Spatial Heterogeneity and Imperfect Mixing in ChemicalReactions: Visualization of Density-Driven Pattern Formation

摘要

Imperfect mixing is a concern in industrial processes, everyday processes (mixing paint, bread machines), and in understandingsalt water-fresh water mixing in ecosystems. The effects of imperfect mixing become evident in the unstirred ferroin-catalyzedBelousov-Zhabotinsky reaction, the prototype for chemical pattern formation. Over time, waves of oxidation (high ferriinconcentration, blue) propagate into a background of low ferriin concentration (red); their structure reflects in part the historyof mixing in the reaction vessel. However, it may be difficult to separate mixing effects from reaction effects. We describe a simplermodel system for visualizing density-driven pattern formation in an essentially unmixed chemical system: the reaction of paleyellow Fe~(3+)with colorless SCN~-to form the blood-red Fe(SCN)~(2+)complex ion in aqueous solution. Careful addition of one dropof Fe(NO3)3 to KSCN yields striped patterns after several minutes. The patterns appear reminiscent of Rayleigh-Taylor instabilitiesand convection rolls, arguing that pattern formation is caused by density-driven mixing.