The liquid-liquid mixing process coupled with chemical reactions in a T-shaped micro-channel was visualized by using 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 signal emitted from the Rhodamine-B dye 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 102 L·mol-1·sec-1. Using the proposed reactive-LIF method, liquid-liquid mixing, mass transfer and reaction in both the single component and the multi-component flows can be easily studied in a quantitative way.
The purely physical mixing and the reactive mixing processes were investigated extensively by comparing the concentration ﬁelds under different operating conditions for liquid mixing (water-water mixing) and liquid-liquid mixing (water-ethanol mixing) in the micro-channels of 0.3×0.2 mm. Distinct differences were revealed in terms of the physical mixing and reactive mixing behaviors between the two mixing processes at low Re numbers. The mass transfer between different liquids can be extremely intensified by the coupled reaction process. The physical mixing process showed that the effect of driving force on liquid-liquid mass transfer due to the concentration difference was huge when the bulk flow was relatively slow. With the increasing of Re numbers, the bulk flow pattern started changing from the stratified flow to the engulfment flow, which gradually became the dominant factor to influence the mixing and reaction process. Compared to the medium Re numbers (100-400), the mixing performance was better at both low (less than 100) and high (more than 400) Re numbers. This phenomenon was also demonstrated by the preparation of nano-drugs using the anti-solvent precipitation process. Smaller curcumin nano-precipitation can be achieved at both low and high Re numbers than at the medium Re numbers.