Nonlinear electrokinetic repulsion effects in combined electroosmotic and Poiseuille flow through microchannels

Recent evanescent-wave particle velocimetry studies in electrokinetically driven flow where aqueous solutions are driven by an electric field of magnitude $E$, have shown that the radius $a=O$(0.1-1 $\mu $m) particle tracers suspended in the solution are subject to a wall-normal force that drives particles away from the wall [Kazoe {\&} Yoda, \textit{Langmuir} \textbf{27}:11481]. The magnitude of this force appears to scale as $E^{2}$ and $a^{2}$, albeit over a limited range of $E$ and $a$, suggesting that particles of different sizes will have different average wall-normal positions, and hence sample different velocity distributions in a shear flow. To verify this hypothesis, evanescent-wave particle velocimetry was used to measure near-wall particle distributions and velocities of $a$ = 0.2 $\mu $m and 0.5 $\mu $m particles in the combined electroosmotic and Poiseuille flow of a bidisperse dilute aqueous solution through fused-silica channels about 30 $\mu $m deep for $E$~$<$~45 V/cm and pressure gradients $\Delta p$/$L$~$\le $~1.3Bar/m. To evaluate the whether this nonlinear electrokinetic force can be used separate particles based on their size, near-wall particle distributions for both particle sizes were measured at different streamwise locations in the combined flow.