Propagating Fronts and Morphological Instabilities in a Precipitation Reaction

Precipitation processes are essential in many natural systems, especially in biomineralization and in geological pattern formation. We observe temporal oscillations in the total mass of the precipitate, the formation of propagating and annihilating waves, and morphological instabilities in a thin precipitation layer in a two-side-fed gel reactor containing the AlCl₃/NaOH reaction–diffusion system. Contrary to the standard Liesegang patterns, these structures form in the lateral direction at the meeting of the counterpropagating diffusion fronts of the electrolytes. The two main ingredients of the system are the amphoteric precipitate and the cross gradient of the chemicals due to the fixed boundary conditions. Simulations with a four-variable precipitation/redissolution model qualitatively reproduce the oscillations in the total mass of the precipitate and point out the stratified three-dimensional structure of the precipitate.