No abstract

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 à g ), which is a ratio of the capillary force to the gravitational force acting on a particle. Tunneling occurred for Bo à g [ 65,000 for all values of bed porosity, whereas Spreading and Crater Formation occurred when Bo à 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.

Granule formation from drop impact on a powder bed can occur by either Tunneling or Spreading/Crater Formation. The governing regime can be specified by the experimentally determined modified Bond number (Bo(g)*), which is a ratio of the capillary force to the gravitational force acting on a particle. It was hypothesized that Tunneling would occur when the capillary and surface tension forces exceeded the weight of a powder aggregate in contact with the drop. To confirm this hypothesis, force balances were derived for a drop in contact with a single particle and separately for a drop in contact with an aggregate to predict when a particle or aggregate will be sucked into the drop. The force ratios derived for each case were compared to the Bo(g)* force ratio used in a previously published regime map that separates Tunneling from Spreading/Crater Formation. The force balance model correctly predicts the trends of the impact of powder and liquid properties on the governing regime. However, the single particle model does not quantitatively predict the critical Bond number for regime change in Tunneling. The aggregate model gave a better prediction of the Tunneling boundary than the single particle model, but it still under predicts the experimentally determined Tunneling criterion given by the Bond number. Potential reasons for this discrepancy are discussed.

Yield stress fluids are widely used in industry, deeply studied as an example of soft matter, and easy to conceptually describe: A solid-like material that can be yielded and made to flow by applying a minimum stress but will re-solidify once the applied stress is removed. Similarly, a particle will be stably suspended against sedimentation by a yield stress fluid if the stress it exerts on the fluid does not exceed the yield stress. In this article, we examine the current approach to predicting particle suspension in a yield stress fluid. We focus on a key cause of variability in both the fluid yield stress and propagation of particle stress: The fluid microstructure. We measure the prevention of particle sedimentation by examples of the two key microstructures used to create a yield stress suspension: A colloidal glass, Carbopol, which forms high volume fraction elastic structures by crowding, and a colloidal gel, microfibrous cellulose or MFC, which forms a sparser low volume fraction elastic network by inter-particle attachments. Comparing the sedimentation behavior of a single sphere in Carbopol and in MFC indicates that fluids with the same yield stress value can differ by a factor of 6 in their stability against particle sedimentation as a result of microstructure differences. Such suspensions cannot be characterized by yield stress alone, so the different fluids' yielding, and possibly recovery, from applied stress must also be studied. The work points to methods of improved design of microstructured fluids in a range of formulated product applications and also links shared goals of the rheology and microrheology communities. V