Decoupled capacitive-gate electro-osmosis: Phase-controlled bidirectional flow in high-conductivity electrolytes via a modified Poisson–Boltzmann–Stokes framework

Alternating-current electro-osmosis in high-conductivity electrolytes (σ ≈ 1.68 S m−1) is constrained by a single Resistance Capacitance (RC)-pole operating window: Flow velocity peaks at the electric double layer charging frequency fRC and collapses steeply on either side, while attempts to widen the band by raising electrode voltage drive Faradaic reactions that are incompatible with physiological media. We present a decoupled electrokinetic architecture in which a buried capacitive gate electrode independently modulates the wall zeta potential through a thin Al2O3 dielectric, while asymmetric propulsion electrodes generate the tangential field in the bulk. The electric double layer is modelled with a Bikerman-modified Poisson–Boltzmann equation that incorporates finite-ion-size steric saturation, necessary because the classical Poisson–Boltzmann distribution predicts near-wall cation concentrations exceeding 1040 mol m-3. The gate–propulsion phase difference θ acts as a controlled parameter: The time-averaged velocity scales as cos⁡θ, enabling continuous flow reversal without hardware reconfiguration. At the representative operating point ψgate = 0.15 V, Vprop = 1.2 V, f = 2.5 kHz, and θ = 0, the dimensionless transport efficiency w~≈0.4 corresponds to a time-averaged slip velocity ⟨U⟩=w~εψgateVprop/(ηL)≈600μm/s in cerebrospinal fluid. These results establish design criteria for tunable electro-osmotic actuation in high-conductivity media and identify decoupled surface-charge control as a route beyond conventional architectures.