Confined Electroconvective Vortices at Structured Ion Exchange Membranes

In
this paper, we investigate electroconvective ion transport at
cation exchange membranes with different geometry square-wave structures
(line undulations) experimentally and numerically. Electroconvective
microvortices are induced by strong concentration polarization once
a threshold potential difference is applied. The applied potential
required to start and sustain electroconvection is strongly affected
by the geometry of the membrane. A reduction in the resistance of
approximately 50% can be obtained when the structure size is similar
to the mixing layer (ML) thickness, resulting in confined vortices
with less lateral motion compared to the case of flat membranes. From
electrical, flow, and concentration measurements, ion migration, advection,
and diffusion are quantified, respectively. Advection and migration
are dominant in the vortex ML, whereas diffusion and migration are
dominant in the stagnant diffusion layer. Numerical simulations, based
on Poisson–Nernst–Planck and Navier–Stokes equations,
show similar ion transport and flow characteristics, highlighting
the importance of membrane topology on the resulting electrokinetic
and electrohydrodynamic behavior.