Use of combined electrokinetic and poiseuille flows to generate organized colloidal structures

Our work with combined Poiseuille and electrokinetic flows shows that particle slip velocity with respect to the fluid can be used to manipulate colloidal particles confined within microchannels. The particle migration was attributed to a new electrophoretic lift-like force that we report here, analogous to the inertial lift forces. The migration of colloidal particles towards the walls occurs for the electric potential gradient and pressure gradient in opposite directions with respect to streamwise flow, a condition we refer to as counter-flow. For dilute colloidal particle suspensions with particle diameters < 1micron, under counter-flow, led to the assembly of distinct colloidal bands within microchannels (100 – 300 wide x 34 deep x 4cm long). Band formation requires a minimum applied potential threshold at a given shear rate. Band formation is a function of particle size and volume fraction, electrolyte concentration, and the minimum electric field thresholds change non-monotonically for particle mixtures. We are able to extract these bands to porous substrates through a continuous flow microfluidic “print-head”. Here, we also discuss the effect of manipulating particle properties and fluid inertia over broad parametric ranges to elucidate robustness of particle migration to and away from microchannel walls, and the formation of particle bands in addition to their extraction and the likely parameters contributing to extracted band structures.