Flow‐Driven Precipitation Patterns with Microemulsions in a Confined Geometry

Abstract

AbstractPrecipitation reactions occurring under fluid flow can self‐organize a variety of complex spatiotemporal patterns. Herein, we investigate the structures formed when a CoCl2‐containing water‐in‐oil microemulsion is injected into an aqueous NaOH solution. The reacting liquids are confined to the thin gap of a horizontal Hele‐Shaw cell and injection is carried out from a central port. Dark‐field detection of the patterns reveals borders that can deviate from smooth circles by either jagged or sinusoidal deformations. The interior of the patterns shows speckle‐like features that can arrange along either spike‐like tracks or concentric circular segments. The area ratio of light scattering regions increases with increasing flow rates but decreases sharply with increasing NaOH concentration. The latter transition is interpreted as a shift from patterns dominated by the physical breakdown of the microemulsion to patterns strongly affected by the precipitation of α‐Co(OH)2.