What is granular segregation?

Granular segregation is a phenomenon commonly observed in granular materials like powders. Powders being a collection of solid grains which differ from each other in size, shape, and/or true density, granular segregation is the ability of grains with the same properties to gather while grains with different properties separate from each other [1]. The size distribution is generally the dominant parameter in this phenomenon. It means that, due to granular segregation, small grains tend to gather with small grains and separate from large grains, and vice versa for large grains.

In general, one would observe small grains going to the bottom of the granular pile. This effect is due to the ability of small grains to pass through the voids between large grains. On the opposite, large grains would move up the granular pile. This effect is called the Brazil nut effect, referring to the nuts with which this phenomenon is often observed. Whether due to a difference in particle size, shape, or true density, granular segregation happens when the powder is mechanically solicited when flow or vibration is undergone by the powder, which is generally the case in industrial manufacturing processes.

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Why granular segregation should be avoided?

Although granular segregation is used in some processes like in mining to separate different materials, this phenomenon is something to avoid for most industrial manufacturing processes [2]. Indeed, different granular materials are generally mixed to obtain a homogeneous powder blend. The degree of homogeneity can be of huge importance. For instance, in the pharmaceutical industry, inhomogeneity in a blended batch is highly problematic for tableting. Indeed, this could result in different doses of medicine, leading to dangerous or ineffective tablets.

Moreover, when segregation happens in a powder going through the different steps of a production line, the properties of this powder change. Processability is thus affected, making handling possibly difficult and changing the quality of the final product. For example, bulk density is an important parameter to control in many industrial applications while it is highly affected by segregation. In addition, the packing dynamic is also observed to be influenced by segregation, impacting the densification of the powder material in a manufacturing process.

Moreover, the inhomogeneity induced by segregation changes the flowability of the powder. Indeed, the separation of particles of different sizes, shapes, and/or true densities completely changes the distribution of interactions and consequently the global flowability of the powder.

Therefore, the handling can be challenging since the flow of powder will be inconstant along the process, making it difficult to adjust the flow rate of powder for conveying or filling. Because granular segregation is hard to avoid in industrial processes, with detrimental effects, it is important to be able to characterize the propensity of segregation to change powder behavior.

Evaluation of granular segregation effect with powder characterization instruments

While it is known that segregation influences the bulk density and the packing dynamic of powder mixtures, most of the works consider the system as homogeneous, leading to discrepancies between predictions and reality. Moreover, most of the devices for measuring the powder bulk density don’t give access to the packing dynamic, resulting in few works focussing on this topic. On the contrary, the GranuPack allows to measure this dynamic to investigate the effect of segregation on powder densification. Moreover, studies realized with this tool have shown how improved tapped density measurement can help to evaluate the effect of segregation in static/quasi-static conditions and the interest in doing so [3].

For dynamical conditions, as encountered during a free-flow in a manufacturing process, the rotating drum method employed by the GranuDrum is ideal to quantify the change in flowability due to segregation. Indeed, the measurement is done by following an increasing rotating speed sequence and then a decreasing speed sequence. Then, the temporal change of flowability will be seen as a hysteretic behavior between the two speeds sequence, quantifying the effect of segregation before and after a flow.

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Conclusion: Powder characterization is necessary to evaluate the effect of granular segregation

Granular segregation is omnipresent in powder due to differences in particle size, shape, and particle true density. It is detrimental to many industrial manufacturing processes. For a better improvement of these processes, it is important to better understand such phenomena. Therefore, adequate characterizations are necessary to evaluate the effect of granular segregation on powder behavior such as packing dynamic, bulk density, and flowability. For this purpose, the GranuPack and the GranuDrum instrument of Granutools are ideal for investigation in quasistatic and dynamic conditions.

References

[1] Particle segregation in dense granular flows, J. M. N. T.Gray,  Annual Review of Fluid Mechanics 50, 407-433 (2018).

[2] An experimental scaling law for particle-size segregation in dense granular flows, T. Trewhela, C. Ancey and J. M. N. T. Gray, Journal of Fluid Mechanics, 916, 55 (2018).

[3] How size ratio and segregation affect the packing of binary granular mixtures, S. Pillitteri, E. Opsomer, G. Lumay and N. Vandewalle, Soft Matter, 16, 9094-9100 (2020).

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