Harnessing Nonlinear Nanofluidic Rectification to Suppress Concentration Polarization

Abstract

Electrokinetic transport in nanoscale systems has garnered significant interest because of its potential applications in desalination, molecular separation, and ion-selective devices. In particular, electroosmotic flow (EOF) through selective membranes offers a promising mechanism for voltage-driven reverse osmosis. However, a fundamental limitation arises from ion concentration polarization, which develops at the membrane extremities, creating a local voltage and increasing osmotic pressure that tend to limit the EOF. Through numerical simulations, we investigate the buildup of polarization at an osmotic diode interface under applied DC and AC electrostatic forces. We demonstrate that the concentration-induced polarization leads to a limitation of EOF efficiency and, under nonlinear rectified and AC fields, gives rise to complex depletion-polarization dynamics that fundamentally differs from recently reported pulsed strategies. We show that imposing a lateral flow to flush the reservoirs effectively mitigates polarization, restoring the EOF efficiency. Our simulations reveal that nonlinear nanofluidic transport, coupled with lateral flow flushing, allows for the suppression of polarization effects and enhances filtering efficiency. These findings provide critical insights into the interplay between electrostatic forcing and CP, offering a deeper understanding of the constraints on voltage-driven reverse osmosis and electrokinetic membrane systems.