Grain-scale modelling of swelling granular materials: the effect of particle shape on porosity, permeability, and retention properties

Swelling of porous materials is a common and important phenomenon in industrial products and earth materials; for example, in paper, hygienic products, and swelling clays. In hygienic products, hydraulic parameters, such as porosity, permeability, and retention properties depend on the degree of swelling of the Super Absorbent Polymer (SAP) particles. SAPs can swell up to 30 times their initial mass, most of which occurs in less than 5 minutes. Therefore, change in hydraulic parameters during swelling cannot easily be identified by experiments unless a quasi-static approach is used. However, we have performed grain-scale simulations of compaction of SAP particles to reconstruct the pore geometry for varying degrees of swelling and various degrees of compaction, using the Discrete Element Method (DEM). The effect of swelling was first studied using spherical elements in DEM [1]. However, to obtain large porosity values (>0.60) of typical SAP packings, the shape of individual particles had to be included. This was achieved by approximating the shape of a non-spherical particle by an assembly of spheres, called a clump of spheres. The real shape was obtained using micro-CT scans of individual SAP particles. Then, the individual particle shapes were extracted and exported into surface plots. Using the surface plots, around 14 spheres were randomly fitted into each particle following the fitting algorithm by [2]. Finally, we obtained a library of 30 particles which are represented by clumps of spheres. In DEM, particles were randomly generated using the particle shape library and based on the particle size distribution of SAP. The particle packing was then compacted under a constant confining stress. The resulting pore geometry was used to obtain the retention properties using Pore Morphology Method, and the permeability using direct simulations. The major advantage of this method is that we can perform dynamic simulations in DEM, where by particles can swell over time. While rearranging among themselves. At intervals, the pore geometry is exported such that we can analyze the hydraulic parameters.