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Granular materials can segregate spontaneously due to differences in particle properties when subjected to vibrations, shear strain or because of the equipment geometries. Although the difference in particle size is the most critical factor that drives segregation, the effects of large density difference may also be detrimental for a lot of industries. In this work, we experimentally investigate density-driven segregation in bi-disperse mixtures of particles having the same size but different density when subjected to non-uniform shear rates. We found that the features of the segregation process are related to the density ratio as well as to the dimensionless loaded mass. The experimental outcomes are then compared with the solution of a simple density-driven segregation model. The model can successfully capture the main features of segregation driven by density for a range of density ratios.

Powders have always been used in industry, spanning from food to metallurgic ones, and in recent decades they have become the protagonists of other innovative process: additive manufacturing (AM). Knowing the flowability of a powder in a specific piece of equipment is fundamental to achieve better manufacturing efficiency and product quality in powder processing industries. A powder used in AM machines is subjected to different flow and stress regimes. There are commercial devices able to measure the flowability of the powder under different levels of consolidation stresses, but they are not able to work at very low stresses. Among the available, alternative, non-commercial tests dynamic BIM is the most promising. In dynamic BIM a spherical indenter impinges on the powder with the velocity of a falling body; it is possible to determine the flowability (through the hardness) of the powder, but the evaluation is complex and must be improved. In this work we investigate the dynamic impact of an indenter in a packed powder, measuring the geometrical characteristic of the crater forming on the powder surface. Furthermore, we try to improve the definition of dynamic hardness and shear rate by rewriting them in terms of granular velocity rather than indenter impact velocity as the most common reports in the literature. [ABSTRACT FROM AUTHOR]

Particles differing in size, under external mechanical activation, typically segregate rather than mix. This phenomenon, known as particle size segregation, is spontaneous and unavoidable in practice. In this work, we present an experimental investigation aiming at better understanding the segregation mechanisms occurring in industrial devices related to the storage of particulate materials. Even if storage in closed vessels is often considered a static operation, loading and unloading steps are always present and they must be considered as inherent part of the whole storage operation. These steps are clearly dynamic in nature. This work concentrates on the discharge step in which the particulate material is strongly sheared, especially close to the outlet of the storage bin giving high chances of segregation to the material. Segregation in dense sheared flows under gravity occurs mainly through the percolation of smaller particles in the voids existing between larger ones. The level of saturation of voids by small particles can strongly impact the segregation rate. It appears therefore that the relative amount of fines with respect to large particles, the level of bed dilatancy induced by shear, the interparticle friction are all variables that can affect fine particles mobility and therefore the extent of segregation. For these reasons, we carried out experiments on small-scale vessels discharging in a funnel flow regime. Binary mixtures at different fine compositions and particle size ratios have been considered. Furthermore, we carried out experiments to a better knowledge of the mixtures voidage. The correlations found in the literature fail to predict the critical fine concentration at which fine particle segregation stops. An Alternative correlation has been therefore proposed based on a micromechanical analysis of particles filling. [ABSTRACT FROM AUTHOR]

This is an experimental study aiming at understanding the relationship between particle diameters measured with different analytical techniques: optical analysis (microscopy aided by computer image analysis), laser light scattering and permeability tests. Permeability test can provide an equivalent mean diameter that can be used in several cases where particle specific surface is the relevant property to focus on (for example problems related to particle reactivity or involving interactions with fluids as in pneumatic transport or in fluidization). This diameter can be determined by measuring the pressure loss in a granular bed of known porosity at various gas flow rate and by using a proper mathematical model (e.g., Darcy or Karman-Kozeny models) to correlate them. Four different materials (glass beads, granulated microcrystalline cellulose, MCC, sodium chloride crystals, tetraacetylethylenediamine powders, TAED) in the size range 700-1000 µm (by sieve analysis) were analysed. A strong discrepancy was observed by calculating the Sauter diameters from image analysis (and laser light scattering) and those from permeability measurements. The introduction of a function considering the pores morphology and connectivity was able to reconcile the size measured with the three different methods. [ABSTRACT FROM AUTHOR]

Granular materials can segregate spontaneously due to differences in particle properties when subjected to process vibration, shear strain or because of the equipment geometries. Among the different properties, difference in particle size is the most relevant factor that drives segregation. Although size-driven segregation has serious technical implications in a lot of industrial processes, a fundamental understanding of the phenomenon is still lacking. Furthermore, models and theories on segregation are often validated with DEM simulations as an alternative to experiments. This leads to a shortage of experimental data. In this paper we experimentally investigate size-driven segregation of ternary mixtures of grains flowing down an inclined plane, for a range of mixture compositions. The segregation process is filmed through the transparent sidewall with a camera, and the evolution of the particle concentration is evaluated by means of post-processing image analysis. Since different-sized particles are of different colors, the post processing procedure consists in associating the color information of each pixel to the respective component. It is found that the segregation features are strictly related to the relative amount of the largest grains: a higher fraction of the larger particles improves mixing and reduces segregation. [ABSTRACT FROM AUTHOR]

Unreliable powder flow is a major problem during processing of powders. The shear cell is the most widely used method for powders subjected to moderate or high stresses, and under quasi-static conditions, with established methods for designing large bins and hoppers based on the measurement. However, this method is not suitable for measuring the flowability of dynamic systems, such as powder mixing. Here, the ball indentation method is investigated as a technique for evaluating powders in the intermediate and dynamic regime of flow. The method, which simply consists of dropping a ball onto a cylindrical bed of powder previously consolidated, directly measures hardness, which is related to the unconfined yield stress of the powder by the constrain factor (Hassanpour and Ghadiri, 2007). The impact of the ball on the bed is recorded with a high-speed camera to determine velocity and penetration depth. The shear rate is varied by using a range of indenter materials and sizes, and a range of drop heights. The hardness against the strain rate is considered for several materials. It was found that the indenter size does not influence the hardness results, which are consistent with the flowability evaluation achieved with the rheometer. Furthermore the hardness, which is independent of the strain rate in quasi-static conditions, becomes shear rate dependent in intermediate regime of flow. Further work is needed to evaluate hardness in the rapid granular flow regime. [ABSTRACT FROM AUTHOR]