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 flow in microscale devices which are typically limited to low Reynolds numbers, turbulence does not readily occur. The mixing of two or more fluid streams in a simple microchannel is dominated by the molecular diffusion effect. The diffusive mixing time is given by tm ~ w2/D and the mixing length (lm) along the downstream channel increases linearly with the Péclet number (i.e. lm ~ Pe×w), where w and D are the channel width and molecular diffusivity. However, the rate of diffusive mixing in microscale channels is very slow compared to the convection of the fluid along the channel since the Péclet number of typical microchannel flows is very high due to biomolecules (e.g. DNA and protein) with relatively lo