Investigation of internal pressure gradients generated in electrokinetic flows with axial conductivity gradients |
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| Authors: | Shankar Devasenathipathy Rajiv Bharadwaj Juan G Santiago |
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| Institution: | (1) Mechanical Engineering Department, Stanford University, 440 Escondido Mall, Stanford, CA 94305, USA;(2) Present address: Intel Corporation, Assembly & Test Technology Development, 5000 W Chandler Blvd, Chandler, AZ 85226, USA;(3) Present address: Caliper Life Sciences, 605 Fairchild Drive, Mountain View, CA 94043, USA |
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| Abstract: | Field amplified sample stacking (FASS) is used to increase sample concentrations in electrokinetic flows. The technique uses
conductivity gradients to establish a non-uniform electric field that accumulates ions within a conductivity gradient, and
can be readily integrated with capillary electrophoresis. Conductivity gradients also cause gradients in near-wall electroosmotic
flow velocities. These velocity gradients generate internal pressure gradients that drive secondary, dispersive flows. This
dispersion leads to a significant reduction in the efficiency of sample stacking. This paper presents an experimental investigation
of internally generated pressure gradients in FASS using micron-resolution particle image velocimetry (μPIV). We measure velocity
fields of particles seeded into an electrokinetic FASS flow field in a glass microchannel with a single buffer–buffer interface. μPIV
allows for the direct quantification of local, instantaneous pressure gradients by analyzing the curvature of velocity profiles.
Measurements show internally generated pressure-driven velocities on the order of 1mm/s for a typical applied electric field
of 100 V/cm and a conductivity ratio of 10. A one-dimensional (1D) analytical model for the temporal development of the internal
pressure gradient generation is proposed which is useful in estimating general trends in flow dynamics.
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