The effect of sand grain roughness on the grain-scale spatial distribution of grain-surface precipitates formed by evaporation

This work examined the precipitation patterns formed by two solutes (sucrose, sodium chloride) on individual sand grains after evaporation from aqueous solution. Experiments were conducted by placing droplets of solution on individual grains, allowing them to evaporate, and then imaging the resulting precipitates. Grains were cleaned and used for multiple experiments with different solution concentrations. Precipitation patterns were studied through a combination of scanning electron microscope (SEM) imaging and energy dispersive x-ray spectroscopy (EDS). The results of the work indicate that sodium chloride precipitates were formed on ridges on the grain surfaces, while sucrose precipitates were formed in valleys. The difference in the behavior was consistent with the hydraulics of film flow out of low points on grain surfaces during evaporation (a phenomenon sometimes referred to as the coffee-ring effect), and the different viscosities of the two solutes near their solubilities. Simulations were able to produce precipitation patterns on grain surfaces consistent with experimental observations. Based on the results of the work, it is likely that most solutes will exhibit precipitation on ridges; only those with extremely high viscosities at their solubility (e.g. hundreds of mPa s) might be expected to be trapped in valleys.