P. Vontobel

Drying front and water content dynamics during evaporation from sand delineated by neutron radiography

Evaporative drying of porous media is jointly controlled by external ( atmospheric) conditions and by media internal transport properties. Effects of different atmospheric potential evaporative demand on observed drying rates were studied in a series of laboratory experiments using sand-filled Hele-Shaw cells. We examined two potential evaporation rates of about 8 and 40 mm per day. The evolution and geometry of the drying front (marking the interface between saturated and partially dry regions) and water content distribution above the drying front were measured every 5 min at 0.1 mm spatial resolution using neutron radiography. Water loss rates decreased with time for both rates, but the decrease was more pronounced for high evaporative demand. External evaporative demand had no effect on drying front geometry or spatial water content distribution for depths below 2 mm. Cycles of roughening and smoothing of drying fronts due to interfacial pinning and unpinning were observed. The water content above the front showed irregular patterns due to formation of isolated liquid clusters with the general profile showing a decrease in mean water content with deepening drying front. Measured water content profiles support the hypothesis that liquid flow supply surface evaporation during stage 1 and water content distribution were not affected by external drying rates. Additionally, observed saturation profiles indicate that the corresponding hydraulic conductivity supports fluxes larger than the highest drying rate measured for sand, suggesting that decreasing drying rate was limited by vapor exchange between progressively drying surface and the viscous boundary layer above.

Investigation of water imbibition in porous stone by thermal neutron radiography

The understanding and modelling of the process of water imbibition is important for various applications of physics (e.g. building or soil physics). To measure the spatial distribution of the water content at arbitrary times is not trivial. Neutron radiography provides an appropriate tool for such investigations with excellent time and spatial resolution. Because of the high sensitivity to hydrogen, even small amounts of water in a porous structure can be detected in samples with dimensions up to 40 cm. Three different porous stones found in Indiana, USA, have been investigated (Mansfield sandstone, Salem limestone and Hindustan whetstone). The imbibition of deionized water and a NaCl solution in up- and downwards directions has been tracked during several hours and radiographed at regular intervals. A correction method to reduce the disturbing effects due to neutron scattering is applied. This allows a quantitative evaluation of the water content in addition to the visualization of the water distribution. The results agree well with theoretical models describing water infiltration and reproduce the water content with a pixel resolution of 272 µm in time steps of 1 min. The comparison with the radiographed structure of the dry stone explains variations in the conduction or retention of the water, respectively. The experimental and correction procedures described here can be applied to other porous media and their uptake and loss of fluids.

Assessment of structural evolution of aggregated soil using neutron tomography

The advanced non-destructive method of neutron tomography, together with image analysis, is used to evaluate the structural evolution of an aggregated soil during one-dimensional compression tests. Aggregation of primary particles is a commonly observed phenomenon in natural and compacted soils that causes an open soil structure with two dominant pore sizes corresponding to macro (inter-aggregates) and micro (intra-aggregate) pores. The evolution of macro porosity and the degradation of structures are evaluated by means of morphological parameters such as volume fraction, size distribution and chord length. Change in the structure is then linked to the macroscopic soil response. It is observed that the major structural modifications are associated with irreversible strains in soil.