Fundamental phenomena in moving granular materials and powders

March 31, 2015

“Well into the 21st century, we still do not possess a fundamental understanding of powder behavior,” says a prominent page on the website of Freeman Technology.

“Well into the 21st century, we still do not possess a fundamental understanding of powder behavior,” says a prominent page on the website of Freeman Technology.

Yet, “Granular materials are ubiquitous in nature and are the second-most manipulated material in industry – the first is water,” says the material scientist Patrick Richard, quoted on Wikipedia.

For many educated generalists, both of the above observations might come as a surprise. But taken together they reinforce the notion that even for scientists and engineers how the world really works remains a mystery.

Of course, neither the physics of powders or granular material-and-powder applications and logistics get the kind of media attention as do the oil, gas and chemicals industries, where the processed particulars tend to be flowing liquids. 

Nevertheless, scientists, equipment and instrumentation developers, engineers and industrial managers working with granular materials and powders have to get things done day-to-day in an information technology, software and automation-dominated world, while at the same time addressing some very “elemental” challenges.

What’s the idea?

What must seem most surprising about granular materials and powders to anyone first taking notice is how they seem to exhibit something that are as much like “behaviors” as they are “properties.”

In some sense — it is said — granular materials don’t constitute a single phase of matter but have characteristics of solid, liquid or gas depending on the average energy per grain. However, in each state, granular materials exhibit unique properties.

Most notably, they exhibit pattern-forming behaviors when excited, vibrated or allowed to flow, including the segregation of unlike grains, formation of surface patterns, ripples, dunes and sandsheets.

Powders are a sub-class of granular material, with particulates having finer grain sizes and a greater tendency to clump. The clumping behavior that causes the individual grains to cling together is called the Van der Waals force.

Besides segregation, clumping and pattern forming, other behaviors said to be common to granular materials include stratification, jamming & unjamming, fragility, loss of kinetic energy, functional shearing, compaction and something called “Reynolds dilatancy.”

On-going mysteries

As Freeman Technology points out, it’s a common misconception that powder behavior can be described by just understanding its flowability, and that flowability is a discrete property that can be quantified by a single number.

This just isn’t the case. Despite comparable physical and chemical properties, loosely-packed and tightly-packed powders don’t act the same. The same is true regarding the influence of aeration and consolidation. What’s more, different materials react to the totality of these conditions in different ways.

Freeman says it’s a matter of how much stress is put on the material, with some being more sensitive than others. Processing speed, ambient moisture and time stored are other important factors.

In fact, if nothing else, when it comes to theory and practice in granular materials and powders, simply saying there “are many factors involved” is something of an understatement.

Freeman Technology is a UK-based maker of systems for measuring powder flow properties, including so-called rheometers, which use an automated shear cell and several bulk-property tests, including density, compressibility and permeability.

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