PORE-SCALE LATTICE BOLTZMANN MODELING OF SOIL WATER DISTRIBUTION

Water in soil controls almost all physical and biogeochemical processes in terrestrial ecosystems and correctly describing its distribution and flow is critical in human development and ecological environment protection. Water distribution at pore scale is modulated by a multitude of abiotic and biotic factors such as the exudates secreted by plant roots and microbes, which could alter soil wettability and water surface tension. The combined impact of all these factors can be described by a single parameter, the contact angle. Practical studies on soil water distribution normally focus on large scale using continuum approaches by volumetrically averaging the microscopic processes out, but it is the physical and biochemical processes occurring in the pores that underpin the emerging phenomena at large scales. Studying the microscopic mechanisms underlying the microscopic water distribution is hence essential to improving the understanding of macroscopic phenomena. Since it is difficult to observe the water distribution at pore-scale due to the complexity of pores structure and the opaque nature of the soils, pore-scale modelling in combination with tomography has been increasingly used to bridge this gap. In this paper, we numerically investigated how a change in the contact angle reshaped water distribution using the Lattice Boltzmann model and X-ray computed tomography. Two soils with contrasting structures were acquired using X-ray computed tomography and they were then segmented to binary images consisting of pore and solid voxels. Water distribution in pore spaces of the soils was assumed to be controlled by capillary force and was simulated using a modified two-phase lattice Boltzmann model. The results show that with the contact angle increasing, the impact of the pore diameter on water distribution in both soils waned, and that a change in the contact angle also led to a change in the channel diameter for fluid flow and interfacial areas between liquid, solid and gas. It was found that as the contact angle decreased, the channel diameter for the liquid decreased while that for the gas increased first followed by a decline. The density of the liquid water was independent of the contact angle, but the density of the vapor decreased significantly as the contact angle increased. The effects of saturation on density of the vapor also increased as the contact angle decreased.