Induced-Charge Electrokinetic Phenomena: Theory and Microfluidic Applications

We give a general, physical description of “induced-charge electro-osmosis” (ICEO), the nonlinear electrokinetic slip at a polarizable surface, in the context of some new techniques for microfluidic pumping and mixing. ICEO generalizes “ac electro-osmosis” at microelectrode arrays to various dielectric and conducting structures in weak dc or ac electric fields. The basic effect produces microvortices to enhance mixing in microfluidic devices, while various broken symmetries—controlled potential, irregular shape, nonuniform surface properties, and field gradients—can be exploited to produce streaming flows. Although we emphasize the qualitative picture of ICEO, we also briefly describe the mathematical theory (for thin double layers and weak fields) and apply it to a metal cylinder with a dielectric coating in a suddenly applied dc field.

Figure 1

Lines of electric field $\mathbf{E}$ (or current $\mathbf{J}=\sigma \mathbf{E}$) around a cylindrical metal wire in an electrolyte (a) before and (b) after double-layer charging in response to a suddenly applied dc field and (c) the resulting ICEO streamlines. The flow around a charged polarizable cylinder is shown in (d).

Figure 2

Simple microfluidic devices exploiting ICEO flows around fixed metal objects (shaded area) driven by small ac voltages at microelectrodes (cross-shaded area).

Figure 3

Sketches of induced diffuse charge ($+,-$) and ICEO slip $\mathbf{u}$ around (a) a circular cylinder, (b) with a partial coating (dashed line) of increased surface capacitance, (c) with an insulating coating (shaded area), and (d) an asymmetric cross section. In (b)–(d), the ICEP velocity $\mathbf{U}$ points up, regardless of whether the applied field ${\mathbf{E}}_b$ points up or down.

Figure 4

Microfluidic devices exploiting asymmetric ICEO.