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Powders are used in many industries. In some cases, they are added in as a raw material, other times they can be an intermediate stage in the manufacturing of a product, and at other times they can be the final product itself. Powders are not the easiest substance to work with because their behavior can change with different manufacturing conditions, the localized environment, and when they are in different mediums. Their behavior can even change while being processed in a manufacturing line, so understanding how powders behave is critical for many industries.
The Rheology of Powders
The rheology of powders is vitally important. The rheology and flowability of a powder can affect many of its properties, from its behavior in specific mediums to how it packs as a solid. The flowability of a powder also affects many things outside of the direct properties of the powder itself, such as how the powders should be handled, transported, and stored. In many powder-based products, the rheology and flowability also showcases effects that are only specific to that one product. One such example of this how the flow of the powder affects the mixing and processing of spice blends in the food industry, which can influence the overall flavor of the product.
Understanding the rheology and flowability is not a simple task. At the particle level (either macroscopic or microscopic), the way in which the particles interact and behave, not only as individuals but as complete particle systems, often dictates the rheology and flowability of the powder in certain scenarios. At the particle level, the size, size distribution, and shape of the individual particles all play a key role in how the powder behaves, as does the surface texture, surface area, surface porosity, density and packing behavior of the particles.
Aside from the particle level interactions, while powders do flow, their flowability is remarkably different to that of liquids. In terms of a powder’s flow behavior, and unlike a liquid (or a semi-solid), it is highly dependent upon an applied consolidating pressure. In simpler terms, this means, that when the pressure is increased—for example in a silo where there are large volumes of powder—due to the weight of the powder, the stress on the material at the bottom of the silo (or other storage vessels) is increased, and this reduces the flowability of the powder. Moreover, the frictional forces between the powder and the vessel it is stored in can also affect the flowability of the powder; hence, there is a lot more parameters to think about when talking about the flow and rheology of a powder compared to a liquid.
How is Powder Flow and Rheology Tested?
One of the more common methods to analyze the rheology and flowability of powders is to use powder flow testing methods. Powder flow testing is widely used in the quality control space to determine a range of properties about a powder, and there are a number of specific tests available using powder rheometers, shear cells, and other powder cells. The tests possible on powders using these instruments/apparatuses include compression tests, wall friction tests, pressure drop tests, permeability tests, tensile strength tests, air-retention tests, and cohesion tests, to name a few of the common ones.
In terms of the information available about powders, these tests can determine the flow index of the powder, the flow behavior in different packing states (aerated, consolidated, and conditioned), how stable the powder is, how the powder behaves under compressive forces, how the flow changes when air is passed through the powder (and how easily it does so), how permeable the powder is (for determining dissolvability), how the powder behaves against different wall surfaces, how much energy is needed to turn the powder into a stressed state, how easy it is to fluidize the powder, how the density changes when stored in silos, and how different environmental conditions affect the flow and behavior of the powder. The above list is not exhaustive but showcases some of the information that is commonly sought after by manufacturers and those in quality control operations.
Effect of Environmental Factors
Moreover, the change in environmental conditions (as mentioned above), such as temperature, humidity, presence of certain gases, or electrostatic charge, also changes the flow behavior of the whole powder system—this can be in either the processing or transport stages—so it is common for powders to be tested at multiple points along the manufacturing and supply chains to ensure that the product is consistent and at the required quality at all times.
Sources and Further Reading