  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.
