Influence of cohesive forces on the macroscopic properties of granular assemblies - 2013

The influence of cohesive forces inside a granular material is analyzed with compaction experiments. To begin, a model cohesive granular material is considered. This granular material is made of millimetric grains with a cohesion induced by an external magnetic field. Therefore, the cohesion between the grains is adjusted through the intensity of the applied magnetic field. Afterward, the cohesion induced by capillary bridges are considered. In the first study concerning capillary forces, the cohesion between neighboring grains is induced by liquid bridges in a wet granular material. The cohesiveness is tuned using different liquids having specific surface tension values. The second study performed with capillary forces concerns initially dry granular materials surrounded by a well controlled air humidity. Then, the cohesion inside the packing is controlled through the relative humidity which influence both triboelectric and capillary effects. The evolution of the parameters extracted from the compaction curves have been analyzed as a function of the cohesiveness. All the results show that the packing fraction of a pile and the compaction dynamics is strongly sensitive to cohesive forces. Therefore, the compaction measurement is a good way to characterize a powder or a granular material for R&D and quality control in industrial applications. Finally, we show that the cohesive forces play an important role when the grain size is typically below 50μm.

Abstract