Pore-scale modeling of water-phase fragmentation in simulated soils with realistic pore geometry

Bacterial diversity in soil is extremely high, and evidence suggests that spatial isolation created by fragmented aquatic microhabitats in unsaturated soil plays a large part in creating this diversity. Soil texture determines the extent and connectivity of hydrated microbial microhabitats through its influence on the pore size distribution. In this study we simulate pore-scale water retention processes in soils of varying textures. We develop a novel methodology for generating realistic soil particle shapes, conforming to experimentallydetermined sphericity and roundness values for sand and silt particles. We then employ the lattice Boltzmann method, using a 3-D single-component multiphase model to simulate equilibrium water configurations in a series of simulated soils, and quantify the fragmentation of the water phase by identifying isolated water pockets that could serve as bacterial microhabitats. Pocket counts at a range of liquid saturations were dominated by filling of small pores created by domains of high silt content, which was a realistic depiction of the fragmentation behavior for the finest-textured soil. Fragmentationmeasures in the coarser soils showed sensitivity to a thresholding parameter controlling thickness of liquid films on rough surfaces of soil grains, indicating the importance of such films in controlling fragmentation in coarse soils. This work highlights the differential mechanisms controlling hydraulic fragmentation in soils of different textures: filling of pores due to capillary forces in finer soils, and disconnected water films on grain surfaces in coarser soils. Follow-onwork will include grid refinement to better resolve the spatial distribution of surface films in coarse soils.