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Do you know how powders clump together due to weak molecular forces?
A powder is a dry solid composed of many very fine particles that flows freely when shaken or tilted. Powders are considered a special subcategory of granular materials, although the terms "powder" and "granular" are sometimes used to distinguish between the different categories of materials. Powders refer specifically to materials that have fine particles and are therefore more likely to form clumps when flowing, while granules generally refer to materials that have coarser particles and generally do not form clumps except when wet.
Many manufactured goods exist in powder form, such as flour, sugar, ground coffee, powdered milk, photocopier toner, gunpowder, cosmetic powders, and some medicines.
In nature, dust, fine sand and snow, volcanic ash, and the uppermost layer of soil on the moon's surface are examples of powders. Because of their importance in industry, medicine, and earth sciences, powders are studied in detail by chemical engineers, mechanical engineers, chemists, physicists, geologists, and researchers in other fields.
Mechanical properties of powders
Powders generally have a much greater overall density in either the compacted or loose range than coarse particle materials. When sprinkled, a powder may appear very light and fluffy; after vibration or compression, it may become very dense and even lose the ability to flow. In contrast, the overall density of coarse sand does not change significantly. The agglomeration behavior of powders originates from molecular van der Waals forces, which make individual particles adhere to each other. This force exists not only in powder, but also in sand and gravel. However, in these coarse-grained materials, the weight and inertia of the individual particles are much greater than the very weak van der Waals forces, so the tiny adhesions between particles do not have a dominant effect on the overall behavior of the material. Only when the particles are very small and lightweight do van der Waals forces dominate, causing the material to agglomerate like a powder.
The boundary across the flow condition and the viscous condition can be determined by simple experiments.
Many other powder behaviors are common characteristics of all granular materials, including classification, stratification, blocking and unblocking, brittleness, kinetic energy loss, frictional shear, compaction, and Reynolds expansion.
Transport of powders
Powders are transported through the atmosphere differently than coarse particulate materials. First, tiny particles have little inertia to contend with from the drag of the surrounding gas, so they tend to move with the flow rather than travel in a straight line. Therefore, the powder may pose an inhalation hazard. Larger particles are unable to penetrate the body's defenses in the nasal and sinus cavities, but will strike and adhere to the mucous membranes. The body then clears the particles by passing mucus out of the body. Smaller particles, on the other hand, can reach the lungs directly and cannot be expelled.
Serious and sometimes fatal diseases such as silicosis can result from exposure to certain powders without adequate respiratory protection.
Also, if the powder particles are small enough, they may remain suspended in the atmosphere for considerable periods of time. The random motion of air molecules and turbulence provide an upward force that may counteract the downward effect of gravity. In contrast, coarse particles are too heavy and fall immediately back to the ground. Once disturbed, the dust can form large dust storms that sweep across continents and oceans before returning to the surface. This explains why there is almost no harmful dust in the natural environment.
Explosion risk
Many common powders manufactured in industry are combustible, particularly metallic or organic materials such as flour. Because powder has such a large surface area, once ignited, it can burn with explosive force. Therefore, facilities like flour mills that do not have proper dust protection measures in place may face vulnerability to such explosions.
Some metals are particularly dangerous in powder form, especially titanium.
Compared to other substances, powders have unique properties. A dry paste or gel may become a powder when thoroughly dried, but it is not considered a powder when wet because it does not flow freely. Materials like dry clay, although dry solids composed of very fine particles, are not considered powders unless they are crushed because the cohesive forces between the particles are too strong to flow freely like a powder. Liquids flow differently than powders because they cannot resist any shear stress and therefore cannot remain stable at an inclined angle (i.e., their angle of rest is zero). A powder, on the other hand, is a solid because it can support shear stress and therefore exhibit an angle of repose.
The behavior of powders is an important area of research in particulate materials, and its understanding is crucial both from an industrial technology and health risk perspective. In this scientific field, what we need to think about more is how to safely utilize the properties of these tiny particles to benefit human life and work?
