Microscopic mechanisms of particle segregation during hindered settling of polydisperse sand-mud mixtures using CFD-DEM

It is well known that particle segregation during the hindered settling of polydisperse sand-mud mixtures significantly influences their sedimentation and consolidation behavior. However, the underlying microscopic mechanisms remain insufficiently understood. This study employes coupled computational fluid dynamics and discrete element method (CFD-DEM) approach to investigate microscopic segregation mechanisms during the hindered settling of polydisperse sand-mud mixtures, with emphasis on the effects of sand content and mud concentration. Both macroscopic behaviors (e.g., interface settling rate, fluid velocity) and microscopic indicators (e.g., average contact number, force chain size) are analyzed. It is found that the settling of coarse particles compresses the fine particles beneath and along their descent paths, reducing local void ratios and increasing density of the fine particles. This compression promotes faster consolidation and denser packing. Coarse particles also create macropores along their paths, which serve as effective drainage channels and contribute to vertical heterogeneity in the void ratio. Additionally, they disrupt floc structures beneath while promoting floc reformation laterally, increasing contact numbers but reducing average contact forces. Furthermore, the downward movement of coarse particles induces transient downward fluid flow, exerting drag on nearby fine particles and enhancing their local settling velocity.