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Apowder is a drysolid composed of many veryfine particles that mayflow freely when shaken or tilted. Powders are a special sub-class ofgranular materials, although the termspowder andgranular are sometimes used to distinguish separate classes of material. In particular,powders refer to those granular materials that have the finergrain sizes, and that therefore have a greater tendency toform clumps when flowing.Granulars refer to the coarser granular materials that do not tend to form clumps except when wet.[citation needed]
Many manufactured goods come in powder form, such asflour,sugar, groundcoffee,powdered milk, copy machinetoner,gunpowder,cosmetic powders, and somepharmaceuticals. In nature,dust, finesand andsnow,volcanic ash, and the top layer of the lunarregolith are also examples.
Because of their importance to industry, medicine and earth science, powders have been studied in great detail bychemical engineers,mechanical engineers,chemists,physicists,geologists, and researchers in other disciplines.
Typically, a powder can be compacted or loosened into a vastly larger range ofbulk densities than can a coarser granular material. When deposited by sprinkling, a powder may be very light and fluffy. When vibrated or compressed it may become very dense and even lose its ability to flow. The bulk density of coarse sand, on the other hand, does not vary over an appreciable range.
The clumping behavior of a powder arises because of the molecularVan der Waals force that causes individual grains to cling to one another. This force is present not just in powders, but in sand and gravel, too. However, in such coarse granular materials the weight and the inertia of the individual grains are much larger than the very weak Van der Waals forces, and therefore the tiny clinging between grains does not have a dominant effect on the bulk behavior of the material. Only when the grains are very small and lightweight does the Van der Waals force become predominant, causing the material to clump like a powder. The cross-oversize between flow conditions and stick conditions can be determined by simple experimentation.[1]
Many other powder behaviors are common to all granular materials. These include segregation, stratification, jamming and unjamming,fragility, loss ofkinetic energy,frictional shearing,compaction andReynolds' dilatancy.
Powders are transported in the atmosphere differently from a coarse granular material. For one thing, tiny particles have little inertia compared to the drag force of the gas that surrounds them, and so they tend togo with the flow instead of traveling in straight lines. For this reason, powders may be an inhalation hazard. Larger particles cannot weave through the body's defenses in the nose and sinus, but will strike and stick to the mucous membranes. The body then moves the mucus out of the body to expel the particles. The smaller particles on the other hand can travel all the way to the lungs from which they cannot be expelled. Serious and sometimes fatal diseases such assilicosis are a result from working with certain powders without adequate respiratory protection.
Also, if powder particles are sufficiently small, they may becomesuspended in the atmosphere for a very long time. Random motion of the air molecules andturbulence provide upward forces that may counteract the downward force of gravity. Coarse granulars, on the other hand, are so heavy that they fall immediately back to the ground. Once disturbed, dust may form hugedust storms that cross continents and oceans before settling back to the surface. This explains why there is relatively little hazardous dust in the natural environment. Once aloft, the dust is very likely to stay aloft until it meets water in the form of rain or a body of water. Then it sticks and is washed downstream to settle asmud deposits in a quiet lake or sea. When geological changes later re-expose these deposits to the atmosphere, they may have already cemented together to becomemudstone, a type of rock. For comparison, the Moon has neither wind nor water, and so itsregolith contains dust but no mudstone.
Thecohesive forces between the particles tend to resist their becoming airborne, and the motion of wind across the surface is less likely to disturb a low-lying dust particle than a larger sand grain that protrudes higher into the wind. Mechanical agitation such as vehicle traffic, digging or passing herds of animals is more effective than a steady wind at stirring up a powder.
The aerodynamic properties of powders are often used to transport them in industrial applications.Pneumatic conveying is the transport of powders or grains through a pipe by blowing gas. A gas fluidized bed is a container filled with a powder or granular substance that isfluffed up by blowing gas upwardly through it. This is used forfluidized bed combustion, chemically reacting the gas with the powder.
Some powders may be dustier than others. The tendency of a powder to generate particles in the air under a given energy input is called "dustiness". It is an important powder property which is relevant to powder aerosolization. It also has implications for human exposure to aerosolized particles and the associated health risks (via skin contact or inhalation) in workplaces.
Many common powders made in industry are combustible; particularly metals or organic materials such asflour. Since powders have a very high surface area, they can combust with explosive force once ignited. Facilities such as flour mills can be vulnerable to such explosions without proper dust mitigation efforts.
Some metals become especially dangerous in powdered form, notablytitanium.
Apaste orgel might become a powder after it has been thoroughly dried, but is not considered a powder when it is wet because it does not flow freely. Substances like driedclay, although dry bulk solids composed of very fine particles, are not powders unless they are crushed because they have too muchcohesion between the grains, and therefore they do not flow freely like a powder. Aliquid flows differently than a powder, because a liquid cannot resist any shear stress and therefore it cannot reside at a tilted angle without flowing (that is, it has zeroangle of repose.) A powder on the other hand is a solid, not a liquid, because it may supportshear stresses and therefore may display an angle of repose.[citation needed]
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