Ion transport in an annular geometry: limiting current, concentration polarization and vortex formation

Ion transport playing a key role for water treatment and energy storage has been extensively studied in a parallel geometry. Here we explore the ion transport in a novel annular geometry experimentally. The limiting current depends on both the direction of electric field and the radius of the inner electrode. In the region of overlimiting current, we directly visualize the vortex formation and dynamical evolution near the cathode side. Moreover, we directly observe the propagation of concentration depletion from the cathode side toward the anode side, and the propagation velocity agrees well with the theory as well. These results have implications for desalination, biomolecular detection and improving battery performance.