Electroosmotic velocity profiles in microchannels

Electroosmotic flows in microchannels of circular and square cross-sections with a range of hydraulic diameters are investigated experimentally. Velocity profiles at the mid-planes of the channels are obtained using a caged-dye based technique with a high degree of resolution in the bulk and near the channel walls. At low electric fields, plug-like velocity profiles are obtained, in agreement with electrokinetic theory. However, high electric fields in channels of larger hydraulic diameter were found to result in concave velocity profiles. The presence of this velocity deficit is shown to correlate with both an increase in pressure sensitivity, and an increase in fluid temperature due to Joule heating. To obtain accurate dye-based velocimetry data in such cases, a new scalar image velocimetry method is presented and verified with concurrent current-based measurements. The velocimetry method compensates for the temperature dependence of the dye mobility without explicit knowledge of the fluid temperature.