A regime map for granule formation by drop impact on powder beds

"Tunneling," "Spreading," and "Crater Formation" are the three granule formation mechanisms known to occur when single drops impact static powder beds. To quantify the conditions under which each mechanism will occur, dimensional analysis was performed, and a new regime map was created that plots the powder bed porosity against the modified granular Bond number (Bo_g*), which is a ratio of the capillary force to the gravitational force acting on a particle. Tunneling occurred for Bo_g* > 65,000 for all values of bed porosity, whereas Spreading and Crater Formation occurred when Bo_g* < 65,000 for all values of bed porosity below the minimum fluidization porosity. The granule formation mechanism regime map provides a useful tool to design and predict wet granulation processes by predicting the granule formation mechanism, and thereby general granule shape, from a few key dimensionless groups involving formulation properties and process parameters that can be calculated a priori.