The presence of inter-particle cohesion in powders and grains, such as capillary bridges due to moisture, can drastically change their mechanical properties. For instance, cohesion causes grains to agglomerate into clumps of sizes much larger than the size of the constituent particles. The agglomeration of granules makes the processing of such materials challenging, particularly during drying under agitation processes, where agglomerates can reduce the overall efficiency of processes. The methods of drying a mixture of particles and liquid affect the state of agglomeration of the final dried product, particularly through the influence of impacts and shear forces on the agglomerates.
Our research direction in this area is based on the development of model experiments to understand and model the mechanics, size distribution, and time evolution of particle agglomerates. By controlling cohesion and grain properties, as well as the input of energy in the system, we hope to shed some light on the mechanical behavior of agglomerates to develop models at that can closely connect cohesion to agglomerate mechanics. Then, thanks to the models that will be developed, we will be able to consider industrial powders, such as calcium carbonate or powders used in the food industry, among other examples.
During the first year of the project, we developed an oscillating system consisting of a mechanical shaker and a quasi-2D transparent box, which allowed us to observe the agglomerates being mechanically agitated. Experiments with glass beads and water have illustrated the role of acceleration and amplitude of oscillation in determining the agglomerate sizes. In parallel, a second prototype, relying on airflow through a deposited bed, was designed. These tools were described in our last annual report, ARR-51-16 (2022-2023).
Following in-depth interactions and suggestions from IFPRI members, we have since modified the airflow setup to produce a much higher shear in a small region of the total setup during Year 2 of the project. Using small-diameter nozzles now allows us to reach considerably larger flow rates and shear stresses than we studied in the first year. This new setup, its characterization and some results are presented in the following. In addition, some members mentioned their need to also consider lower shear, and we have thus finalized the construction and characterization of a rotating drum, which was in our initial proposal to study the effects of low and moderate amounts of shear on drying wetted grains.
During this year, we were also invited to write a review article for the Royal Society of Chemistry journal Soft Matter, discussing the current state-of-the-art experimental techniques to study model cohesion in granular systems and some perspectives on future research. Since this review may likely be of general interest to some IFPRI members and was also initially strongly motivated by the interactions with IFPRI members, we have included the peer-reviewed version of this manuscript at the beginning of this annual report.
Finally, since our understanding of the evolution of the inter-particle force during drying is very limited, we have further developed an apparatus to measure the temporal evolution of the forces between individual particles during drying in our laboratory. These measurements will also help us to design model agglomerates to better estimate the shear rate and the effect of agitation on the time-evolution of particle agglomerates.