Throat-gated ionic current rectification in soft-layer hourglass nanochannels

Abstract

Hourglass nanochannels provide a confined throat where geometric focusing and soft fixed charge can interact strongly. However, the role of spatially patterned polyelectrolyte layers (PELs) in controlling transport through this junction remains insufficiently resolved. Here, ionic current rectification is investigated using coupled Poisson–Nernst–Planck and Stokes–Brinkman/Navier–Stokes simulations for uniform, convergent-only, and bipolar PEL architectures. The results show that the hourglass throat acts as a localized electrostatic–hydrodynamic gate. In the bipolar configuration, oppositely charged PEL domains form an internal soft junction that switches with voltage polarity between a strongly depleted, low-conductance state and an enriched, high-conductance state. This switching is traced to the coupled effects of concentration polarization, electrostatic selectivity, electric-field localization, and throat-focused electrohydrodynamic flow. The PEL fixed-charge density exhibits an optimum rather than a monotonic effect, yielding a peak rectification factor of approximately 1120 within the charge-density sweep at a bulk electrolyte concentration of 0.3 mol/m³ for the reference geometry with a convergent angle of 4.57°. The geometry provides an additional control mechanism: changing the convergent-section length can reverse the preferred transport direction. In a separate convergent-angle sweep conducted at a bulk electrolyte concentration of 10 mol/m³, decreasing the convergent angle from 20° to 4.57° increases the rectification factor from approximately 470 to 1630. These results establish the hourglass throat as a tunable soft junction for controlling both the magnitude and polarity of ionic current rectification.