Grain-Scale Modelling of Swelling Granular Materials Using the Discrete Element Method and the Multi-Sphere Approximation

In swelling materials, the hydraulic parameters, such as porosity, permeability, and retention properties depend on the degree of swelling. An example of swelling granular materials are Super Absorbent Polymer (SAP) particles, which can absorb 30-40 times their initial weight of saline fluids within a few minutes. Due to their fast absorption rate, measurements of the hydraulic parameters of a bed of SAP particles is complex or not feasible unless a quasi-static approach is used. An alternative is to use a grain-scale model to estimate hydraulic properties for various packings. We have performed compaction simulations of SAP particles, at the grainscale, to reconstruct various packings with varying degrees of swelling and varying mechanical parameters, using the Discrete Element Method (DEM). DEM models are commonly based on packings of spheres. But, SAP particles have very irregular shapes and this affect their hydraulic properties significantly. Therefore, the shapes of particles were represented by sets of overlapping spheres also known as clumps. An algorithm was developed to make realistic clumps that are representative of real SAP particles. An inventory of real shapes was obtained using micro-CT scans of individual SAP particles. In DEM, particles were randomly generated based on 20 clumps and the particle size distribution. The particles were then compacted under a constant confining stress in order to obtain a packing. The resulting pore geometry was used to obtain the porosity value which was compared to experimental data.