Near-infrared imaging of interfacial instability between aqueous acid and base solutions

Abstract Buoyancy-driven instabilities often occur at the interface between two miscible reactive solutions due to complex density stratifications arising from differences in solutal concentrations and diffusion coefficients. In this study, to visualize and characterize these instabilities, two-dimensional concentration maps of sulfuric acid (H 2 SO 4 ), sodium hydroxide (NaOH), and sodium sulfate (Na 2 SO 4 ) were simultaneously obtained for a system where an aqueous H 2 SO 4 solution was layered over aqueous NaOH in a microfluidic channel and a modified Hele–Shaw cell. This measurement method exploits the distinct spectral variations of the near-infrared water absorption band induced by these species. The resulting maps revealed that the H 2 SO 4 and NaOH regions remained separated, with the product Na 2 SO 4 forming at the interface. Subsequently, the interface underwent fluctuations, triggering upward convection plumes from the H 2 SO 4 –Na 2 SO 4 boundary and downward finger-like plumes from the Na 2 SO 4 –NaOH boundary. Density profiling revealed distinct driving mechanisms for these asymmetric instabilities: a locally unstable stratification (heavy over light) at the upper interface drove Rayleigh–Taylor convection, whereas the lower interface remained statically stable, indicating that the downward fingers were driven by double-diffusive instability. Graphical abstract