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Microfluidic blood plasma separation via bulk electrohydrodynamic flows
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View: Figures


Image of FIG. 1.

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FIG. 1.

Schematic representation of the experimental apparatus used to generate the bulk electrohydrodynamic air thrust and, hence, liquid rotation.

Image of FIG. 2.

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FIG. 2.

(a) Ionic wind mechanism. The grey circles indicate the electroneutral air molecules that the counter-ions collide with when they are repelled away from the corona electrode. (b)-(d) Side and plane views of the different surface and bulk liquid flow configurations in which the corona electrode is mounted in a (b) symmetrically centered vertical position , (c) symmetrically centered inclined position , and (d) lateral inclined position .

Image of FIG. 3.

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FIG. 3.

Numerical solution of the governing hydrodynamic equation of motion subject to an angular rotation at the top liquid surface and no slip conditions at the side and bottom walls of the microfluidic chamber. Panels (a), (b), and (c) are flow field traces that illustrate the helical swirl-like motion inwards toward a stagnation point at the bottom of the chamber and the subsequent rise of the liquid up a central column due to flow conservation. Panel (b) is a cross-sectional plan view of panel (a), whereas panel (c) shows the flow field a small distance above the base. Panels (d), (e), and (f) are the circumferential , radial , and vertical flow velocity profiles, respectively. The warm colors (red, orange, and yellow) indicate motion along the respective axis directions, whereas the cool colors (light and dark blue) indicate motion against the axis direction.

Image of FIG. 4.

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FIG. 4.

Sketch of the azimuthal and radial velocity profiles depicting the mechanism by which a recirculating flow arises, attributed to the decrease in the azimuthal velocity and, hence, centrifugal force and the increase in the inward radial velocity at the fluid layer near the bottom of the chamber. The flow in the thin Bödewadt layer just below the free surface is omitted for simplicity.

Image of FIG. 5.

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FIG. 5.

Successive images obtained by high speed video microscopy at showing the spiral-like trajectory of the RBCs near the bottom of the microfluidic chamber toward a stagnation point. The applied field and frequency are and , respectively, and the initial hematocrit is 0.4%.

Image of FIG. 6.

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FIG. 6.

Delineation of the operating regimes for RBC collection in the electric field-frequency space. In region 1, the applied potential is below the threshold ionization voltage. Thus, there is no bulk electrohydrodynamic air thrust and, hence, no liquid motion. Although the primary surface recirculation is present in region 2, it is too weak to generate the bulk meridional recirculation vortices for RBC trapping. Region 3 is the optimum operating zone, where RBC collection is most efficient. If the field is increased further, however, the strong convective flow associated with the bulk meridional recirculation overwhelms the gravitational force on the particle that pins it down to a stagnation point. As such, RBC resuspension back up the liquid recirculation path occurs in region 4. Close to the boundaries between regions 3 and 4, however, a moderately strong convection may not lead to complete resuspension but will cause shifting in the stagnation point to a new location. RBC collection can still take place, but is less efficient than in region 3.

Image of FIG. 7.

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FIG. 7.

Image sequences acquired at showing the effects of a stronger local convective flow at and ; the initial hematocrit is 0.4% The gravitational force acting on the RBCs is no longer sufficient to provide a downward force to pin the particle down to the stagnation point and, hence, partial resuspension of the particles together with the flow up the recirculation column occurs. The operating parameters in this case, however, are close to the boundaries between the stable operating zone of region 3 and the unstable operating zone of region 4. As such, the bulk convective effects are insufficient to cause complete RBC resuspension but adequately strong to cause shifting of the stagnation point to a new location.

Image of FIG. 8.

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FIG. 8.

RBC collection at the stagnation point as a function of time as measured by a pixel intensity analysis for various values of the applied electric field and frequency. The pixel intensities at different times, extracted from the relevant image frames, are normalized by the pixel intensity of the initial frame at time . The data when the electric field is not present are also shown; in this case, the particles sink to the bottom of the chamber purely due to gravitational sedimentation effects alone.

Image of FIG. 9.

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FIG. 9.

Effect of increasing the electric field at a fixed applied frequency, i.e., , on the normalized pixel intensity as a function of time. Above , the strong convective effects associated with region 4 begin to cause RBC resuspension back into the bulk flow recirculation path.

Image of FIG. 10.

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FIG. 10.

Effect of increasing the electric field at a fixed applied frequency, i.e., , on the normalized pixel intensity as a function of time. Above , the partial resuspension and shifting of the stagnation point occurs as indicated by the dip in the normalized pixel intensity at about . Once the shift occurs, RBC trapping resumes at the new location as indicated by the reincrease in the pixel intensity at approximately .


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An effective mechanism for rapid and efficient microfluidic particle trapping and concentration is proposed without requiring any mechanically moving parts. When a voltage beyond the threshold atmospheric ionization value is applied on a sharp electrode tip mounted at an angle above a microfluidic liquid chamber, the bulk electrohydrodynamic air thrust that is generated results in interfacial shear and, hence, primary azimuthal liquid surface recirculation. This discharge driven vortex mechanism, in turn, causes a secondary bulk meridional liquid recirculation, which produces an inward radial force near the bottom of the chamber. Particles suspended in the liquid are then rapidly convected by the bulk recirculation toward the bottom, where the inward radial force causes them to spiral in a helical swirl-like fashion toward a stagnation point. In particular, we show that these flows, similar to Batchelor flows occurring in a cylindrical liquid column between a stationary and rotating disk, can be used for the separation of red blood cells from blood plasma in a miniaturized device.


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Scitation: Microfluidic blood plasma separation via bulk electrohydrodynamic flows