View source for Chemistry:Powder

{{short description|Dry, bulk solid composed of fine, free-flowing particles}}
[[File:Iron powder.JPG|thumb|right|250px|Iron powder|alt=A small canister of metallic powder]]
A '''powder''' is a dry, bulk [[Physics:Solid|solid]] composed of many very fine particles that may [[Physics:Particle-laden flow|flow freely]] when shaken or tilted. Powders are a special sub-class of [[Chemistry:Granular material|granular material]]s, 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 the finer [[Earth:Grain size|grain size]]s, and that therefore have a greater tendency to [[Chemistry:Particle aggregation|form clumps]] when flowing. ''Granulars'' refer to the coarser granular materials that do not tend to form clumps except when wet.

== Types ==

Many manufactured goods come in powder form, such as [[Biology:Flour|flour]], [[Chemistry:Sugar|sugar]], ground coffee, powdered milk, copy machine [[Engineering:Toner|toner]], [[Chemistry:Gunpowder|gunpowder]], cosmetic powders, and some [[Chemistry:Pharmaceutical|pharmaceutical]]s. In nature, dust, fine sand and [[Physics:Snow|snow]], [[Earth:Volcanic ash|volcanic ash]], and the top layer of the lunar regolith are also examples.

Because of their importance to industry, medicine and earth science, powders have been studied in great detail by [[Engineering:Chemical engineer|chemical engineer]]s, [[Engineering:Mechanical engineering|mechanical engineers]], [[Chemist|chemist]]s, [[Physicist|physicist]]s, [[Geologist|geologist]]s, and researchers in other disciplines.

==Mechanical properties==

Typically, a powder can be compacted or loosened into a vastly larger range of [[Physics:Bulk density|bulk 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 molecular [[Physics:Van der Waals force|Van 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.<ref>Smalley, I.J. 1964. Flow-stick transition on powders. Nature 201, 173–174. doi:10.1038/201173a0</ref>

Many other powder behaviors are common to all granular materials. These include segregation, stratification, jamming and unjamming, [[Chemistry:Fragile matter|fragility]], loss of [[Physics:Kinetic energy|kinetic energy]], [[Physics:Friction|friction]]al shearing, [[Engineering:Soil compaction|compaction]] and Reynolds' dilatancy.

==Transport==

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 to ''go 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 mucous 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 as silicosis are a result from working with certain powders without adequate respiratory protection.

Also, if powder particles are sufficiently small, they may become [[Chemistry:Suspension|suspended]] in the atmosphere for a very long time. Random motion of the air molecules and [[Physics:Turbulence|turbulence]] 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 huge [[Earth:Dust storm|dust storm]]s 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 as [[Earth:Mud|mud]] 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 become mudstone, a type of rock. For comparison, the Moon has neither wind nor water, and so its regolith contains dust but no mudstone.

The [[Chemistry:Cohesion|cohesive]] 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. [[Engineering:Pneumatics|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 is ''fluffed up'' by blowing gas upwardly through it. This is used for [[Physics:Fluidized bed combustion|fluidized 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 "[[Physics:Dustiness|dustiness]]". It is an important powder property which is relevant to powder aerosolization process. It also has indications for human exposure to aerosolized particles and associated health risks (via skin contacts or inhalation) at workplaces. Various dustiness testing methods have been established in research laboratories, in order to predict powder behaviors during aerosolization.<ref>{{Cite journal |doi = 10.1080/02786826.2015.1114999|title = Dustiness and Deagglomeration Testing: Interlaboratory Comparison of Systems for Nanoparticle Powders|journal = Aerosol Science and Technology|volume = 49|issue = 12|pages = 1222–1231|year = 2015|last1 = Ding|first1 = Yaobo|last2 = Stahlmecke|first2 = Burkhard|last3 = Jiménez|first3 = Araceli Sánchez|last4 = Tuinman|first4 = Ilse L.|last5 = Kaminski|first5 = Heinz|last6 = Kuhlbusch|first6 = Thomas A. J.|last7 = Van Tongeren|first7 = Martie|last8 = Riediker|first8 = Michael|bibcode = 2015AerST..49.1222D|s2cid = 53998736|url = https://serval.unil.ch/resource/serval:BIB_CF268F729869.P001/REF.pdf}}</ref><ref>{{cite journal|doi=10.1093/annhyg/mem059|pmid=18056087|title=Combined Single-Drop and Rotating Drum Dustiness Test of Fine to Nanosize Powders Using a Small Drum|journal=The Annals of Occupational Hygiene|volume=52|issue=1|pages=23–34|year=2007|last1=Schneider|first1=T.|last2=Jensen|first2=K. A.|doi-access=free}}</ref><ref>{{cite journal|doi=10.1016/j.jaerosci.2015.06.001|title=A system to assess the stability of airborne nanoparticle agglomerates under aerodynamic shear|journal=[[Physics:Journal of Aerosol Science|Journal of Aerosol Science]]|volume=88|pages=98–108|year=2015|last1=Ding|first1=Yaobo|last2=Riediker|first2=Michael|bibcode=2015JAerS..88...98D|doi-access=free|url=https://serval.unil.ch/resource/serval:BIB_5FC5894C3EF2.P001/REF.pdf}}</ref><ref>{{cite journal|doi=10.1016/j.powtec.2013.05.040|title=Experimental study of the aerosolization of fine alumina particles from bulk by a vortex shaker|journal=Powder Technology|volume=246|pages=583–589|year=2013|last1=Morgeneyer|first1=Martin|last2=Le Bihan|first2=Olivier|last3=Ustache|first3=Aurélien|last4=Aguerre-Chariol|first4=Olivier}}</ref> These methods (laboratory setups) allow application of a wide range of energy inputs to powdered materials, which simulates different real-life scenarios.

==Explosion risk==
{{Main|Chemistry:Dust explosion}}
Many common powders made in industry are combustible; particularly metals or organic materials such as [[Biology:Flour|flour]]. 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, notably [[Chemistry:Titanium|titanium]].

==Comparison with other substances==

A [[Chemistry:Paste (rheology)|paste]] or [[Physics:Gel|gel]] 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 dried [[Earth:Clay|clay]], although dry bulk solids composed of very fine particles, are not powders unless they are crushed because they have too much [[Chemistry:Cohesion|cohesion]] between the grains, and therefore they do not flow freely like a powder. A [[Physics:Liquid|liquid]] 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 zero ''[[Engineering:Angle of repose|angle of repose]].'') A powder on the other hand is a solid, not a liquid, because it may support [[Physics:Shear stress|shear stress]]es and therefore may display an angle of repose.

==See also==
{{div col}}
* [[Chemistry:Fragile matter|Fragile matter]]
* [[Chemistry:Granular material|Granular material]]
* [[Physics:Liquefaction|Liquefaction]]
* [[Earth:Grain size|Grain size]]
* [[Chemistry:Paste (rheology)|Paste (rheology)]]
* [[Physics:Powder mixture|Powder mixture]]
{{div col end}}

==References==
{{Reflist}}
* 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}}.
*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}}
* [[Biography:Ralph Bagnold|Bagnold, R.A.]], ''Physics of Blown Sand and Desert Dunes'', First Springer edition, 1971, {{ISBN|0-412-10270-6}}.
* [http://www.particles.org.uk/particle_technology_book/particle_book.htm Fundamentals of Particle Technology – free book]

==External links==

{{Dosage forms|state=expanded}}

[[Category:Granularity of materials]]
[[Category:Dosage forms]]

{{Sourceattribution|Powder}}