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Effective mixing in supersonic and hypersonic flow conditions is critical for developing next-generation high-speed air-breathing transport systems. Efficient fuel mixing with fast-moving air leads to a better economy and fewer pollutants. Ultimately the mixing is a molecular diffusion problem. However, the macroscopic phenomena, such as entrainment and vorticity dynamics resulting from the shear layer instabilities of the mixing fluids, play a significant role in the overall efficiency of the process. This paper studies a novel, coaxial injector system integrated with a high-frequency microjet actuator operating at 15.5 kHz for improving mixing in extreme flow conditions. This co-flow system consists of a highfrequency supersonic actuation air jet at the inner core that provides large mean and fluctuating velocity profiles in the shear layers of a fluid stream injected surrounding the core through an annular nozzle. The high-frequency streamwise vortices and shockwaves tailored to the mean flow significantly enhanced supersonic flow mixing between the fluids compared to a steady co-axial configuration operating at the same input pressure. This paper reports the flow mixing characteristics of the injection system captured using planar laser-induced fluorescence (PLIF).