Rapid mixing by turbulent-like electrokinetic microflow

- The work characterizes novel rapid turbulent mixing in a microchannel at relatively low Re in the viewpoint of statistical turbulence.
- The work opens a new avenue by redefining the limits where turbulent mixing can be present.
- The work provides a new perspective view on turbulent mixing at microfluidic length scales.

Rapid mixing has been achieved in a pressure-driven microflow which is forced under AC electric field. In our previous investigations, the mixing on large scale at the centerline of microchannel has been studied. Here, we show that the evaluation on mixing effect by flow visualization could be misleading, if its temporal resolution is not sufficiently high. By using single-point laser induced fluorescence (LIF) method, the mixing on both large and small scales can be investigated with high spatiotemporal resolution. It is found fast mixing is not only achieved at the centerline, but also in the majority of cross-sectional area, even near the bottom wall region. This is resulted from the large scale secondary flow due to unbalanced AC electroosmotic flow near the bottom wall. The temporal −5/3 spectrum of concentration measured with LIF in our previous investigations is also supported by the spatial spectrum of concentration. The physical process of mixing on small scales is further investigated by the flatness of concentration gradients.