The liquid-liquid mixing process coupled with chemical reactions in a mini-scale jet mixer was visualized by reactive laser induced fluorescence (reactive-LIF) technique for a deep understanding of the interplay between the mixing and the simultaneous reactions. A novel approach for implementing the reactive-LIF measurements was advanced in this work, where the principle was based on the quenching of the fluorescence using the mechanism of Fenton reaction. Different from the literature reports using the expensive dyes or UV-laser for visualizing the concentration field, Rhodamine B (absorption spectrum: 460 - 590 nm, max = 550 nm; emission spectrum 550 - 680 nm, max = 590 nm) was employed as the fluorescent tracer, and a diode pumped solid state continuum laser with the characteristic wavelength at 532 nm was adopted in our experiments. Fenton reaction was introduced to the mixing process as a model reaction, where the reaction rate for quenching the fluorescence signal was on the order of magnitude of 100 L/(mol•sec). By comparing the concentration fields under different operating conditions, i.e., the different momentum ratios between the jet and the bulk flows, and the different Reynolds numbers in the mixing channel, the purely physical mixing and the reactive mixing processes were investigated extensively. The results revealed that the transient dynamics in the reactive mixing process cannot be precisely understood based on the time-averaged concentration fields from either the physical or the reactive mixing measurement. While, the mixing and the reaction can be decoupled into two isolated processes by tuning the operation conditions.

Using the proposed reactive-LIF method, liquid-liquid mixing, mass transfer and reaction in either the single phase or the multiphase flows can be easily studied in a quantitative way. We are also exploring the similar measurement technique to visualize the gas-liquid mass transfer and reaction behaviors.