Crossover between Spatially Confined Precipitation and Periodic Pattern Formation in Reaction Diffusion Systems

We have observed a crossover between a spatially confined precipitation regime and periodic pattern formation regime. This unusual behavior was observed when electrolyte solutions of ${\mathrm{Na}}_2{\mathrm{HPO}}_4$ and ${\mathrm{CaCl}}_2$ were allowed to diffuse into an agarose gel from opposite ends. The formation of the confined precipitate occurs when the electrolyte flux $J$ is the same at both sides of the gel. The time of formation and the width of the precipitate are a function of $J$ and both follow the scaling relation $\omega \sim (J/D{)}^{-\beta }$ with $\beta =0.40\mathrm{}\pm 0.2$ and $D$ the diffusion coefficient. The growth of periodic bands of precipitate was observed when $J$ was different at both gel ends.