Visualization and modeling of the evaporation process using representative 2D Micromodels and universal scaling laws

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Evaporation is a key process controlling the soil water budget and is characterized by a complex interplay of atmospheric boundary conditions and the flow- and transport properties of the 3-phase system "solid phase - water - gas". The central objective is the experimental and theoretical investigation of the evaporation process for real soil structures using representative 2D-micromodels and network models. The 3D-pore structure is measured by micro-X-ray computer tomography (micro-CT) and mapped to the 2D-micromodel by a new topological 2D-3D transformation. The experimental results are compared with the theoretical results of an evaporation-network model, which for the first time takes into account the evaporation over the thick-film surface and the experimental pore size distribution. The 2D-micromodels have both an inner and an outer roughness as natural soils and loose rock. For the first time the thick-film flow is investigated as a function of the microstructure of the solid surface. Therefore, REV-micromodels (REV: representative elementary volume) are produced in silicon (inner roughness = 0.1 micro-m), glass (0.5) and glass ceramic (3). As a working hypothesis it is assumed that (i) different atmospheric conditions (diffusive flow, laminar flow and turbulent flow; increasing potential evaporation rate) cause different drying dynamics and that (ii) the critical water potential depends on the thick-film flow and on the thick-film-surface. Two soil types from agricultural land (Fuhrberger Feld) are investigated: (i) sandy soil (A-horizon) and (ii) medium sand (C-horizon). The pore structure is analyzed using micro-CT (maximal resolution: 5 micro-m) and methods of mathematical morphology (Minkowski functions). The micro-model experiments are carried out for both the horizontal and the vertical water flow in the range of relevant capillary numbers; (10^-7 ... 10^-4; increasing evaporation rate). The visualization experiments are carried out with two high-resolution SLR cameras (18 megapixels) and a fluorescence microscope. The Thick-film dynamics is described by the standard theory of "wetting on rough surfaces". For all experiments (i) a cluster analysis is conducted, (ii) the gravitational correlation length is calculated by universal scaling laws, and (iii) the fractal dimension of the drying/displacement front is determined. Based on a representative network model the two important parameters of the continuum theory (REV-scale) - the effective permeability and the effective diffusion coefficient - are derived. The expected results are both of fundamental interest and of great practical relevance, as they improve process understanding and consequently the predictive modeling of evaporation at REV-scale.