Lift forces on colloidal particles in combined electroosmotic and Poiseuille flow.

Colloidal particles suspended in aqueous electrolyte solutions flowing through microchannels are subject to lift forces that repel the particles from the wall due to the voltage and pressure gradients commonly used to drive flows in microfluidic devices. There are very few studies that have considered particles subject to both an electric field and a pressure gradient, however. Evanescent-wave particle tracking velocimetry was therefore used to investigate the near-wall dynamics of a dilute suspension of 245 nm radius polystyrene particles in a monovalent electrolyte solution in Poiseuille and combined electroosmotic (EO) and Poiseuille flow through 30-μm-deep fused-silica channels. The lift force observed in Poiseuille flow, which is estimated from the near-wall particle distribution, appears to be proportional to the shear rate, a scaling consistent with hydrodynamic lift forces previously reported in field-flow fractionation studies. The estimates of the lift force observed in combined flow suggest that the force magnitude exceeds the sum of the lift forces observed in EO flow at the same electric field or in Poiseuille flow at the same shear rate. Moreover, the force magnitude appears to be proportional to the electric field magnitude and have a power law dependence on the shear rate with an exponent between 0.4 and 0.5. This unexpected scaling suggests that the repulsive lift force observed in combined electroosmotic and Poiseuille flow is a new phenomenon, distinct from previously reported electroviscous, hydrodynamic lift, or dielectrophoretic-like forces, and warrants further study.