Controlling salt crystallization in evaporating thin films of colloidal liquids

Studying crystallization kinetics of mineral salts during desiccation of thin films of the complex colloidal salt matrix is a topic of considerable research interest due to its direct relations to the structures, property of materials, and utmost importance for controlling crystallization and colloidal products design. In this article, the shape and size distribution of macro-structures induced by the crystallization mechanism were systematically investigated using experimental and numerical methods. In the experiments, Sodium Chloride crystallization in starch or Xanthan solutions is considered as a model system. It is found that quantified parameters such as evaporation rate and local viscosity of the medium mainly govern nucleation and propagation of the ramified dendritic patterns leading to several categories of fractal aggregates, such as river patterns, cross-shaped patterns, and finger-like structures. The fractal dimension of the patterns is independent of the salt concentrations but changes with the initial concentration of the thickener. The effect of evaporation rate and the medium viscosity on the dynamics of crystallization for pattern formation was simulated numerically by two modified models based on diffusion-limited aggregation (DLA) and the deposition, diffusion, and aggregation (DDA) algorithms which enable us to successfully reproduce the experimental results. Our experimental results and numerical approaches open avenues for controlling the size and shape of crystals and patterns formed in a complex gel matrix.