DEM analysis of the effect of particle shape, cohesion and strain rate on powder rheometry

Abstract

Discrete Element Method (DEM) is used to simulate the flow of particles addressing the influence of shear strain rate, particle shape and cohesion on the flow characteristics. For this purpose, the dynamics of particle motion in the Freeman Technology FT4 rheometer is analysed. The simulations are first validated by comparison with experiments with cohesive particles, i.e. silanised glass beads, from the literature. Particles with faceted shapes, sharp corners and edges are then simulated and found to require significantly higher energy to flow compared to spherical particles. The presence of truncated vertices, typical of active pharmaceutical ingredients, influences the flow behaviour drastically. The results of this analysis therefore reveal the importance of considering the actual particle shape in DEM simulations when faceted particles are considered. Finally, a rheological model describing the relationship between the dimensionless shear stress and the inertial number for several particle shapes, cohesion values and blade tip speeds is proposed. The outcome of this study may lead to a unified rheological description of powder flow, which incorporates the effect of cohesion, shape and shear strain rate.

Cohesive Powder Flow: Trends and Challenges in Characterisation and Analysis

“…They measured the work (basic flow energy) required to penetrate the rotating impeller into the cohesive powder bed, and showed that this could also be quantitatively predicted by DEM using the elasto-plastic adhesive model of Pasha et al (2014). Bulk cohesion simply lifts up the baseline of powder resistance to bulk motion and the functional dependence on the shear strain rate has the same trend, as recently analysed by Vivacqua et al (2019). They simulated the FT4 test using DEM for both cohesive (two levels) and cohesionless contacts, using Luding's contact model (Luding, 2008) and calculated the shear stress on the impeller blade as a function of the impeller tip speed, as shown in Fig.…”

“…Nevertheless, to use FT4 and Anton Paar instruments as powder rheometers, it is clearly necessary to analyse the mechanics of powder flow induced by the impeller and relate the expended work to the bulk powder rheological characteristics. For this purpose, Ghadiri and co-workers have carried out a systematic study of the effects of cohesion, air drag and strain rate, and particle shape on bulk powder rheology by analysing the stress and strain fields within FT4 using numerical simulations by a combined DEM-CFD approach (Nan et al, 2017a, b, c;Vivacqua et al, 2019). With current computer power and memory, it is too challenging to use fine cohesive powders with a complex shape in such simulations.…”

“…For crystalline solid structures with faceted shapes, sharp edges and corners, it is inappropriate to use clumped spheres. Recently, Vivacqua et al (2019) used bonded Polyhedra to construct faceted shapes in the DEM simulation software package ROCKY-DEM, ESSS, Brazil, and followed the same approach as Hare et al (2015) and Nan et al (2017c) to analyse the effect of faceted particle shape on the powder flow rheology for both adhesive and non-adhesive particles using the FT4 rheometer. They investigated the effect of the shapes as illustrated in Table 1.…”

“…They investigated the effect of the shapes as illustrated in Table 1. The results obtained by Vivacqua et al (2019) are shown in Figs. 4 and 5.…”

“…There are only three instruments available for powder rheometry: the Couette device (Tardos et al, 2003), the FT4 Powder Rheometer of Freeman Technology (Freeman, 2007), as analysed recently by a large number of workers, see e.g. Bharadwaj et al (2010), Hare et al (2015), Nan et al (2017a-c), Wilkinson et al (2017) and Vivacqua et al (2019) and the powder cell of Anton Paar Modular Compact Rheometer, as recently analysed by Salehi et al (2018). There are of course other instruments, such as GranuDrum of GranuTools (Lumay et al, 2012), Hall Flow Tester (ASTM B213-17) and Hosokawa Micron Powder Tester (ASTM D6393-08), with the latter integrating several test methods into one instrument, giving indirect measures of bulk behaviour under flowing conditions.…”