Precipitation-dissolution cycling drives intermittent salt creeping

Abstract

Salt creeping --- the precipitation of salt crystals away from an evaporating liquid interface along surrounding surfaces --- occurs across settings from geology and cultural-heritage weathering to inkjet printing and carbon sequestration. Yet why its dynamics are sometimes smooth and sometimes violently intermittent has remained unexplained. Here we investigate the confined evaporation of salt solutions from a capillary with unidirectional water loss, and show that intermittent salt creeping is driven by a cycle of precipitation and dissolution sustained entirely by liquid transport. Time-resolved measurements reveal that each burst of crystal growth coincides with episodic imbibition of solution into the growing salt deposit on the outer capillary surface, which couples evaporation to crystallization. Combining experiments with a minimal theoretical model, we show that the difference between the supersaturation at which crystals nucleate and the saturation concentration at which they redissolve --- the metastable zone of the salt --- is sufficient to generate this cycle. The same mechanism accounts for the cases in which creeping is not intermittent: where the concentration at the capillary exit never reaches the nucleation threshold the liquid meniscus recedes smoothly, and where it cannot subsequently fall back below saturation the deposit grows continuously, exhibiting self-amplified creeping. Our results recast salt creeping as a relaxation oscillator, and identify precipitation-dissolution cycling as a generic route to intermittency in evaporating multicomponent fluids.