Heterogeneous swelling of couscous particles exposed to a high relative humidity air, as revealed by TD-NMR and X-ray tomography

The mechanical behaviour of hygroscopic granular materials is highly influenced by water. The strong dependence of individual particle properties on moisture content affects the response of their packings, which can agglomerate and eventually lead to loss of product functionality. Despite the frequent occurrence of this problem in industry, only few studies tried to explore the underlying phenomena and investigate the fundamental connection between particle and packing behaviour. This work presents an experimental study that aims to link the water uptake to the microstructural changes in assemblies of couscous, a material selected to be representative of hygroscopic granular materials. In the experiments, the samples are exposed to high relative humidity (RH), and the moisture content increase is quantified with TD-NMR, along with the shift in molecular mobility. An analogous test is performed, during which x-ray tomographies are acquired continuously. We analyse the volumetric response of the sample and of thousands of particles. Despite the oversupply of water molecules available to saturate the system, the formation of a swelling heterogeneity is observed, which we attribute to the pressure gradient affecting locally the adsorption kinetics. Combining the results obtained from the two techniques, water uptake and particle swelling are found to be linearly correlated.