A study on slow evaporation of liquids in a dual-porosity porous medium using square network model

Slow evaporation of a liquid is studied in a two-dimensional pore square network of aspect ratio 1 with three sides insulated and one side exposed to air for drying. In this study, the external transfer resistance and liquid-film effects are ignored while the capillary effects dominate viscous and gravity forces in the hydrophilic network. The square domain is divided into two layers with distinct porosities and particle sizes such that the two layers are exposed to drying alternately. A 100 × 100 network simulation of two cases of the exposed larger-pore layer shielding the smaller-pore layer, and the exposed smaller-pore layer shielding the larger-pore layer, lead to dramatically different responses in terms of the liquid evaporation plots and saturation distributions. The former case retains moisture in the inner smaller-pore layer till the entire outer larger-pore layer is dry, and is characterized by decaying liquid evaporation plots. The latter case leads to loss of moisture in both the exposed smaller-pore layer (due to evaporation) and the inner larger-pore layer (due to capillary pumping), and is characterized by bilinear evaporation plots (with an initial faster evaporation followed by a subsequent slower one). A case study that imposes uniform porosity in the two layers but keeps particle sizes different in the two layers indicate that though the pattern of saturation distribution during evaporation may remain similar to the earlier cases, but the evaporation plots are significantly different. An experimental validation of the simulation is undertaken with the help of a smaller 12 × 12 network where saturation patterns and evaporation plots are replicated well by the simulation. However the presence of surface liquid films created due to surface roughness as well as the capillary-suction driven liquid redistribution may be the cause of the large mismatch in the drying time of the network.