Forces on near-wall dielectric microparticles in combined electroosmotic and Poiseuille flow through microchannels

Recent studies of electroosmotic (EO) flows have shown that neutrally buoyant radii $a \quad = \quad O$(0.1-1 $\mu $m) particles experience a ``dielectrophoretic-like'' repulsive force whose magnitude scales as $a^{\mathrm{2}}$ [\textit{Phys. Fluids} \textbf{18}:031702; \textit{Langmuir} \textbf{27}:11481]. Tracers with different sizes could then have different velocities in the same nonuniform flow. Evanescent-wave particle velocimetry was therefore used to study $a \quad =$ 125 nm and 245 nm fluorescent polystyrene (PS) tracers in combined EO and Poiseuille flow, which is effectively the superposition of simple shear and uniform flows within 1 $\mu $m of the wall. For ``coflow,'' where the EO and Poiseuille flows are in the same direction, the larger particles are strongly repelled from the wall; surprisingly, estimates of the magnitude of the repulsive force exceed the sum of the dielectrophoretic-like force and the shear induced electrokinetic lift force [\textit{J Colloid Interf Sci} \textbf{175}:411]. For ``counterflow,'' where the EO and Poiseuille flows are in opposite directions, these particles are instead \textit{attracted} to the wall. These unexpected results suggest that the nonlinear interaction between the electric field and shear could be used to manipulate near-wall microparticles.