Crystallisation triggered by mass diffusion at a lower local supersaturation

Crystallisation is fundamental to many natural and industrial processes. It is influenced by various non-equilibrium factors such as thermal history, mechanical perturbations, and flow, yet the effect of imposed mass fluxes on the supersaturation ration at which crystallisation first becomes macroscopically observable remains uncharacterised. Here, we show experimentally that thermodiffusive and isothermal diffusive mass fluxes can cause aqueous potassium chloride to crystallise at lower local supersaturation ratios than in spatially isothermal reference systems. A reference supersaturation ratio was first established using cooling crystallisation, where temperature varies in time but remains spatially uniform. Under thermophobic thermodiffusion, the first appearance of crystals occurred at a lower local supersaturation ratio than this equilibrium benchmark. Likewise, under isothermal diffusion between a supersaturated solution and a lower-concentration reservoir, crystallisation occurred at lower concentrations and higher temperatures than expected under spatially uniform conditions. In both configurations, crystallisation consistently initiated in regions of steep concentration gradients rather than at locations of maximum supersaturation ratio. These results provide macroscopic evidence that non-equilibrium mass fluxes can narrow the metastable zone width, emphasising the importance of spatially varying temperature and concentration fields in controlling crystallisation. The findings have broad implications for processes requiring precise crystallisation control.