Matalon–Packter law for stretched helicoids formed in precipitation processes

Helicoid-like precipitation structures emerging in the wake of reaction–diffusion fronts are studied experimentally as well as theoretically. We find that the helicoids are stretched, their local pitch behind the advancing front increases exponentially. We compare this result to the exponential increase of the band spacing in Liesegang phenomena. The spacing coefficient (p) characterizing the exponential increase satisfies the same Matalon–Packter law in both cases, i.e. p ∼ 1/a0 where a0 is the initial concentration of the outer electrolyte in the experimental setup. Our experiments also reveal that, at the microstructure level, the helicoids are assembled from building blocks of micron-size achiral spherulites.