Crystallisation triggered by mass diffusion at a lower local supersaturation

Crystallisation is fundamental to many natural and industrial processes and is strongly influenced by temperature and concentration. While extensively studied under equilibrium conditions, crystallisation under non-equilibrium conditions remains less explored. Here, we demonstrate crystallisation of potassium chloride in aqueous solution driven by thermodiffusion and isothermal diffusion. The supersaturation at which crystallisation occurs is first determined through cooling crystallisation. Under thermophobic thermodiffusion, crystals formed at a lower local supersaturation compared to isothermal systems. Similarly, under isothermal diffusion between the same supersaturated solution and a lower-concentration solution, crystallisation occurred at lower concentrations and a higher temperature; thus lower local supersaturations. In particular, crystals formed in regions of the maximum concentration gradient rather than the maximum supersaturation. These findings provide macroscopic experimental evidence that non-equilibrium processes can narrow the metastable zone width, underscoring the critical role of spatial temperature and concentration control in triggering crystallisation. This work has broad implications for technologies that require precise crystallisation control.