A grain-scale study of unsaturated flow in highly swelling granular materials

Abstract

Unsaturated flow in swelling porous materials are common and important phenomena in industrial products and earth materials; for example, in paper, hygienic products, swelling clays, and foods. Swelling causes porous media to expand and to deform, which results in a change in pore structure and thus in distributions of water and air. This work focusses on the swelling of Super Absorbent Polymer (SAP) particles, which are absorbent particles capable of absorbing large amounts of fluids, typically 30x its initial weight, in a relatively short time frame (order of minutes). Characterization of a bed of SAP particles is complex, because of its large deformation in a short time as well as the complexity of the process itself; i.e. unsaturated flow in a deforming and swelling material. To characterize swelling, continuum-scale models can be used, but they require hydraulic parameters to have dependencies on both swelling and state-of-stress of the particle packing. Another approach is to employ a pore-scale model that applies physics and rules at the grain-scale, which can be used to upscale grain-scale processes to continuum-scale phenomena. However, a pore-scale model for unsaturated flow in deformable granular materials does not exist, yet. Therefore, we developed a pore-sale methodology that enables simulation of a deforming and swelling bed of particles and the subsequent redistribution of air and water in the pores. Deformation is simulated using the Discrete Element Method (DEM), which can describe motions of individual particles, while unsaturated flow is simulated using a newly developed Pore-Unit Assembly (PUA) methodology. The specific objects in this work are as follows: i) to study the applicability and versatility of the discrete element method towards simulation of a bed of SAP particles; ii) to identify swelling kinetics of individual SAP particles; iii) to couple the discrete element method with the pore-unit assembly method in order to construct capillary pressure-saturation curves of SAP particle beds; iv) to study the effect of swelling and porosity change on the retention properties of SAP particle beds; v) to investigate the role of particle shape in generating packings of SAP particles, using the discrete element method; and vi) to identify dynamic effects of unsaturated flow in rigid and swelling particle packings.