Motivation:
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. Current instruments 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.
Project Description:
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. |
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