Discrete salt crystallization at the surface of a porous medium

Abstract

Efflorescence refers to crystallized salt structures that form at the surface of a porous medium. The challenge is to understand why, as is commonly observed, these structures do not form everywhere at the surface of the porous medium but at some specific locations and why there exists an exclusion distance around an efflorescence where no new efflorescence forms. To better understand efflorescence, experiments have been performed to investigate the factors that control the spatial organization of the salt crystals. A porous material is created by packing millimeter-sized glass beads into a Teflon cylinder. The cylinder is suspended vertically, with the bottom portion submerged in a sodium chloride solution. By capillary action, liquid wicks up to the top through the spaces between the beads. As the water evaporated from the top surface, it leaves salt crystals behind. By varying the evaporation flux distribution at the porous medium surface, it is first shown that the salt crystallizes preferentially where the evaporation flux is higher. Then pore network simulations are performed to show that the velocity fluctuations in the porous material due to the spatial fluctuations in sizes of pores and throats induce spatial fluctuations in the salt concentration at the porous medium surface. The concentration maxima correspond to the locations of crystallization onset and explain the discrete nature of efflorescence formation. From previous works, it is known that efflorescence is porous. As a result, instead of blocking evaporation at the pore, a salt structure boosts the rate of evaporation by providing more surface area from which the fluid could evaporate. The increased evaporation draws up fluid even faster along this high-speed pathway. In response, the flow through neighbouring pathways would slow, and the corresponding pores would be “starved” of salt ions. This process explains why salt crystals continue to growth in separated structures rather than forming a uniform coat.