Electroosmotic and Pressure-Driven Flow in Open and Packed Capillaries:  Velocity Distributions and Fluid Dispersion

Abstract

The flow field dynamics in open and packed segments of capillary columns has been studied by a direct motion encoding of the fluid molecules using pulsed magnetic field gradient nuclear magnetic resonance. This noninvasive method operates within a time window that allows a quantitative discrimination of electroosmotic against pressure-driven flow behavior. The inherent axial fluid flow field dispersion and characteristic length scales of either transport mode are addressed, and the results demonstrate a significant performance advantage of an electrokinetically driven mobile phase in both open-tubular and packed-bed geometries. In contrast to the parabolic velocity profile and its impact on axial dispersion characterizing laminar flow through an open cylindrical capillary, a pluglike velocity distribution of the electroosmotic flow field is revealed in capillary electrophoresis. Here, the variance of the radially averaged, axial displacement probability distributions is quantitatively explained by longitudinal molecular diffusion at the actual buffer temperature, while for Poiseuille flow, the preasymptotic regime to Taylor-Aris dispersion can be shown. Compared to creeping laminar flow through a packed bed, the increased efficiency observed in capillary electrochromatography is related to the superior characteristics of the electroosmotic flow profile over any length scale in the interstitial pore space and to the origin, spatial dimension, and hydrodynamics of the stagnant fluid on the support particles' external surface. Using the Knox equation to analyze the axial plate height data, an eddy dispersion term smaller by a factor of almost 2.5 than in capillary high-performance liquid chromatography is revealed for the electroosmotic flow field in the same column.

Dispersion in electroosmotic flow generated by oscillatory electric field interacting with oscillatory wall potentials

“…These features of steady velocity component with varyingk are well known in the literature. The plug flow profile was experimentally verified by Taylor and Yeung (1993), Tallarek et al (2000), and Herr et al (2000), among others. On losing velocity differentials in the core region, the dispersion becomes weaker ask increases.…”

Laser induced fluorescence photobleaching anemometer for microfluidic devices

“…30,31 Nuclear magnetic resonance has also been used to measure flow velocity in the research group. [32][33][34][35] The most successful method that applies the traditional advanced anemometer, is micro Particle Image Velocimetry (mPIV), [36][37][38] but it is difficult to apply to commercial instruments. Photobleached Fluorescence Visualization [39][40][41] and the line writing technique [42][43][44] with photobleaching are also developed to measure the flow velocity.…”

Modelling the relation between the species retention factor and the C‐term band broadening in pressure‐driven and electrically driven flows through perfectly ordered 2‐D chromatographic media

“…This might seem surprising (ED flows are considered to be superior because of their plug flow-like velocity profile in the through-pores), but, as already clearly remarked by Wen et al [46], the true advantage of ED flows in packed bed columns is that they lead to a much smaller eddy dispersion. In packed bed columns, the pore-averaged local velocity of ED flows does not depend on the local packing density and pore size, in big contrast with PD flows, whose local velocity is highly sensitive to the local packing density and pore size [46][47][48][49]), which is precisely the very source of eddy dispersion. Since PACs display nearly no eddy dispersion, the PD flow is no longer ''handicapped'', thus explaining why the difference between PD and ED flows is much smaller in PACs than in real packed bed columns.…”