Powder

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.

In nature, dust, fine sand and snow, volcanic ash, and the top layer of the lunar regolith are also examples.

Typically, a powder can be compacted or loosened into a vastly larger range of bulk densities than can a coarser granular material.

The clumping behavior of a powder arises because of the molecular Van der Waals force that causes individual grains to cling to one another.

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.

These include segregation, stratification, jamming and unjamming, fragility, loss of kinetic energy, frictional shearing, compaction and Reynolds' dilatancy.

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.

Larger particles cannot weave through the body's defenses in the nose and sinus, but will strike and stick to the mucous membranes.

Mechanical agitation such as vehicle traffic, digging or passing herds of animals is more effective than a steady wind at stirring up a powder.

It also has implications for human exposure to aerosolized particles and the associated health risks (via skin contact or inhalation) in workplaces.