Hygroscopic hysteresis drives intermittent salt creeping
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 salt creeping is an intrinsically intermittent, out-of-equilibrium process. By systematically varying the initial salt concentration and the ambient relative humidity, we identify regimes in which crystal deposition on the outer capillary surface goes hand in hand with non-monotonic, intermittent dynamics. Time-resolved measurements reveal that these intermittent dynamics are sustained by episodic water imbibition into the growing salt structures on the outer surface of the capillary, which sets up a self-amplifying feedback between evaporation and crystallization. Combining experiments with a minimal theoretical model, we demonstrate that hysteresis between deliquescence and efflorescence concentrations is sufficient to generate oscillatory salt accumulation and intermittent dynamics. Hygroscopic hysteresis, in other words, is the switch that turns steady evaporation into intermittent creeping. Our results recast salt creeping as a relaxation oscillator, and point to the hysteretic phase change as a generic route to intermittency in evaporating multicomponent fluids.