Some striking, and potentially useful, effects in electrokinetics occur for
bipolar membranes: applications are in medical diagnostics amongst other areas.
The purpose of this talk is to describe the experiments, the dominant features observed
and then model the phenomena: This uncovers the physics that control this process.
Time-periodic reverse voltage bias
across a bipolar membrane is shown to exhibit transient hysteresis.
This is due to the incomplete depletion of mobile ions, at the junction
between the membranes, within two adjoining polarized layers; the layer thickness depends on
the applied voltage and the surface charge densities. Experiments
show that the hysteresis consists of an Ohmic linear rise in the
total current with respect to the voltage, followed by a
decay of the current. A limiting current is established for a long
period when all the mobile ions are depleted from the polarized layer.
If the resulting high field within the two polarized layers is
sufficiently large, water dissociation occurs to produce proton and
hydroxyl travelling wave fronts which contribute to another large jump
in the current. We use numerical simulation and asymptotic analysis
to interpret the experimental results and
to estimate the amplitude of the transient hysteresis and the
water-dissociation current.