Lattice Boltzmann modelling of capillarity, adsorption and fluid retention in simple geometries: Do capillary and film water have equal matric suction or not?

Abstract

The pore water retained in unsaturated soil includes film water attached on the solid surface and capillary water in corners or pores, which are mainly controlled by adsorptive force from the solid surface and capillary force from the water-gas interface, respectively. The soil water retention (SWR) curve represents the fundamental characteristic of unsaturated soil, in which the connected capillary and adsorptive water are conventionally presumed to have equivalent suction values as matric suction. Here, a long-range adsorptive fluid-solid interaction force is developed in the mesoscopic multiphase lattice Boltzmann model (LBM) framework to model the macroscopic processes of capillarity and adsorption. The pressure tensors of capillary and film water derived based on mechanical equilibrium and the results of numerical simulations combine to show that the adsorptive suction is much higher than the capillary suction, not following the classical relationship. We attribute this inequality to the different adsorptive interaction potentials incorporated in the capillary and film water pressures, due to the fluid density profiles varying differently with the separation distance from solid surface, and, from the perspective of thermodynamic equilibrium, the deviation of film water and capillary water densities from free water density. The film thickness almost does not change for the given radii of meniscus curvature in simple geometries (i.e., slits and corners). The adsorption effects on the matric suction upscaled from the intrinsic phase average method and on the equivalent pore size distribution are investigated for both single-sized slits and complex pore networks. The findings reveal the influences of capillarity and adsorption on the shape of SWR curve, and help establish the SWR function with accurate physical meanings in the field of soil physics and hydrology and measure the disjoining pressure isotherm properly in colloid and interface chemistry.