A well-known decolourization technique (refer to Takahashi and Motoda, 2009), involving the neutralization reaction of NaOH and HCL was used to visualize the IMRs. A passive tracer fluorescent dye, as a pH indicator, was injected and homogenized in the working fluid. In order to remove all the air bubbles, the working fluid mixed with fluorescent dye was allowed to be settled for about 12 h before conducting the experiments. Refer to Fig. 3a, the working fluid mixed with fluorescent dye was made basic by blending it with a basic solution consisting of 10 ml of 2 M NaOH. The working fluid was then stirred at 500 rpm for ∼2 h in order to fully disperse the base solution throughout the tank, leading to a uniform colour distribution (refer to Fig. 3a). After the agitator reached the test speed, a small amount of acidic solution (HCl: 10 ml 2 M) was injected at the blade tip and consequently the decolourization took place due to neutralization reaction in the active mixing regions (i.e. transparent area), as shown in Fig. 3c–f.