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Granular Materials
A granular medium is an assembly of many solid elements, which can interact for example in collisions. But also under the effect of electromagnetic or gravitational forces...
Granular State
A granular medium is an assembly of many solid elements, which can interact for example in collisions. But also, under the effect of electromagnetic or gravitational forces. Those elements, also known under the generic term grains, are generally arranged in a disordered way and often have differences in shape, size, and surface condition.
Granular media are found at all scales: fine compacted powders composing aspirin tablets, dunes of the desert, the ballast of the railways and the rings of Saturn. The structure and properties of these environments do not depend only on the characteristics of the grains, but also on the history of the environment. Granular media exhibit a variety of behaviours that make them unclassifiable among the three usual states of matter: namely solid, liquid, and gaseous. Take the simple example of the pile of dry sand.
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It is not a solid, because if it resists compression (therefore, it is not a gas), it does not resist stretching. Yet, as long as its surface slope does not exceed a certain limit, nothing happens, and it is akin to a solid. But if the slope increases too much, avalanches occur on its surface, behaving almost like a liquid.
However, it is not a liquid, since, at rest, its surface is not horizontal. This ambivalence has made some authors say that this is a fourth state of matter, located between the solid and the liquid. These environments also have properties that are not found in any of the other three states among which we can cite the effect of arching, dilatancy and segregation.
Granular Temperature
Granular materials, whether it is sand, snow, or powdered sugar, often have strange properties that make them behave like solids, liquids, or even gases. In The New Journal of Physics, a team of four researchers (Swiss Federal Institute of Technology Lausanne – EPFL – in Switzerland) has revealed how to measure the “granular temperature” of these materials.
Take for example the solid snow cover of a ski slope. When the snow is still, it stays in a solid state. But as soon as it begins to sink like an avalanche, the snow flows is more like a liquid. For a sandstorm, the grains are lifted off the ground and behave more like gaseous molecules.
While most materials are usually listed as solids, liquids or gases, granular systems do not visibly belong to any of these categories. Plus, they are considered to have their own state of matter. Variable granular materials make it extremely difficult to conceive of a general theory valid for the observed phenomenon. The temperature of an object reflects the movement of its constituent elements.
Thus, the faster the molecules of a gas move, the higher the temperature of the gas. However, unlike liquids, the temperature of the granular material varies with the depth of the material. Being able to make this type of measurement could allow researchers to better understand the properties of the granular material. That could be used by all industries that handle particulate or powdered materials such as the pharmaceutical and construction industries.
Granular Material Nowadays
Granular material and fine powders are widely used in many industry (pneumatic conveying, dry powder inhalers, silo conveying, …). To control and to optimize processing methods, these materials must be precisely characterized. The characterization methods are related either to the properties of the grains (size, chemical composition, …). But also, to the behaviour of the bulk powder (flowability, density, electrostatic properties, …).
However, many old tests are still being used to measure the physical properties of powders. Such as the angle of repose, bulk/tapped density, flow through an aperture, …
Nowadays, it is well-known that these tests have important limitations (user dependency, bad reproducibility, …). But they are still used due to their simplicity. These old tests can be improved to meet current requirements of industries. The development of electronic and modern technologies can be used to this end. These tests were improved by GranuTools with images acquisition systems or electronic sensors to give reproducible and interpretable results. The classical repose angle measurement method has been revisited to measure powders cohesivity from the heap irregularities (GranuHeap instrument).
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A dynamic version of the repose angle is presented with a rotating drum cell (GranuDrum instrument).
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The tapped density measurement method has been automatized to precisely measure the bulk density/tapped density. But also, the powders compaction kinetic (GranuPack instrument).
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Finally, the mass flowrate through different aperture sizes is investigated electronically to study powder flowability (GranuFlow instrument).
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Moreover, GranuTools developed also new technologies. Indeed, a new instrument able to measure with great accuracy (± 0.5nC) the electrical charges acquired by powders during a flow in contact with various pipe materials was developed. The influence of powders ageing can also be investigated. This new device is known as the GranuCharge.