In microfluidic devices, electrokinetic phenomena can be engaged to manipulate liquids, particles, or cells. In general, electrokinetic phenomena are related to the existence of electrical double layers (EDL), which are present e.g. where solid walls are in contact with electrolytes. Due to surface charges of the solid, ions of opposite charge are attracted and lead to a thin electrically non-neutral zone — the EDL. The application of an electrical field, consequently, leads to forces onto the fluid inside the EDL, capable of driving a flow within the microchannel. This particular flow is termed electroosmotic. If further the electrodes are placed inside the microchannel, due to small distances, strong inhomogeneous electrical fields can be induced, which in turn cause complex flow fields. This is true despite low applied electrode potentials of a few Volts. Numerical simulations of various authors have shown the potential for pumping and mixing of such complex flows in microchannels with internal electrodes.
We focus on the electroosmotic flow within rectangular microchannels of 100 × 200 μm cross section. In each microchannel eight pairs of electrodes are placed onto the top and bottom glass covers, whereas the spatial offset of the electrodes is chosen differently in each of the microchannels. Due to the limited optical access through the glass covers, only the two velocity components which are tangential to the glass cover (i.e. the x– and y–components) are accessible by means of the micro–particle–image velocimetry (μPIV). The expected electroosmotic flow, however, features vortices which cannot be recognized within this x–y velocity plane. Therefore, a number of two–dimensional, two–component velocity fields is measured at different heights of the microchannel. From an integration of the continuity equation, subsequently, the third velocity component in the z–direction, which is normal to the glass cover, can be determined at reasonable accuracy. A further difficulty arises from the fact that the microparticles, used for μPIV, are not electrically neutral. Hence, in addition to drag forces from the electroosmotic flow, they experience electrophoretic forces. Hence, the particle movement appears to be a superposition of electroosmotic and electrophoretic effects.
