Direct 3D observation and unraveling of electroconvection phenomena during concentration polarization at ion-exchange membranes

Abstract

A decade ago, two-dimensional microscopic flow visualization proved the theoretically predicted existence of electroconvection roles as well as their decisive role in destabilizing the concentration polarization layer at ion-selective fluid/membrane interfaces. Electroconvection induces chaotic flow vortices injecting volume having bulk concentration into the ion-depleted di↵usion layer at the interface. Experimental quantification of these important flow patterns have so far only been carried out in 2D. Numerical direct simulations suggest 3D features, yet experimental proof is lacking. 3D simulations are also limited in covering extended spacial and temporal scales.This study presents a new comprehensive experimental method for the time-resolved recording of the 3D electroconvective velocity field near a cationexchange membrane. For the first time, the spatio-temporal velocity field can be visualized in 3D at multiples of the overlimiting current density. In contrast to today's simulations, these experiments cover length and time scales typical for actual electrodialytic membrane processes.We visualize coherent vortex structures and reveal the changes in the velocity field and its statistics during the transition from vortex rolls to vortex ⇤

Localized Electroconvection at Ion-Exchange Membranes with Heterogeneous Surface Charge

“…The regular structure vanished, showing vortex rings or ellipses of different shapes instead. Additionally, the rotational direction in the vortex ring centers changes towards the electrode, similar to the direction of EC vortex rolls at an untreated Nafion membrane at higher current densities [2]. Later, the chaotic vortex rings approximately double in diameter between 50 s and 62.5 s, which stays constant in the steady-state at 112.5 s. The changed rotational direction is dominant throughout the steady-state.…”

“…To validate the findings of the 2D simulations, namely the change in rotational direction and localization of vortex field features, 3D experiments were conducted with membranes modified with patches of defined surface charge contrast to the base material. The experiment shows a typical, stable vortex roll pattern for relatively small overlimiting current densities visible as linearly stretched coherent vortex structures [2,24,25]. These rolls occur as counter-rotating pairs visible as tube-like coherent structures.…”

“…We conduct experiments using an electrochemical cell that is suitable to evoke simultaneously and optically record electroconvective vortices [2]. We record and analyze the difference in vortex fields evolving at different modified cation-exchange membranes in this cell.…”

“…b) Electric field lines at a homogeneous and heterogeneous membrane surface. limiting current density due to the formation of convective 3D vortices, recently quantified by Stockmeier et al [2], which mix the depleted layer close to the membrane. In fact, EC balances with unwanted water splitting and a maximum contribution of EC to the overlimiting current density is desired [3].…”

“…In this paper, we evaluate the effect of patterning a cation-exchange membrane surface, with the technique described by Roghmans et al, on the 3D hydrodynamics of EC using micro particle tracking velocimetry (µPTV) [2,11]. Our numerical and experimental investigation aims at revealing the effect of patterning the membrane surface on the electroconvective vortex field.…”