Coarsening of Precipitation Patterns in a Moving Reaction-Diffusion Front

Precipitation patterns emerging in a 2D moving front are investigated on the example of NaOH diffusing into a gel containing AlCl_3. The time evolution of the precipitate Al(OH)_3 can be observed since the precipitate redissolves in the excess outer electrolyte NaOH and thus it exists only in a narrow, optically accessible region of the reaction front. The patterns display self-similar coarsening with a characteristic length, xi(t), increasing with time t as xi(t) ~ t^(1/2). A theory based on Cahn-Hilliard phase-separation dynamics including dissolution is shown to yield agreement with the experiments.