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The flow and the distribution of water and air in the subsurface are on all length scales strongly influenced by the structure of the porous media. The structure is the spatial arrangement of the constituents of the porous media. To reveal the impact of structure on flow processes, the structure must be mapped and quantified. The technique to measure soil structure and its effect on two phase flow depend on the observation scale:
At the microscale, porous media consists of solid grains and voids in between. The voids are denoted as pores and are filled with gas or liquid. The pore structure can be determined using synchrotron light with the resolution of a few microns.

A cylinder (5mm in diameter) was packed with sand particles with sizes ranging from 100 to 200µm. A section of thickness 0.7mm is shown. The sample was scanned at the Swiss Light Source, Paul Scherrer Institute, Villigen Switzerland.

At the mesoscale, the porous medium can be characterized as a homogeneous material. Constitutive relationships between fluid saturation, energetic state and fluid flow for the homogeneous material must be determined to predict flow processes. These relationships must be measured in the laboratory or can be predicted with numerical experiments at the microscale.

At the macroscale, porous media like soils are not homogeneous but consist of different soil materials. The properties of the individual materials and their spatial arrangement must be known to predict flow processes. To improve the understanding of the relationship between flow and structure at the macroscale, cubes filled with coarse or fine sand material are arranged randomly in a column. The fluid flow depends on the constitutive relationships of the two materials and on the geometrical properties of the spatial arrangement. The goal of this project is to analyze the relation between geometric measures of the soil structure and the volume averaged flow behavior. Experiments based on the neutron transmission technique will be performed to measure the water distribution in space and time.

Sand cubes of size 14mm with coarse or fine sand material are arranged randomly in a column with a quadratic cross-section of size 10cm. The image shows the vertical cross-section through the column, when suction is applied at the lower boundary of the water saturated column. The transmission of a neutron beam was measured at the NEUTRA facility at Paul Scherrer Institute. Dark colors correspond to high attenuation of neutrons in the partially air-filled coarse sand material. The bright color represents the water filled fine sand material.