Flow of magnetized grains in a rotating drum

Experimental models for cohesive granular materials: a review

Granular materials are involved in most industrial and environmental processes, as well as many civil engineering applications. Although significant advances have been made in understanding the statics and dynamics of cohesionless grains over the past decades, most granular systems we encounter often display some adhesive forces between grains. The presence of cohesion has effects at distances substantially larger than the closest neighbors and consequently can greatly modify their overall behavior. While considerable progress has been made in understanding and describing cohesive granular systems through idealized numerical simulations, controlled experiments corroborating and expanding the wide range of behavior remain challenging to perform. In recent years, various experimental approaches have been developed to control inter-particle adhesion that now pave the way to further our understanding of cohesive granular flows. This article reviews different approaches for making particles sticky, controlling their relative stickiness, and thereby studying their granular and bulk mechanics. Some recent experimental studies relying on model cohesive grains are synthesized, and opportunities and perspectives in this field are discussed.

Packing dynamics of powders at high temperature

In industrial powder processing, precise control of environmental temperature is difficult especially for large production facilities. A better knowledge of powder behavior modification due to temperature variation will help to improve product quality and consistency. From a fundamental point of view, the effect of temperature on powder flow and packing dynamics is still poorly understood. In particular, because temperature modifies the complex interplay between the different forces acting at the contact between the grains. The packing dynamics of a set of different powders (lactose, cacao and dried milk powder) has been investigated with a tapping experiment. After the filling procedure, the sample is heated and the evolution of the density is measured after each tap. We show that the packing dynamics is drastically influenced even for low temperature changes. Slight increase of temperature affects both the packing kinematics and the packing range characterised by the classical Hausner ratio analysis. Finally, the different physical mechanisms that could explain these modifications are discussed. The results of this preliminary study demonstrate the importance of temperature in powder behaviour and strongly motivate further investigations.

Lactose monohydrate flow characterization using shear cell method

The flow properties of pharmaceutical powders have a great importance in the manufacturing of solid dosage forms. In order to ensure the performance in the production line this parameter must be determined. There are several methods described in European Pharmacopeia that are used to measure these properties. Some of them were used in this study and the results obtained from conventional methods (Conv) and shear cell using the powder flow tester (PFT) showed differences that were more evident in fractions with smaller particle size (F < 63) and for bulk powder (F_Total). The various powder behaviors showed to be related with the size of the particles. An increase of the ff_c (Flow Index) was observed with the increase of the particle size. It was also found for the different fractions that the ff_c always increases with increasing major principal consolidation stress (σ₁). This study shown to be predictive because it also allowed the behavior profiles of other LactMN fractions to be known by interpolation of the median size (Dv50) or σ₁ values ranged between the studied intervals. Furthermore, it was also observed that ff_c of the F_Total was similar to the F < 63, showing the same behavior under σ₁. The occurrence of caking was not observed.

Experimental study of a vertical column of grains submitted to a series of impulses

We report physical phenomena occurring in a vertical Newton's cradle system. A dozen of metallic spheres are placed in a vertical tube. Therefore, the gravity induces a non-uniform pre-compression of the beads and a restoring force. An electromagnetic hammer hits the bottom bead at frequencies tuned between 1 and 14Hz. The motion of the beads are recorded using a high-speed camera. For low frequencies, the pulses travel through the pile and expel a few beads from the surface. Then, after a few bounces of these beads, the system relaxes to the chain of contacting grains. When the frequency is increased, the number of fluidized beads increases. In the fluidized part of the pile, adjacent beads are bouncing in opposition of phase. This phase locking of the top beads is observed even when the bottom beads experience chaotic motions. While the mechanical energy increases monotically with the bead vertical position, heterogeneous patterns in the kinetic energy distribution are found when the system becomes fluidized.