Electroosmotic flow in a dissimilarly charged slit microchannel containing salt-free solution

A theoretical study on the electroosmotic flow through a dissimilarly charged slit microchannel containing a salt-free solution is presented. The exact analytical solutions for the electric potential distribution and the electroosmotic flow velocity are derived by solving the nonlinear Poisson–Boltzmann equation and the Navier–Stokes equation when both surfaces maintain constant charge or constant potential or a mix of constant charge and constant potential. Based on these solutions, a systematic parametric study on the characteristics of the electroosmotic flow is detailed. In a salt-free solution, the regime where the electric double layer field exists cannot be identified from the electric potential profile. In contrast, such information is clearly revealed in the counterion concentration profile. Furthermore, the effect of finite electric double-layer thickness on the Smoluchowski equation with an average zeta potential can be ignored inside the oppositely charged slit microchannel containing a salt-free solution, which significantly differs from that observed in an electrolyte solution.