Evaporation with Salt Crystallization in Capillaries of Different Cross Sections

Evaporation-induced salt crystallization in complex porous structures is highly important for diverse scientific and industrial fields. Individual capillary tubes are elementary components used for investigating flow and transport in the interparticle interstices of porous media. In this study, the effects of the angularity and size of capillary tubes on water evaporation and salt crystallization were investigated through monitoring the receding meniscus, salt crystal morphology and growth process in capillary tubes with different cross sections. The Stefan diffusive and two-regional models were used to simulate evaporation from capillaries of different cross-sectional shapes in the absence and in the presence of salt, respectively. The evaporation process of deionized water in round tubes could be divided into two stages: the falling rate and the receding front stages. However, the evaporation process of deionized water for the square tubes, where a liquid film was formed, could be divided into three stages: a constant rate, receding front and falling rate stages. The salt evaporation rate was lower than that of deionized water owing to the lower water activity and obstruction from the salt crystals. The evaporation rate was proportional to the tube diameter for the round capillaries and inversely proportional to the inner side length of the square capillaries for both deionized water and the salt solution. Owing to the effect of thick liquid films on both the drying rate and ion transport, crystallization occurred in the bulk meniscus within a round tube, while crystallization preferentially occurred at the tube entrance for the square tubes. The agreement between the experimental observations and model simulations revealed that the two-region model was capable of describing the evaporation-induced salt crystallization in the square tubes.