  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
diffusion 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
cation-exchange 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 rings with increasing current density. The transition is characterized
by changes in the rotational direction, mean square velocity, and temporal
energy spectrum with only little influence on the spatial spectrum. These
findings indicate a more significant impact of EC's structural change on the
mean square velocities and temporal spectra than on the spatial spectra. This
knowledge is a prerequisite for engineering ion-selective surfaces that will
enable the operation of electrically driven processes beyond the
diffusion-limited Nernst regime.
