Understanding solute transport in drying porous media plays an important role in a variety of processes including soil salinization, durability of building materials, and preservation of arts and monuments. During drying of porous media filled with saline solution, solutes are transported to the vaporization plane via capillary-induced liquid flow, while diffusion tends to homogenize the concentration laterally throughout the pore space. The interaction between the two determines the dynamics of solute distribution in porous media (Shokri-Kuehni et al. (2018), Sci. Rep., 10, 10731). 

We combine a variety of experimental and computational tools (including synchrotron X-ray micro-tomography, thermal imaging, pore-network modelling and direct numerical simulations) to understand and describe the physics and pore-scale dynamics of solute transport in porous media under a given boundary condition. Particularly speaking, we are interested in developing quantitative tools capable of describing the effects of transport properties of porous media (such as wettability, permeability, pore shape and size distribution), the properties of the evaporating solution and the external conditions (e.g. relative humidity and temperature) on the general dynamics of solute transport and crystallization in porous media. The following figures illustrate typical examples of the results we obtained using state-of-the-art synchrotron X-ray micro-tomography imaging technology to delineate how transport properties of porous media influence dynamics of solute transport and distribution in sandy media during evaporation. Pore-scale visualization and quantification enable us to extend the physical understanding required to describe the mechanisms constraining the macroscopic responses. 

Figure caption. 4D synchrotron X-ray microtomography was used to investigate the dynamics of salt transport and deposition during evaporation from porous media. Three-dimensional rendering of the reconstructed volume of the packed (a) coarse and (b) fine sands with the corresponding porosity presented in (c). The inset shows the particle-size distributions of the sand grains used in the drying experiments.

Figure caption. This figure illustrates a typical example showing the distribution of solute concentration within the top 1 mm of (a) coarse- and (b) fine-textured sands 6 hours after the onset of the evaporation experiments. The color map represents the solute concentration percentage, such that the closer to red, the higher the concentration. The images were recorded using 4D synchrotron X-ray microtomography (see more details in Shokri-Kuehni et al. (2018), Sci. Rep., 10, 10731, London: Nature Publishing Group).

Figure caption. (a) Salt precipitation patterns (white color) at the surface of fine sand in the presence of a water table fixed at 4 cm below the surface after 10 days from the onset of evaporation experiment. (b) The corresponding temperature distribution at the surface of fine sand. The color map indicates temperature in degrees centigrade. The closer to yellow (brighter color), the higher the temperature (see more details in Shokri-Kuehni et al. (2020), Water Resour. Res., 56, e2019WR026707. doi.org/10.1029/2019WR026707).