The Effects of Evaporation and Capillary Forces on Fluid Motion, Solute Transport, and Water Content From the Grain Scale to the Continuum Scale in Unsaturated Porous Media
Abstract
This work aimed to explore how fluid motion, solute transport, and water content are influenced by evaporation and capillary forces in unsaturated porous media, across a range of scales. Experiments were conducted from the grain scale to the continuum scale to explore a range of related flow phenomena.
The effect of solid surface roughness on precipitate formation has been widely studied for several years in different disciplines. However, few studies exist which studied precipitation on natural sand grains. In this study, the patterns of deposits formed on individual natural sand grain surfaces from solutions of sodium chloride and sucrose were studied. The patterns were studied using Scanning Electron Microscope (SEM) imaging and Energy Dispersive Spectroscopy (EDS). Results showed that as evaporation progressed, flow tended to occur towards the ridges, likely as a result of the discrepancy between the water configuration demanded by capillarity and the spatial distribution of evaporation. In the case of the sodium chloride solution, this led to sodium chloride precipitates being deposited on the ridges. For the sucrose solution, the outward flow was retarded by its viscosity at high concentrations. As a result, the sucrose was trapped in the valleys and precipitated there.
The impact of surface roughness on liquid wicking over a dry surface was also studied. While several studies exist on wicking on engineered surfaces, very few studies exist on wicking over natural surfaces. This study examined wicking patterns on dry individual sand grains. The roughness of the grains was studied using a combination of SEM imaging and edge detection to generate edge density maps. The wicking pattern was studied by taking regular images of the grain using an optical microscope. Sulforhodamine B dye was used in the water to make it easy to see the water on the sand grains. The results suggest that microporosity is required for rapid wicking. On grains which exhibited wicking, the liquid was wicked onto the rougher portion of the grain as shown by the edge density map. However, between grains, edge density maps alone are not sufficient to predict flow.
Finally, the effect of evaporation on capillary pressure-saturation (Pc-S) curves of two porous media was studied. The media were saturated and then allowed to drain. A test sample was exposed to evaporation after drainage while the control sample was not. Both samples were backlit, and images were taken at regular interval as the experiment progressed. The images were analyzed using a computer program which related light transmission to saturation. Results showed that evaporation lowered the drainage curve of both samples towards the imbibition curves. The rate of the lowering depended on the rate of evaporation experienced by the sample relative to the volume of the sample.
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