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 the finer grain sizes, and that therefore have a greater tendency to form clumps when flowing.
Granulars refers to the coarser granular materials that do not tend to form clumps except when wet.
Examples of powders would include ground coffee, powdered milk, cosmetic powders, gunpowder, powdered sugar, fine snow, household dust,salt, volcanic ash, the top layer of the lunar regolith, copy machine toner, and many pharmaceuticals.
Because of their importance to industry, medicine and earth science, powders have been studied in great detail by chemical engineers, mechanical engineers, chemists, physicists, geologists, and researchers in other disciplines.
Typically, a powder can be compacted or loosened into a vastly larger range of 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.
