The characterisation of bulk behaviour of cohesive powders is very important in processing of particulate solids, e.g. for reliable powder flow out of storage vessels. Therefore the bulk mechanical properties of cohesive powders have been analysed extensively at large operational scales and applied stresses, typical of those prevailing in storage vessels. However for filling and dosing of small quantities of powders in capsules and for dispersion in dry powder inhalers, the interest is on the behaviour of loosely-compacted powders in small quantities and under very low applied loads. Furthermore at the early stages of drug development, the quantity of the powder available is often very small and the traditional bulk testing methods are neither possible nor applicable. Therefore the ability to successfully predict the flowability of fine cohesive powders from a small quantity or even from the properties of individual particles and their interactions under various physical conditions and in various processes is of great importance. This forms the scope of an on-going research programme at the Institute of Particle Science and Engineering, University of Leeds.
In this presentation the current progress on evaluating the indentation method on a powder bed for assessing bulk powder flow properties is reported (Fig. 1a). The proposed test method can be carried out on very small quantities, typically a few mm3 and at low consolidation pressures (less than 1 kPa). During indentation, the deformation area around the indentation zone is constrained by the rest of powder bed. The constraining of the deformation in the indentation process in powder beds is far more complicated as compared to continuum solids due to the discrete nature and degree of freedom of particle movement for which no published work has been reported in the literature. In order to gain an understanding of the process of deformation around the indentation zone the Discrete Element Method (DEM) is used to simulate the indentation process (Fig. 1b). Single particle properties are characterised using the common existing techniques and used as input parameters in DEM simulation with a view to predict cohesive powder behaviour. In this work, particle adhesion is characterised using the newly developed “drop test” technique at the Institute of Particle Science and Engineering (IPSE), University of Leeds.
Fig. 1 Ball indentation on a powder bed (a) and DEM simulation of the stress distribution around the indentation zone (b).
