Granule formation mechanisms and morphology from single drop impact on powder beds

Single drops impacting static powder beds were studied to explain the different resulting granule structures. Three chemically similar powders with different physical properties formed static beds with porosities of 0.66–0.69, 0.54, and 0.30–0.35 respectively. Three different binder fluids (distilled water and silicone oils with viscosities of 9.3 and 96 mPa s) were released onto these powder beds from two different heights (0.5 and 30 cm). The initial drop impact, as well as complete penetration of the droplet into the bed, was recorded with a high speed camera. The high speed camera videos were analyzed, and three different granule formation mechanisms were identified: Tunneling, Spreading, and Crater Formation. Tunneling occurred for loose, cohesive powder beds. Powder aggregates were sucked into the drop which then tunneled into the beds. For coarser powders, granules were formed by a Spreading mechanism at a low impact velocity. At a high impact velocity, the drop formed a crater in the bed surface and deformed elastically in the crater, coating the drop in a layer of powder before penetrating into the bed by capillary action. Using all three dimensions of the granule, a new shape factor, the vertical aspect ratio (the ratio of the granule's projected area diameter to its maximum vertical height), was proposed as a more accurate descriptor of granule shape than currently used descriptors such as the horizontal aspect ratio. The different granule shapes observed were explained by the granule formation mechanisms. The Tunneling mechanism always produced round granules, the Spreading mechanism always produced flat disks, and the Crater Formation mechanism produced granules of varying shapes that were dependant on liquid binder properties. The results of this study have important implications for being able to predict granule structure from granule formation mechanisms, and to be able to choose the desired granule properties by operating in the appropriate regime.

Graphical AbstractThree different granule formation mechanisms were defined for single drop impact on static powder beds. Drop impact and penetration were captured visually with a high speed camera, and the shapes of the resulting granules were evaluated with a novel prism technique. The following video sequence depicts the Crater Formation mechanism.Figure optionsDownload as PowerPoint slideResearch highlights
► Three different granule formation mechanisms were determined: Tunneling, Spreading, and Crater Formation.
► Granule formation mechanisms were found to be dependant on powder properties, powder bed structure, and drop impact velocity.
► Each granule formation mechanism resulted in a distinct set of granule shapes.