Unsaturated flow in a packing of swelling particles; a grain-scale model

•This work investigates unsaturated flow in a drying bed of swelling particles.•The material of interest are particles of Super Absorbent Polymers (SAP).•Swelling particles are simulated using the Discrete Element Method (DEM).•Unsaturated flow at the pore-scale is simulated using an IMPES scheme•Results show that the rate of absorption determines unsaturated flow properties. In this work, a grain-scale modelling technique is introduced for the simulation of unsaturated flow in deforming and swelling granular materials. To do so, a pore-scale model for unsaturated flow is coupled to the discrete element method (DEM). It is assumed that initially a dry packing of particles is quickly invaded by a liquid and becomes fully saturated. Particles start absorbing the liquid and this causes a rearrangement of particles, entrance of air into the packing, and a redistribution of the liquid (i.e. unsaturated flow). Flow was computed using a scheme of implicit pressure solver and explicit saturation update (IMPES), whilst particle movement was modelled using DEM. Simulations are continued until the packing is dry again. This is the first time that such a pore-scale model has been developed. We have used the model to investigate unsaturated flow during drying of a bed of swelling particles. Results indicated that the characteristic time scales of unsaturated flow and water absorption determine the swelling behaviour of the particle packing.