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.