Kinetics of spatially confined precipitation and periodic pattern formation

We have performed experiments and computer simulations of the formation of two types of precipitate profiles which appeared when electrolyte solutions of Na2HPO4 and CaCl2 are allowed to diffuse into an agarose gel from both ends of the gel. The formation of a spatially confined precipitate occurred when the electrolyte flux J was the same at both sides of the gel, whereas the growth of periodic bands was observed when J was different at both ends of the gel. The kinetics of formation of both types of precipitation profiles indicated that the initial position of the lateral bands in the precipitation front, controlled by the electrolyte flux, would determine if the precipitate is periodic or spatially confined. For the case of equal flux, in the investigated time interval, all the bands grow linearly in intensity and the initial narrowing of the central band reverses this tendency after the appearance of the lateral bands. For the case of unequal flux, the exponential intensity increase of the Liesegang band leads to a saturation of the intensity of the broad central band which widens as a function of time. Using the experimental conditions as a guideline for the simulations, we were able to reproduce both types of precipitation patterns.