Three-dimensional visualization and modeling of capillary liquid rings observed during drying of dense particle packings

•The time-evolution of morphology of liquid in drying particle packings is revealed.•The crucial role of liquid rings in drying capillary porous media is quantified.•Three-dimensional pore network model accounting for the ring effect is developed. Experimental and numerical pore network simulation studies on convective drying of capillary porous media under slow isothermal conditions are presented in this work. As a physical model of a real capillary porous medium, two dense packings of particles filled with monodisperse spherical glass beads (mean diameter 0.8 mm) and initially saturated either with distilled water or with a salt solution are prepared. Two controlled drying experiments with these packings are carried out using a custom-made setup installed in a lab-scale X-ray microtomograph. Based on in-situ tomograms (voxel size 16.4 μm) the time evolution of three-dimensional (3D) structures of liquid and salt deposit in the packings during drying are characterized. The respective results clearly demonstrate the formation of capillary liquid rings at the wedge-shaped pores located near the particle-particle contacts. The rings remain connected over a long distance to the packing surface during a significant period of drying. To highlight the crucial impact of liquid rings on drying, a 3D discrete pore network model that explicitly accounts for the ring effect is developed. Pore network simulations in the presence and absence of this effect are compared with measurements in terms of drying kinetics and saturation profiles. It is found that liquid rings act as additional hydraulic pathways for moisture transport from the interior of the pore/particle network to the surface and thus notably accelerate the drying process, and they lead to a spatially homogeneous distribution of the liquid down to low saturation. This study can be considered as a step forward in discrete modeling of drying of capillary porous media with 3D secondary capillary structures and should be of interest for various applications in the field of complex multiphase flow phenomena in porous media.