Pore-scale simulation of mucilage drainage

Abstract

Compared with bulk soil, rhizosphere has different properties because of the existence of root mucilage, which affects physical, chemical, and microbial processes. The slow response of rhizosphere to changes in water potential buffers water content changes and leads the rhizosphere to be wetter than bulk soil during drying. By affecting connectivity of the liquid and gas phases, mucilage can also influence solute transport and gas diffusion. Overview of the literature and previous models shows the lack of a model that describes the connectivity between different phases in the rhizosphere pore space during wetting and drying processes. A major challenge is that mucilage shows a complex behavior, which at low concentrations is more like a liquid, whereas at higher concentration, dry mucilage becomes a solid. In between, a viscoelastic state is observed where mucilage can be considered as a hydrogel. In this study a three-dimensional pore-scale model based on the lattice spring method is introduced and used to simulate drying of mucilage between two soil particles. The model is capable of reproducing spider-web-like structures that are specific for mucilage. This three-dimensional mucilage drying model is qualitatively validated via environmental scanning electron microscopy (ESEM) images of dry mucilage between glass beads. The proposed model may provide us with a new perspective on hydrodynamic processes within the pore space of the rhizosphere. In addition, the model may help to better understand further important processes that strongly depend on rhizosphere hydraulic dynamics, such as solute transport, connectivity of the liquid phase, root penetration resistance, rhizosheath formation, and microbial activity.