A grain silo can fill up with so much grain that it will engulf a human body and kill the person. If the grain has formed an arch over an outlet, the sudden collapse of the arch can cause the grain to behave like quicksand and pull the human down into the silo. There have been reported cases of people being killed while standing on top of a grain silo trying to clear a clog. This is another reason why grain is not solid.
They are not liquids either. As a form of matter, they are solids of some sort, and, as with all solids, they can be moved from one place to another by some form of engineering. And, as with all solids, there are certain properties of powders that need to be taken into account by that engineering. In the last hundred years or so, the engineering needed to move powders around has largely come to terms with the properties of the solids that it is dealing with.
The internal friction of a powder (i.e. the resistance between individual particles in contact with each other) and the cohesiveness of a powder (i.e. the resistance between individual particles that are not in contact with each other) both affect how a powder can be moved. Damp powders will tend to clump together because of their cohesiveness, whereas dry, non-adhesive powders such as dry sand do not form clumps. The shape of individual particles is a major factor affecting the flow of a powder. Very irregularly shaped particles with jagged edges (e.g. many agricultural materials) can form a solid arch and ‘bridge’ across an outlet in a hopper, whereas smooth, spherical particles (e.g. some plastics) are generally free flowing. The moisture content of a powder can also affect its flowability, as can the electrostatic charge on individual particles. Some powders can even become fluidised by air and flow like a liquid, which can be very good or very bad, depending on the application.
The work of Andrew Jenike of the Utah Engineering Experiment Station laid the foundation for the engineering of bulk solids. His 1964 report provided a basis for design of hoppers and silos of various forms. His work on the ‘angle of repose’ (the maximum angle at which a pile of a particular powder will stand before sliding) is still today referenced in hopper design. The HSE’s guidance on bulk storage bins, storage and handling of powders and bulk solids, is one such example that relies heavily upon this work to establish ‘design rules’ for bins of various forms and sizes.
Vacuum conveying for example has evolved as a method of powder conveying as a compromise to some of the major problems encountered in using other methods. The method uses negative pressure or vacuum to draw powders into a pipe and transport them to a required destination. The method of using vacuum to pull a powder through a pipe as opposed to pushing it through the pipe reduces the chance of pressure surges that can cause problems such as arching and blockages. Although the problems of moisture and size of particles still need to be addressed the major problems have been overcome.
A useful reference for Vacuum conveying is https://www.aptech.uk.com/pneumatic-conveying-systems/vacuum-conveying.
Powders are just crowds of individual particles, each behaving as they see fit under the influences of pressure, of moisture, of their neighbours.