Mimicking some features of a root-soil system by dual porosity media

We report an experimental study of evaporation from a non-homogeneous dual porous system consisting of a finer pore size material at the centre surrounded by coarser particles in a glass beaker. This configuration can be considered to mimic, to a first order, the coupling between the plant roots and soil water. Similar to how roots extract water from the soils, here also water moves, driven by the capillary pressure gradient, from larger to the smaller voids. We explore two different configurations (a) when the small pore sizes span the entire porous column along the gravitational direction and (b) when they are limited to a thin annular region at the porous medium top surface. We measured the mass loss from the system, surface temperature using a thermal camera, and the ambient conditions. We compared the results with two standard systems - (a) a homogeneous porous medium and (b) a texturally layered configuration of finer particles at the top and coarser at the bottom. Compared to the homogeneous case, the duration of stage 1 (high evaporation rate regime) of evaporation and thus the characteristic evaporative length increased significantly in the case of root-mimics; this outcome shows the ability of the root-mimics in sustaining the surface water content induced by the strong capillary pressure gradient in such a dual porosity system. Finally, we track the evaporation sites using the fluorescein dye particles method and assess its effect (if any) on the overall evaporation characteristics.