote plant growth; the main nutrients present in fertilizer are nitrogen, phosphorus, and potassium and other nutrients ('micronutrients') are added in smaller amounts. Fertilizers are usually directly applied to soil, and also sprayed on leaves ('foliar feeding'), and ball bearings A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the moving parts of the bearing. Powders A powder is a dry, bulk solid composed of a large number of very fine particles that may flow freely when shaken or tilted. Powders are a special sub-class of granular materials, although the terms powder and granular are sometimes used to distinguish separate classes of material. In particular, powders refer to those granular materials that have are a special class of granular material due to their small particle size, which makes them more cohesive Cohesion or cohesive attraction or cohesive force is a physical property of a substance, caused by the intermolecular attraction between like-molecules within a body or substance that acts to unite them and more easily suspended In chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase (fluid) through mechanical agitation, with the use of certain excipients or suspending agents. Unlike colloids, in a gas Gas is one of four classical states of matter. Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons are so energized that they leave their parent atoms. Granular materials are commercially important in applications as diverse as pharmaceutical A pharmaceutical drug, also referred to as medicine, medication or medicament, can be loosely defined as any chemical substance intended for use in the medical diagnosis, cure, treatment, or prevention of disease industry, agriculture Agriculture is the production of food and goods through farming. Agriculture was the key development that led to the rise of human civilization, with the husbandry of domesticated animals and plants creating food surpluses that enabled the development of more densely populated and stratified societies. The study of agriculture is known as, and energy production. Research into granular materials is thus directly applicable and goes back at least to Charles-Augustin de Coulomb Charles-Augustin de Coulomb was a French physicist. He is best known for developing Coulomb's law, the definition of the electrostatic force of attraction and repulsion. The SI unit of charge, the coulomb, was named after him, whose law of friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, or material elements sliding against each other. It may be thought of as the opposite of "slipperiness" was originally stated for granular materials^[2].

The soldier A soldier is a member of the land component of national armed forces; whereas a soldier hired for service in a foreign army would be termed a mercenary. In most languages, "soldier" includes commissioned and non-commissioned officers in national land forces/physicist A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole (cosmology). One of the world's best known physicists is Albert 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"^[3] remains an important reference to this day.

According to material scientist Materials science is an interdisciplinary field involving the properties of matter and its applications to various areas of science and engineering. This science investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It includes elements of applied physics and chemistry Patrick Richard, "Granular materials are ubiquitous in nature Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general. It ranges in scale from the subatomic to the cosmic and are the second-most manipulated material in industry (the first one is water Water is a chemical substance with the chemical formula H2O. Its molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state, water vapor or steam)"^[4].

In some sense, granular materials do not constitute a single phase of matter In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition. A simple description is that a phase is a region of material that is chemically uniform, physically distinct, and ( but have characteristics reminiscent of solids Solid is one of the major states of matter. It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does. The atoms in a solid are tightly bound to each other,, liquids Liquid is one of the three classical states of matter. Like a gas, a liquid is able to flow and take the shape of a container, but, like a solid, it resists compression. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly constant density. A distinctive property of the liquid state is surface tension,, or gases Gas is one of four classical states of matter. Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons are so energized that they leave their parent atoms 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 behaviours when excited (e.g. vibrated or allowed to flow). As such granular materials under excitation can be thought of as an example fo a complex system A complex system is a system composed of interconnected parts that as a whole exhibit one or more properties not obvious from the properties of the individual parts.

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 A Vault is an architectural term for an arched form used to provide a space with a ceiling or roof. The parts of a vault exert a thrust that require a counter resistance. When vaults are built underground, the ground gives all the resistance required. However, when the vault is built above ground, various replacements are employed to supply the and arching An arch is a structure that spans a space while supporting weight . Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.

Granular Liquids

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. When freely flowing, granular materials have flow characteristics that roughly resemble those of ordinary Newtonian fluids A Newtonian fluid is a fluid whose stress versus strain rate curve is linear and passes through the origin. The constant of proportionality is known as the viscosity. However, granular materials dissipate energy quickly, so techniques of statistical mechanics Statistical mechanics is the application of probability theory (which contains mathematical tools for dealing with large populations) to study the thermodynamic behavior of systems of a large number of particles. It provides a framework for relating the microscopic properties of individual atoms and molecules to the macroscopic or bulk properties that assume conservation of energy are of limited use. Bulk flow characteristics of granular materials do differ from those of homogeneous fluids In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of liquids in motion). Fluid dynamics has a wide range of applications, in several important ways^[5]:

• Shearing or shaking a granular material may result in its becoming inhomogeneous in space and time (see Brazil nut effect The Brazil nut effect is the name given to a phenomenon in which the largest particles end up on the surface when a granular material containing a mixture of objects of different sizes is shaken).
• Granular materials tend to clog when forced through a constriction (as in a salt cellar Salt is a dietary mineral composed primarily of sodium chloride that is essential for animal life, but can be toxic to many land plants. Salt flavor is one of the basic tastes, making salt one of the oldest, most ubiquitous food seasonings. Salting is an important method of food preservation)
• A compacted granular material must expand (or dilate) before it can deform (A process known as dilatency).
• Turbulence In fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time. Nobel Laureate Richard Feynman describes turbulence as "the most important unsolved problem of is almost impossible to achieve in granular materials
• Granular materials can support (small) shear stresses indefinitely
• Granular materials are often inhomogeneous and anisotropic Isotropy is uniformity in all directions, it is derived from the Greek iso and tropos (direction). Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix an, hence anisotropy. Anisotropy is also used to describe situations where properties vary systematically, dependent on direction
• Granular materials exhibit avalanches An avalanche is a rapid flow of snow down a slope, from either natural triggers or human activity. Typically occurring in mountainous terrain, an avalanche can mix air and water with the descending snow. Powerful avalanches have the capability to entrain ice, rocks, trees, and other material on the slope. Avalanches are primarily composed of^[6]. Avalanching sand is a paradigm example of the process of Self-organized criticality In physics, self-organized criticality is a property of (classes of) dynamical systems which have a critical point as an attractor. Their macroscopic behaviour thus displays the spatial and/or temporal scale-invariance characteristic of the critical point of a phase transition, but without the need to tune control parameters to precise values.

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 Maxwell's demon is a thought experiment formulated by the Scottish physicist James Clerk Maxwell intended to "show that the 2nd Law of Thermodynamics has only a statistical certainty," and is used as a tool to present the possibility of its violation. The concept first appeared in a letter Maxwell wrote to Peter Guthrie Tait on 11 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^[7].

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 The Brazil nut effect is the name given to a phenomenon in which the largest particles end up on the surface when a granular material containing a mixture of objects of different sizes is shaken 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 (and some other) materials move in a circular pattern. some larger materials (Brazil nuts) 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 exitations of the surface known as oscillons.
• The formation of sand ripples, dunes In physical geography, a dune is a hill of sand built by aeolian processes. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind. The valley or trough between dunes is, and sandsheets

Acoustic effects

Some beach sands such as those of the aptly named squeaky beach A tidal river is a river, or more typically a stretch of a river, whose flow and level is influenced by tides. An example of a tidal river is the portion of the Connecticut River flowing from Windsor Locks, Connecticut, to the Atlantic Ocean exhibit squeaking when walked upon. Some dessert dunes In physical geography, a dune is a hill of sand built by aeolian processes. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind. The valley or trough between dunes is 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

1. ^ Duran, J., Sands, Powders, and Grains: An Introduction to the Physics of Granular Materials (translated by A. Reisinger). November 1999, Springer-Verlag New York, Inc., New York, ISBN 0-387-98656-1.
2. ^ Rodhes, M (editor),Principles of powder technology, John Wiley & Sons, 1997 ISBN 0-471-92422-9
3. ^ Bagnold, R.A. 1941. The physics of blown sand and desert dunes. London: Methuen,
4. ^ Richard, P., Slow relaxation and compaction of granular systems. Nature Materials 4, 121–128 (2005) doi:10.1038/nmat1300
5. ^ Fayed, M.E., Otten L. (editor), Handbook of powder science & technology, second edition, Chapman & Hall, ISBN 0-412-99621-9
6. ^ Pudasaini, Shiva P., Hutter, Kolumban, Avalanche Dynamics: Dynamics of Rapid Flows of Dense Granular Avalanches. Springer, Berlin, New York, 2007, ISBN 3-540-32686-3
7. ^ Haye Hinrichsen, Dietrich E. Wolf (eds), The Physics of Granular Media. 2004, Wiley-VCH Verlag GmbH & Co. ISBN 978-3-527-60362-6

External links

• Fundamentals of Particle Technology - free book

• Lu, Kevin; et al. (November 2007). "Shear-weakening of the transitional regime for granular flow". J. Fluid Mech. 587: 347–372. doi:10.1017/S0022112007007331. http://journals.cambridge.org/production/action/cjoGetFulltext?fulltextid=1318128.

• Mester, L., The new physical-mechanical theory of granular materials. 2009, Homonnai, ISBN 96383-43-87

See also

• Fragile matter
• Liquefaction
• Particulate
• Paste (rheology)
• Sandcastle
• Singing Sands

Categories: Granular materials | Discrete-phase flow

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