Electrokinetic mixing in microfluidic systems

The applications of electrokinetics in the development of microfluidic devices have been widely attractive in the past decade. Electrokinetic devices generally require no external mechanical moving parts and can be made portable by replacing the power supply by small battery. Therefore, electrokinetic-based microfluidic systems can serve as a viable tool in creating a lab-on-a-chip (LOC) or micro-total analysis system (μTAS) for use in biological and chemical assays. Mixing of analytes and reagents is a critical step in realizing lab-on-a-chip. This step is difficult due to the low Reynolds numbers flows in microscale devices. Hence, various schemes to enhance micro-mixing have been proposed in the past years. This review reports recent developments in the micro-mixing schemes based on DC and AC electrokinetics, including electrowetting-on-dielectric (EWOD), dielectrophoresis (DEP), and electroosmosis (EO). These electrokinetic-based mixing approaches are generally categorized as either active or passive in nature. Active mixers either use time-dependent (AC or DC field switching) or time-independent (DC field) external electric fields to achieve mixing, while passive mixers achieve mixing in DC fields simply by virtue of their geometric topology and surface properties, or electrokinetic instability flows. Typically, chaotic mixing can be achieved in some ways and is helpful to mixing under large Péclet number regimes. The overview given in this article provides a potential user or researcher of electrokinetic-based technology to select the most favorable mixing scheme for applications in the field of micro-total analysis systems.