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.

In a similar fashion to Einstein's tea leaf paradox, the rotational liquid flow induced by ionic wind above a liquid surface can trap suspended microparticles by a helical motion, spinning them down towards a bottom stagnation point. The motion is similar to Batchelor [Q. J. Mech. Appl. Math. 4, 29 (1951)] flows occurring between stationary and rotating disks and arises due to a combination of the primary azimuthal and secondary bulk meridional recirculation that produces a centrifugal and enhanced inward radial force near the chamber bottom. The technology is thus useful for microfluidic particle trapping/concentration; the authors demonstrate its potential for rapid erythrocyte/blood plasma separation for miniaturized medical diagnostic kits. © 2006 American Institute of Physics.