Mixing time of homogeneous/heterogeneous solutions in a micro-mixer with free impinging jets

We have developed a micro-mixer based on a free impinging liquid-sheet jet technique. We identified a mixing position where two different solutions mixed uniformly and evaluated a corresponding mixing time in the liquid-sheet jets with homogeneous combination (water and water) and heterogeneous combination (ethanol and water). Both combinations could produce the liquid-sheet jet with a length of 3-4 mm, which corresponds to a time-range of longer than 100 μs. By observing a quenching reaction of N-Acetyl-L-tryptophan amide by N-Bromosuccinimide, the mixing times were evaluated to be 36 μs for the homogeneous combination (H2O/H2O), and 46 μs for the heterogeneous combination (C2H5OH/H2O). To clarify the mixing mechanism in the liquid-sheet jet, the theoretical mixing times were calculated by two different models assuming laminar and turbulence flows. Simulations based on molecular diffusion across a well-defined interface for the laminar flow showed a large discrepancy with the experimental results. The calculated mixing times based on energy dissipation in the turbulence flow are in great agreement with the observed mixing times for both H2O/H2O and C2H5OH/H2O combinations. These results indicate that turbulent mixing is a dominant mixing mechanism in the liquid-sheet jet, and that no clear interface is formed between H2O solutions and between C2H5OH and H2O solutions. The liquid-sheet jet technique provides a windowless and ultra-thin target, ideal for applications with X-ray or intense laser pulses, and would be useful to investigate intermediates in mixing-driven chemical reactions such as an oxidation in solution and a folding reaction of proteins proceeding in a microsecond time scale.