Shape Matters - Comparison of Block- and Spherical-Shaped Soil Particle-Meniscus Dynamics Under Wetting and Drying Conditions and their Effect on Soil Mechanical Properties

Soil structure plays a crucial role in mediating several soil physical and biological processes that underpin important ecosystem services. Soil structure metrics are typically inferred either empirically and/or based on bulk scale soil hydraulic or mechanical properties. Such large-scale measurements obscure fundamental pore-scale processes that govern soil aggregation and the evolution of soil structure based on hydrodynamic cycles. This study develops a mathematical model that describes the evolution of a liquid meniscus between two solid soil particles and the resulting forces that cause the particles to displace. The liquid meniscus was simulated using a multiphase two fluid model considering gas (air and water vapor) and liquid (water), evolving under a series of variably imposed wetting and drying cycles. The meniscus curvature was then used to estimate the tensile and expansive forces driving particle movement, as well as an effective soil elastic modulus. We consider both block and spherical-shaped solid soil particles, identifying differences resulting from their geometry. Our results identify the magnitude of forces associated with shrink-swell processes that influence soil structure over several drying and wetting cycles. Importantly, shrinking and swelling only occurred for the block shaped particles, while changes in effective soil elastic modulus for varying moisture conditions was only present between spherical particles. Effective soil elastic modulus provides information associated with aggregate stability, and thus these results may point to moisture conditions that can enhance soil carbon sequestration and generally lead to healthier soil.