Continuous Focusing of Particles by AC-Electroosmosis\nand Induced Dipole Interactions

Continuous particle\nfocusing by using microfluidics is an effective\nmethod for separating particles, cells, or droplets for analytical\npurposes. Previously, it was shown that an alternating current across\nrectangular microchannels with slightly deformed side walls results\nin vortex flow patterns caused by alternating current electroosmosis\n(AC-EOF) and could lead to particle focusing. In this work, we explore\nthis mechanism by experimentally studying the particle focusing behavior\nfor various fluid flow velocities through a microchannel. Since it\nis unlikely that the particles are kept in their focused position\nsolely by convection, a theoretical force balance between the hydrodynamic\nand the induced dipole force was determined. In our experiments, it\nwas found that there is no substantial effect of the pressure-driven\nfluid velocity on the particle focusing velocity within the studied\nrange. From the theoretical force balance calculations, it was determined\nthat while the addition of the induced dipole force can still not\ncompletely describe the experimentally observed particle focusing,\nthe induced dipole can be strong enough to overcome the hydrodynamic\nforce. Finally, it is hypothesized that under specific circumstances,\nincluding a repulsive electrostatic force between a particle and electrode\nwall can complete the theoretical particle focusing force balance.\nAlternative phenomena that could also play a role in particle focusing\nare proposed.