Precipitate Patterns in a Hele-Shaw Cell with Small Sinusoidal Height Variations

Inorganic precipitation reactions can self-organize a multitude of complex macroscopic patterns. The striking precipitate tubes in chemical gardens are a classic example which recently have been studied in the context of spatially confined, thin layers. Here, we introduce stationary, spatially periodic perturbations to this type of pattern formation by using computerized milling procedures for the production of nonplanar Hele-Shaw cells. The injection of NiCl2 solutions into NaOH-filled cells forms precipitate patterns that include lobes, deformed circles, worms, and filaments. Of these structure types, lobes and deformed circles respond most strongly to the introduced height variations, whereas filamentsthe closest analog of chemical garden tubesare essentially unresponsive. The effect of the perturbations also strongly depends on the density difference between the injected and displaced fluids and involves preferred penetration directions for lowered regions. Our experimental technique and observations could be of relevance to research on fluid flow and precipitation reactions in sedimentary rock with inhomogeneous porosities.