The flowability and aerodynamic dispersion of cohesive powders

The relationship between bulk powder cohesion and ease of powder dispersion is of interest for many applications including particle characterisation and drug delivery from dry powder inhalers. Quasi-static bulk powder testing is used to evaluate powder cohesion and its link with powder dispersion is explored. The flow function of a range of powders (defined as the ratio of major principal stress to unconfined yield strength) has been determined using a shear tester. The dispersion performance of the powders has been analysed using a proprietary dry powder disperser (Scirocco, manufactured by Malvern Instruments, UK). It is shown that as the flow function decreases the dispersion efficiency decreases. For cohesive powders, full dry dispersion is not easily achieved. A quantitative relationship has been established between the unconfined yield strength and the dispersion efficiency for the powders tested. In addition, it is possible to use this relationship to evaluate the performance of different dispersers. In this study, two other dispersers in addition to the Scirocco were evaluated; a right angle disperser, which performed the best, and a co-axial disperser, which had the lowest dispersion efficiency.