Evaporation Dynamics and NaCl Precipitation on Capillarity-Coupled Heterogeneous Porous Surfaces

What is it about?

Evaporation of saline water from porous media and associated salt deposition are important for many applications ranging from soil salinization to the protection of archeological and civil structures. We investigate the effects of textural heterogeneity of porous media on evaporative salt precipitation patterns. Textural heterogeneity has been introduced into sand columns to form capillary-interacting porous domains comprised different particle sizes. Several levels of textural contrasts were experimentally studied in terms of their effects on drying rates, salt precipitation patterns, and drying front displacement rates. Results show that in contrast with evaporation of pure water, evaporation of saline solutions from media with textural contrasts exerted only a minor effect on evaporation rates. This difference is attributed to enhanced liquid transport within the highly porous precipitated salt on the surface. The presence of textural contrasts significantly enhanced preferential spread of salt precipitation on the surface. The enhancement is attributed to low surface water content of the coarse domain and larger spacing between remaining evaporating wet pore clusters in which greater local evaporation rates are accelerating salt precipitation. The study illustrates the interactions between the water characteristic curve, preferential drying front displacement and subsequent salt deposition patterns, as also seen in high resolution X-ray Computed tomography (CT) used to verify the suggested mechanisms. The study provides new insights into mechanisms of saline water evaporation and the complex effects of porous media heterogeneity on salt precipitation patterns and dynamics; all critical ingredients for enhanced understanding of the salt distribution in and transport in soils.

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