(472c) Dynamic Analysis of the FT4 Powder Rheometer

Unreliable powder flow arising from bulk cohesion is a challenge to many particle processing industries.  The onset of powder flow is commonly assessed by shear cell or uniaxial compression methods.  These techniques operate at low strain rates in the quasi-static regime. However, there is a great interest in shear deformation of powder beds beyond the quasi-static regime, due to the nature of many dynamic industrial processes, e.g. mixing, conveying, filling of cartons or tableting dies.  The Couette rheometer of Tardos et al. (2003) has previously been employed to measure rheological properties of granular materials.  It consists of two co-axial cylinders operating with different rotational speeds, between which the powder is sheared.  However, this device requires a large sample, which may not be available at the early development stage of a product.  More recently, the Freeman FT4 Powder Rheometer has become widely used to assess the flow behaviour of powders.  However its underlying dynamics is very complex due to the highly non-uniform stress and strain fields induced by the rotation of the impeller, thus it is highly desirable to elucidate what it actually measures. 

In this paper we present our analysis of the dynamics of particle motion of cohesive powders in the FT4 using a two-pronged experimental and numerical simulation approach based on the Distinct Element Method.  The experimental approach uses spherical glass beads, made cohesive to various extents by silanisation with different functional groups (hexane, NH₂, CH₃ and CF₂).  These particle ensembles are separately analysed using the FT4 intsrument to assess their flow energy, and other flow properties.  In parallel DEM simulations of the system, using EDEM (DEM Solutions) have been carried out to determine the stress and strain fields generated by the FT4 Powder Rheometer.  Using the elasto-plastic-adhesive model of Pasha et al. (2014) the adhesive forces of the experimental material are implemented into the DEM.  From this, the effect of single particle cohesion on bulk powder flow is characterised under dynamic conditions.

Keywords: Flowability; Distinct Element Method; Cohesion; Rheometer

References:

1)      Pasha, M., Dogbe, S., Hare, C., Hassanpour, A., Ghadiri, M., 2014. A linear model of elasto-plastic and adhesive contact deformation. Granular Matter 16:151–162, DOI 10.1007/s10035-013-0476-y.

2)      Tardos, G.I., McNamara, S. & Talu, I., 2003. Slow and intermediate flow of a frictional bulk powder in the Couette geometry. Powder Technology, 131(1), pp.23–39.