Both current bench-top biological instruments and emerging miniaturized chemical instrumentation rely on electrokinetic mechanisms for fluid transport and sample separation in fluidic channel dimensions on the order of 10 - 100 microns. Tradional devices use freestanding glass capillaries, while new miniaturized devices are fabricated on glass, quartz, silicon, or plastic substrates using techniques that originated in the semiconductor industry and the micro electro-mechanical systems community. The flow phenomena in each of these devices are a complex function of the applied electric fields, the physical characteristics of the microchannels, and the physical properties of the often multi-component fluids. Flow phenomena in continuous flow devices are also strongly dependent on both the upstream and downstream conditions. An accurate understanding of dispersive phenomena is necessary for the design of microscale flow control schemes, as well as for the design of integrated microfluidic systems.
We have conducted an analytical and experimental study of electroosmotic flow (EOF) in cylindrical capillaries with non-uniform wall surface charge (z-potential) distributions. In particular, this study investigated perturbations of electroosmotic flow in open capillaries that are due to induced pressure gradients resulting from axial variations in the wall z-potential. The experimental inquiry focused on electroosmotic flow under a uniform applied field in capillaries with an EOF-suppressing polymer adsorbed onto various fractions of the total capillary length. This fractional EOF-suppression was achieved by coupling capillaries with substantially different z-potentials. The resulting flow fields were imaged with a non-intrusive, caged-fluorescence imaging technique. Simple analytical models for the velocity field and rate of sample dispersion in capillaries with axial z-potential variations are presented. The resulting induced pressure gradients and the associated band broadening effects are of particular importance to the performance of chemical and biochemical analysis systems such as capillary electrokinetic chromatography and capillary zone electrophoresis.
A.E. Herr, J.I. Molho, J.G. Santiago, M.G. Mungal, T.W. Kenny, M.G. Garguilo, "Electroosmotic Capillary Flow with Nonuniform Zeta-Potential," Analytical Chemistry, Vol.72, No.5, March 1, 2000, pp.1053-1057.