Liquid-phase continuity and solute concentration dynamics during evaporation from porous media: pore-scale processes near vaporization surface

Evaporation-Driven Density Instabilities in Saturated Porous Media

Soil salinization is a major cause of soil degradation and hampers plant growth. For soils saturated with saline water, the evaporation of water induces accumulation of salt near the top of the soil. The remaining liquid gets an increasingly larger density due to the accumulation of salt, giving a gravitationally unstable situation, where instabilities in the form of fingers can form. These fingers can, hence, lead to a net downward transport of salt. We here investigate the appearance of these fingers through a linear stability analysis and through numerical simulations. The linear stability analysis gives criteria for onset of instabilities for a large range of parameters. Simulations using a set of parameters give information also about the development of the fingers after onset. With this knowledge, we can predict whether and when the instabilities occur, and their effect on the salt concentration development near the top boundary.

Modeling of Evaporation-Driven Multiple Salt Precipitation in Porous Media with a Real Field Application

Soil and groundwater salinization are very important environmental issues of global concern. They threaten mainly the arid and semiarid regions characterized by dry climate conditions and an increase of irrigation practices. Among these regions, the south of Tunisia is considered, on the one hand, to be a salt-affected zone facing a twofold problem: The scarcity of water resources and the degradation of their quality due to the overexploitation of the aquifers for irrigation needs. On the other hand, this Tunisian landform is the only adequate area for planting date palm trees which provide the country with the first and most important exportation product. In order to maintain the existence of these oases and develop the date production, a good understanding of the salinization problem threatening this region, and the ability to predict its distribution and evolution, should not be underestimated. The work presented in this paper deals with the Oasis of Segdoud in southern Tunisia, with the objective of modeling the evaporation-driven salt precipitation processes at the soil profile scale and under real climatic conditions. The model used is based on the one developed and presented in a previous work. In order to fulfil the real field conditions, a further extension of the geochemical system of the existing model was required. The precipitated salts considered in this work were halite (NaCl), gypsum (CaSO4) and thenardite (Na2SO4). The extended model reproduces very well the same tendencies of the physico-chemical processes of the natural system in terms of the spatio-temporal distribution and evolution of the evaporation and multiple-salt precipitation. It sheds new lights on the simulation of sequences of salt precipitation in arid regions. The simulation results provide an analysis of the influence of salt precipitation on hydrodynamic properties of the porous medium (porosity and permeability). Moreover, the sensitivity analysis done here reveals the influence of the water table level on the evaporation rate.

The Evapotranspiration of Tamarix and Its Response to Environmental Factors in Coastal Saline Land of China

(1) Background: As a halophytic species, Tamarix (Tamarix chinensis) can be used for saline soil rehabilitation in China. The reclamation and rehabilitation of saline soil depend on the water consumption of plants. However, whether water resources in saline soil can support the construction of Tamarix vegetation is still unknown. (2) Methods: In this study, we measured the transpiration (T) of Tamarix for 3 years using sap flow and the evaporation (E) for 1 year using a micro-lysimeter in Tamarix land. The evaporation values in 2016 and 2017 were estimated with the soil crop coefficients obtained in 2018. (3) Results: The evapotranspiration (ET) ranged from 514.2 to 573.8 mm and was greatly affected by the wind speed, VPD and groundwater table. Transpiration was the main form of water consumption in this region, accounting for 60.2% of the total evapotranspiration. Compared with bare land, vegetation construction increased soil moisture dissipation by 377.6 mm in 2018. According to on-site measurements and estimates, the water shortage in the dry year was 107.2 mm, and the residual water values in the normal year and wet year were 77.8 mm and 187.5 mm, respectively. May and September were months of widespread water shortages in different precipitation years. Although the cultivation of this plant increased water consumption, the groundwater table remained at approximately 0.5 m during the study year. (4) Conclusions: These results indicated that planting Tamarix in coastal saline soil was feasible for the reclamation and rehabilitation of saline soil. In the dry year (2017), the consumption of evapotranspiration exceeded the precipitation. The inverse occurred in the normal year (2016) and wet year (2018). Taken together, our findings showed that the water resources in the coastal saline soil of China could tolerate vegetation construction and laid a strong foundation for saline soil rehabilitation.

Iodine k-edge dual energy imaging reveals the influence of particle size distribution on solute transport in drying porous media

Increasing salinity in groundwater and soil poses a threat to water and land resources. With the expectation of major changes to the hydrological cycle through climate change, the need for understanding the fundamental processes governing solute transport through soil has grown significantly. We provide experimentally verified insights into the influence of particle size distribution on solute transport in porous media during evaporation at the pore- and macro-scales. To do so, we utilized four-dimensional (space plus time) synchrotron X-ray tomography for iodine k-edge dual energy imaging to obtain solute concentration profiles in every single pore during saline water evaporation from coarse- and fine-grained sands. Close to the surface of the coarse-grained sand significantly higher salt concentrations were observed when compared to fine-grained sand with the same porosity under similar cumulative evaporative mass losses. The physics behind this behaviour was delineated using the recorded data with high spatial and temporal resolutions. Moreover, the measured data enabled us to quantify the variations of the effective dispersion coefficient during evaporation and how it is influenced by the particle size distribution. We show that, contrary to common assumption in modelling of solute transport during evaporation, the effective dispersion coefficient varies as a function of liquid saturation and the length of the invaded zone during evaporation from porous media, and that it increases as liquid saturation decreases.

Drying in a microfluidic chip: experiments and simulations

We present an experimental micro-model of drying porous media, based on microfluidic cells made of arrays of pillars on a regular grid, and complement these experiments with a matching two-dimensional pore-network model of drying. Disorder, or small-scale heterogeneity, was introduced into the cells by randomly varying the radii of the pillars. The microfluidic chips were filled with a volatile oil and then dried horizontally, such that gravitational effects were excluded. The experimental and simulated drying rates and patterns were then compared in detail, for various levels of disorder. The geometrical features were reproduced well, although the model under-predicted the formation of trapped clusters of drying fluid. Reproducing drying rates proved to be more challenging, but improved if the additional trapped clusters were added to the model. The methods reported can be adapted to a wide range of multi-phase flow problems, and allow for the rapid development of high-precision micro-models containing tens of thousands of individual elements.

Morphometric analysis of polygonal cracking patterns in desiccated starch slurries

We investigate the geometry of two-dimensional polygonal cracking that forms on the air-exposed surface of dried starch slurries. Two different kinds of starches, made from potato and corn, exhibited distinguished crack evolution, and there were contrasting effects of slurry thickness on the probability distribution of the polygonal cell area. The experimental findings are believed to result from the difference in the shape and size of starch grains, which strongly influence the capillary transport of water and tensile stress field that drives the polygonal cracking.

Adiabatic burst evaporation from bicontinuous nanoporous membranes

Evaporation of volatile liquids from nanoporous media with bicontinuous morphology and pore diameters of a few 10 nm is an ubiquitous process. For example, such drying processes occur during syntheses of nanoporous materials by sol-gel chemistry or by spinodal decomposition in the presence of solvents as well as during solution impregnation of nanoporous hosts with functional guests. It is commonly assumed that drying is endothermic and driven by non-equilibrium partial pressures of the evaporating species in the gas phase. We show that nearly half of the liquid evaporates in an adiabatic mode involving burst-like liquid-to-gas conversions. During single adiabatic burst evaporation events liquid volumes of up to 10(7) μm(3) are converted to gas. The adiabatic liquid-to-gas conversions occur if air invasion fronts get unstable because of the built-up of high capillary pressures. Adiabatic evaporation bursts propagate avalanche-like through the nanopore systems until the air invasion fronts have reached new stable configurations. Adiabatic cavitation bursts thus compete with Haines jumps involving air invasion front relaxation by local liquid flow without enhanced mass transport out of the nanoporous medium and prevail if the mean pore diameter is in the range of a few 10 nm. The results reported here may help optimize membrane preparation via solvent-based approaches, solution-loading of nanopore systems with guest materials as well as routine use of nanoporous membranes with bicontinuous morphology and may contribute to better understanding of adsorption/desorption processes in nanoporous media.

Pore-scale dynamics of salt precipitation in drying porous media

We study the pore-scale dynamics of salt precipitation in three-dimensional drying porous media, utilizing high resolution x-ray microtomography and scanning electron microscopy. Our results illustrate that the salt precipitation patterns in drying porous media are nonuniform, manifesting the influence of the spatial distribution of pore sizes on the dynamics of salt crystallization and formation of discrete efflorescence. Results reveal that during stage-1 evaporation from saline porous media, the salt precipitation rate initially increases which is followed by a constant precipitation rate. This non-linear behaviour is attributed to the preferential liquid vaporization and salt precipitation in finer pores located at the surface of the porous medium contributing in evaporation according to the pore sizes. We also show that, contrary to common practice, the macroscopic convection-diffusion equation cannot provide accurate predictions for the dynamics of salt precipitation, at least at the early stages, due to the microscale heterogeneity of evaporation sites at the surface that results in salt precipitation exclusively in the finer pores.

Structure of drying fronts in three-dimensional porous media

Evaporation in a three-dimensional (3D) porous medium, a sand column saturated by water, was studied using synchrotron x-ray tomography. Three-dimensional images of the medium with a resolution of 7 μm were obtained during the evaporation. The entire column was scanned seven times, resulting in nearly 10(4) 2D cross sections and illustrating the spatial distribution of air, liquid, and solid phases at the pore scale. The results were analyzed in order to gain new insights and better understanding of the characteristics of the drying front that was formed when the liquid-filled pores were invaded by air, as well as the structure of the liquid phase as it was dried. The analysis indicates that the liquid phase has a self-similar fractal structure, with its fractal dimension D(f) in all the cross sections being a function of the water content or saturation. In addition, D(f) for the 3D liquid structure, as well as its density correlation function, were computed using the 3D images. A crossover length scale ξ was identified that separates the fractal regime from the compact geometry. For length scales r>ξ, the density correlation function approaches asymptotically the water content of the porous medium. The drying front is shown to be rough and multi-affine, rather than self-affine. Its properties were also computed using the 3D images. The roughness characteristics agree with those for imbibition in porous media, but not with those of fracture surfaces and crack lines.

Spreading profile of evaporative liquid drops in thin porous layer

Spreading of evaporative liquid drops in a thin porous layer has been studied. The entire spreading process can be divided into three distinct phases according to the change of the wetted porous region size. The first phase is characterized by the expansion of the wetted porous region and shrinking of the liquid drop. Contact line pinning is observed in the wetted porous region in the second phase even with the liquid drop totally absorbed into the porous layer. The third phase sees the shrinkage of the wetted porous region until it is not observable. Based on these observations, a model is devised to simulate the spreading of a liquid drop under the studied conditions. Partial differential equations are used to describe the relation between liquid drop volume and other important parameters of a fluid flow, including maximum wetted region diameter achieved, time taken to reach each spreading process phase, and evaporation rate. Calculated results are in good agreement with the experimental data.