Physics of Viscous Bridges in Soil Biological Hotspots

Plant roots and bacteria alter the soil physical properties by releasing polymeric blends into the soil pore space (e.g., extracellular polymeric substances and mucilage). The physical mechanisms by which these substances interact with the soil matrix and alter the spatial configuration of the liquid phase and the related hydraulic properties remain unclear. Here, we propose a theory to explain how polymer solutions form one-dimensional filaments and two-dimensional interconnected structures spanning across multiple pores. Unlike water, primarily shaped by surface tension, these polymeric structures remain connected during drying due to their high viscosity. The integrity of one-dimensional structures is explained by the interplay of viscosity and surface tension forces (elegantly characterized by the Ohnesorge number), while the formation of two-dimensional structures requires consideration of the interaction of the polymer solution with the solid surfaces and external drivers (e.g., drying rate). During drying, the viscosity of the liquid phase increases and at a critical point, when the friction between polymers and solid surfaces overcomes the water absorption of the polymers, the concentration of the polymer solution at the gas-liquid interface increases asymptotically. At this critical point, polymers are deposited as two-dimensional surfaces, such as hollow cylinders or interconnected surfaces. A model is introduced to predict the formation of such structures. Viscosity of the soil solution, specific soil surface, and drying rate are the key parameters determining the transition from one-to two-dimensional structures. Model results are in good agreement with observed structures formed in porous media during drying.