Colloidal migration in microchannels under combined potential and pressure gradients

Colloidal particle migration in combined Poiseuille and electrokinetic flows away from or towards the microchannel walls has recently been shown to be due to the particle slip velocity with respect to the fluid. The particle migration was attributed to an electrophoretic lift-like force, analogous to the inertial lift forces in sedimentation flows. The migration of colloidal particles towards the walls for dilute colloidal particle suspensions with diameters < 1 micron led to the assembly of distinct colloidal bands within microchannels (100 – 300 microns wide x 34 microns deep x 4cm long). Band formation requires opposing Poiseuille and electrokinetic flows. Band formation also requires a minimum applied potential threshold at a given shear rate. Band formation is a function of particle size and volume fraction, particle and channel wall zeta potential, electrolyte concentration, and the minimum electric field thresholds change non-monotonically for particle mixtures. Here, we discuss the effect of manipulating particle properties and fluid inertia over broad parametric ranges to elucidate robustness of particle migration to and away from walls, and the formation of particle bands.