Real-Time Visualization of Turbulent Micromixing in a T-Mixer: Submillisecond Homogenization Revealed by...

The increasing adoption of continuous-flow chemistry has shifted attention toward transport-controlled phenomena, particularly mixing, as critical determinants of reaction performance. Mixing time is a fundamental yet often ill-defined parameter in flow chemistry, particularly under turbulent, high-throughput conditions where mixing is commonly assumed rather than directly measured. Here, we demonstrate direct measurement of the mixing time in a turbulent T-junction micromixer by using chemiluminescence as a real-time mixing clock that reports hydrodynamic homogenization. Because the intrinsic reaction time of the luminol chemiluminescence system is orders of magnitude shorter than the residence time, the temporal evolution of light emission directly reflects the progression of scalar homogenization, separated from chemical kinetics or molecular diffusion. Applying this concept to a transparent T-mixer with an effective channel diameter of 500 μm, we access Reynolds numbers exceeding 6000 while maintaining pressure drops below 0.5 MPa, a regime relevant to practical flow synthesis. Under these conditions, submillisecond micromixing is directly observed, with mixing times below 0.5 ms. This micromixing time is shorter than the previously reported micromixing time of 1.3 ms in the 250 μm T-mixer. The Péclet number of the system exceeds 10⁷, indicating that turbulent stretching-dominated homogenization drives such quick micromixing even in the large-diameter T-mixer used in flow chemistry. By experimentally defining mixing time through direct observation of homogenization, without invoking reaction models or assumed micromixing parameters, this work establishes a general framework for designing mixers and reaction lengths for better utilization of short-lived intermediates and high-throughput flow chemistry.