Vortical structures of electroconvection on ion-selective surfaces during the steady-to-chaotic transition*

When an electric field is applied to an ion-selective membrane, exclusive transport of positive or negative ions induces hydrodynamic instability, initiating electrokinetic flows known as electroconvection(EC). In this study, we directly observe convective states of EC during the steady-to-chaotic transition for the first time. By conducting a tomography scan with 3D confocal microscopy, we classify two hitherto-unknown states between two well-known initial and final states: i) single-layer steady EC, ii) double-layer EC with steady primary and secondary vortices. iii) double-layer EC with steady primary and chaotic secondary vortices, and iv) fully chaotic EC. First, as described in previous studies, single-layer primary EC occurs on the membrane at the threshold voltage. Secondly, with increased voltage, smaller vortices arise between the larger primary vortices. These primary vortices lead to current hotspots between them, in which the influx of vortices presents towards the membrane. Consequently, in these hotspots, the ion flux and electric field become concentrated, inducing EC once again. Initially, this secondary EC is stable while keeping the balance with the primary EC. In the third state, however, the secondary EC becomes chaotic, spreading to the beneath primary EC and causing the current fluctuations of a finite value. Finally, when voltage is far beyond the threshold, both primary and secondary EC become chaotic.