Large-Scale Flow in Micro Electrokinetic Turbulent Mixer

A Bi-Directional Acoustic Micropump Driven by Oscillating Sharp-Edge Structures

This paper proposes a bi-directional acoustic micropump driven by two groups of oscillating sharp-edge structures: one group of sharp-edge structures with inclined angles of 60° and a width of 40 μm, and another group with inclined angles of 45° and a width of 25 μm. One of the groups of sharp-edge structures will vibrate under the excitation of the acoustic wave generated with a piezoelectric transducer at its corresponding resonant frequency. When one group of sharp-edge structures vibrates, the microfluid flows from left to right. When the other group of sharp-edge structures vibrates, the microfluid flows in the opposite direction. Some gaps are designed between the sharp-edge structures and the upper surface and the bottom surface of the microchannels, which can reduce the damping between the sharp-edge structures and the microchannels. Actuated with an acoustic wave of a different frequency, the microfluid in the microchannel can be driven bidirectionally by the inclined sharp-edge structures. The experiments show that the acoustic micropump, driven by oscillating sharp-edge structures, can produce a stable flow rate of up to 125 μm/s from left to right, when the transducer was activated at 20.0 kHz. When the transducer was activated at 12.8 kHz, the acoustic micropump can produce a stable flow rate of up to 85 μm/s from right to left. This bi-directional acoustic micropump, driven by oscillating sharp-edge structures, is easy to operate and shows great potential in various applications.

Mixing and Flow Transition in an Optimized Electrokinetic Turbulent Micromixer

Micromixer is a key element in a lab on a chip for broad applications in the analysis and measurement of chemistry and engineering. Previous investigations reported that electrokinetic (EK) turbulence could be realized in a "Y" type micromixer with a cross-sectional dimension of 100 μm order. Although the ultrafast turbulent mixing can be generated at a bulk flow Reynolds number on the order of unity, the micromixer has not been optimized. In this investigation, we systematically investigated the influence of electric field intensity, AC frequency, electric conductivity ratio, and channel width at the entrance on the mixing effect and transition electric Rayleigh number in the "Y" type electrokinetic turbulent micromixer. It is found that the optimal mixing is realized in a 350 μm wide micromixer, under 100 kHz and 1.14 × 10⁵ V/m AC electric field, with an electric conductivity ratio of 1:3000. Under these conditions, a degree of mixedness of 0.93 can be achieved at 84 μm from the entrance and 100 ms. A further investigation of the critical electric field and the critical electric Rayleigh number indicates that the most unstable condition of EK flow instability is inconsistent with that of the optimal mixing in EK turbulence. To predict the evolution of EK flow under high Ra_σ and guide the design of EK turbulent micromixers, it is necessary to apply a computational turbulence model instead of linear instability analysis.

Rapid AC Electrokinetic Micromixer with Electrically Conductive Sidewalls

We report a quasi T-channel electrokinetics-based micromixer with electrically conductive sidewalls, where the electric field is in the transverse direction of the flow and parallel to the conductivity gradient at the interface between two fluids to be mixed. Mixing results are first compared with another widely studied micromixer configuration, where electrodes are located at the inlet and outlet of the channel with electric field parallel to bulk flow direction but orthogonal to the conductivity gradient at the interface between the two fluids to be mixed. Faster mixing is achieved in the micromixer with conductive sidewalls. Effects of Re numbers, applied AC voltage and frequency, and conductivity ratio of the two fluids to be mixed on mixing results were investigated. The results reveal that the mixing length becomes shorter with low Re number and mixing with increased voltage and decreased frequency. Higher conductivity ratio leads to stronger mixing result. It was also found that, under low conductivity ratio, compared with the case where electrodes are located at the end of the channel, the conductive sidewalls can generate fast mixing at much lower voltage, higher frequency, and lower conductivity ratio. The study of this micromixer could broaden our understanding of electrokinetic phenomena and provide new tools for sample preparation in applications such as organ-on-a-chip where fast mixing is required.