The formation of isolated mixing regions, IMRs, corresponding to the green areas shown in Fig. 3f, is a consequence of a lack of mixing with the acidic fluids in the AMRs, causing the pH to remain basic and fluorescent dye green in the IMRs. The colour of AMRs, on the other hand, changed significantly due to substantial change in pH as a result of the neutralization reaction by mixing with acid fluid injected at the blade tip, located in the AMRs. The “doughnut-shaped” IMR structure was found to be unchanged in size for a few hours (∼3 h) in our experiments, implying that materials exchange between AMRs and IMRs is extremely poor.