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The Three Phases of Powders
Powders are unique materials consisting of three distinct phases. They comprise solids in the form of the particles, air between the particles, and often water on the surface of, or within the particle.
There is a common misconception that powder behaviour can be described by just understanding its flowability and that flowability is a discrete property that can be quantified with a single number.
Unfortunately neither of these assumptions are correct, which explains why well into the 21st century we still do not possess a fundamental understanding of powder behaviour. Consider a loosely packed powder in a glass jar and visualise its behaviour whilst the jar is tumbled. Then consider how differently it would behave if the jar was first tapped a number of times on a hard surface. Any differences in behaviour from a loosely packed state to a tapped state would be due to the characteristics of the powder. If the powder was dry sand, then it would behave in a similar way before and after tapping. However, if the powder were flour for example, it would flow very differently after it had been tapped. This is an important characteristic and one very typical of powders.
In each state illustrated above, the physical and chemical properties of the particles are the same, but the way the powder flows is very different, simply as a result of changing the air content and contact stresses between the particles. Click here to view animation.