Effective mixing in a microfluidic oscillator using an impinging jet on a concave surface

In this study, we present a microfluidic oscillator design that employs an impinging jet on a concave surface to enhance the microscale mixing process. The Coandă effect along with the Görtler instability proves to incite sustainable flapping motion beyond the obstacle and mixing is profoundly improved. From the flow visualization results, four different regimes are identified and we find that the primary enhancement of mixing performance is always linked to the transition of flow regime. Moreover, incorporating a sudden-expansion confluence provokes flow three dimensionality and elevates the mixing level significantly at low Reynolds numbers. For a Reynolds number as low as 70, the tail flow behind the concave obstacle successfully exhibits a periodic oscillation and Hopf bifurcation is induced, leading to a drastic augmentation in the time-average mixing efficiency. By utilizing the spectrum analysis, the characteristic frequency of flapping motion is found to vary linearly with the throat velocity, resulting in a constant Strouhal number of 3.8 × 10⁻⁵.