The influence of water sorption on the microstructure of a hydro-sensitive granular material (couscous) deduced from simultaneous neutron and X-ray tomography

Abstract

The effects of water on the behaviour of granular materials can be significant. Besides capillary bridges, several other chemo-hydro-mechanical processes can affect the response of hydro-sensitive granular assemblies, when water sorption critically alters the individual particles properties (i.e., swelling, deterioration of mechanical properties). It is very common to find such materials in food and pharmaceutical industries, where water sorption can often lead to important resources waste while processing or storing the product. It is therefore necessary to understand the phenomena that affect the material’s functionality, often related to particle agglomeration and degradation. However, despite the relevance of the problem, our knowledge about these phenomena is still relatively poor. With this study we aim to explore the link between water content increase and particle, contacts and assembly scale phenomena. Simultaneous neutron and X-ray tomography allows us to investigate respectively the water uptake and microstructure evolution of two couscous assemblies exposed to high relative humidity while subjected to constant stress, a configuration chosen to simulate the conditions in an industrial silo-storage. We acquire a data-set of images, from which we follow and quantify the variations of water content distribution and the resulting volumetric response of thousands of particles through bespoke algorithms. Despite the abundance of water provided, we observe spatial gradients in water content distribution and consequently in particle swelling. We find that the relation between these two variables can be described as (quasi-)linear. The contact area growth also seems to follow a similar trend.