Although stirred tanks have been the most commonly used fluid mixing devices since the beginnings of the industrial era, little is known about how and why they mix, and, therefore, about how to improve their performance, particularly when operated in the laminar regime. In laminar conditions, chaos is the only route to achieve efficient mixing. From the body of research in 2-D chaotic flows, very minute perturbations in the velocity field can lead to widespread chaos and substantial enhancement of mixing performance, but these observations have not been systematically applied to industrially relevant 3-D flows. Using planar laser induced fluorescence and direct numerical simulations, the mechanisms of creation and evolution of mixing structures were studied in Newtonian and non-Newtonian flows. The observed dynamical behaviors are related to geometric features of the system. In Newtonian flow systems, the passing of the impeller blades triggers the onset of chaos by introducing small perturbations to the underlying regular 2-D flow that is observed when impellers are substituted with discs. In non-Newtonian viscoelastic systems, nonlinearity in the stress field introduced by the fluid rheology drives the system to spontaneous chaos even when concentric discs are used to stir the fluid.