Zig-zag structures in silver dichromate precipitate

Precipitation patterns are commonly concentric rings forming in a Petri dish or parallel bands appearing in a test tube (Liesegang phenomenon). The rings frequently consist of a number of convex segments that are separated from each other by spaces devoid of precipitate resulting in small gaps (dislocations). Along these gaps, the so-called zig-zag structures can form, which connect one side of a gap with its opposite side. We observe that the occurrence of zig-zags requires a minimum thickness of the reactive layer (≥ 0.8 mm). This fact together with microscopic evidence indicates their three-dimensional character. One finds that at the very beginning of the precipitation reaction a curling process starts in the corresponding contour lines. These observations suggest structures of a helicoid with the axis perpendicular to the plane of the reaction–diffusion front to pass through the layer. Zig-zags are not parallel to the reaction plane, i.e., they are not formed periodically, but evolve continuously as a rotating spiral wave. Thus, their topology is closely related to helices in a test tube.