Wall repulsion during electrophoresis: Testing the theory of concentration-polarization electro-osmosis

Abstract

We experimentally study the repulsion of charged microscopic particles with the channel walls during electrophoresis in microfluidic devices. For low frequencies of the electric fields (<10 kHz), this repulsion is mainly due to the hydrodynamic interaction caused by the flow vortices that arise from the slip velocity induced by the electric field on the particle surface, as shown in a recent publication [Fernandez-Mateo et al., Phys. Rev. Lett. 128, 074501 (2022)]. The maximum slip velocity on the particle surface is inferred from measurements of wall-particle separation. Importantly, this procedure allows us to infer very small slip velocities that, otherwise, are too weak to be measured directly. Data at small electric field amplitudes (E0) agree with theoretical predictions using the model of Concentration Polarization Electro-osmosis (CPEO), which has recently been proposed as the mechanism behind the flow vortices on the surface of the particles. Data for higher electric fields show that the predictions of the CPEO theory for weak electric fields are not valid beyond E0∼60 kV/m. Additionally, we also show that, for sufficiently strong electric fields, the quadrupolar flow structures become disrupted, leading to a weaker wall repulsion.