Direct-current (DC) electroosmotic flow through uncharged channels

Usually, electroosmotic flow (EOF) arises when an electric field is applied along an electrolyte-filled channel whose walls are charged. In scenarios with broken (spatial) symmetry, a net EOF can also be induced along uncharged bodies. We study a scenario in which a DC electric field is applied along a channel or pore (in the following denoted as channel), where the channel walls do not carry any charge and the symmetry is broken via the different valences of anions and cations. The focus is on wall materials with high dielectric permittivity. Using numerical simulations and analytical solutions of the coupled Poisson-Nernst-Planck-Stokes equations, we show that in such systems, a substantial net EOF arises, which refers to the flow averaged over the cross sectional area of the channel. The flow is due to the asymmetry of the induced Debye layer, i.e., the reflection symmetry with respect to the channel midplane between the two reservoirs is broken. The asymmetric Debye layer results in a non-vanishing average zeta potential at the walls even if the wall material is not charged. The numerical simulations show that very significant flow velocities of the order of millimeters per second can be achieved for electric field strength magnitudes commonly applied to nanopores. The analytical calculations, relying on the Debye-Hückel approximation and the assumption of thin Debye layers, yield an explicit expression for the average zeta potential and the net EOF velocity. The average zeta potential scales as E², the net EOF velocity as E³, where E denotes the magnitude of the applied electric field.