On the Creeping of Saturated Salt Solutions

Creeping is a well-known but annoying phenomenon in the preparation of crystals from solution, where growing crystallites gradually extend up the walls of the growth vessel. In this process, solution is transported toward the tip of the creeping crystallites, where solvent evaporation takes place and solid material is deposited. In this study, the growth of crystal aggregates extending from evaporating droplets of saturated aqueous solutions of ionic salts, placed on different substrate materials, has been investigated using optical microscopy. It is shown that the rate determining step of the crystallization process is the evaporation of solution, following Fick's laws. Fresh solution, necessary to continue the growth process, is supplied by liquid flow along the growing crystallites. This can take place aside and on top of the crystallites (top supplied creeping, TSC) or in the narrow space between the crystallites and the substrate (bottom supplied creeping, BSC). The occurrence, mode (TSC or BSC), and velocity of creeping is shown to be determined by the relative humidity of the ambient air, the various interfacial energies involved, and the shape and size of the growing crystallites. In a number of cases, seaweed-like patterns are formed by repeated side-branching of the growing aggregates, induced by three-dimensional nucleation of secondary crystallites.