Field-Unmasked Surface Charge Enables Programmable Nanofluidics

A central challenge in the development of programmable nanofluidic devices lies in achieving dynamic control over surface charge density, which governs ionic conductance at the nanoscale. A prevailing view in electrokinetics is that the surface charge density remains invariant under axial electric fields of varying magnitudes. Here, we report a mechanism by which axial electric fields actively reshape interfacial charge in nanofluidic systems through field-driven detrapping of wall-bound counterions, exposing additional surface charges. This reversible unmasking of hidden charges triggers large, real-time modulations in electroosmotic conductance, far exceeding classical electrokinetic expectations. By accounting for the discrete origin of the charge sites on ionizable surfaces, we demonstrate the field-coupled nature of the ion-site equilibria. A modified Smoluchowski-Langevin framework quantitatively captures this emergent behavior. Experiments on silicon nitride nanopores confirm the field-dependent modulation of ionic conductance, while all-atom non-equilibrium molecular dynamics reveal the underlying coupling between solvent structuring, ion adsorption, and surface heterogeneity. By overturning the long-held view of surface charge as a static property, our findings establish field-programmable nanofluidics as a powerful strategy for dynamic control of ionic transport, without chemical modification or complex nanofabrication.