  For a microchamber filled with a binary electrolyte and containing a flat
un-biased center electrode at one wall, we employ three numerical models to
study the strength of the resulting induced-charge electro-osmotic (ICEO) flow
rolls: (i) a full nonlinear continuum model resolving the double layer, (ii) a
linear slip-velocity model not resolving the double layer and without
tangential charge transport inside this layer, and (iii) a nonlinear
slip-velocity model extending the linear model by including the tangential
charge transport inside the double layer. We show that compared to the full
model, the slip-velocity models significantly overestimate the ICEO flow. This
provides a partial explanation of the quantitative discrepancy between observed
and calculated ICEO velocities reported in the literature. The discrepancy
increases significantly for increasing Debye length relative to the electrode
size, i.e. for nanofluidic systems. However, even for electrode dimensions in
the micrometer range, the discrepancies in velocity due to the finite Debye
length can be more than 10% for an electrode of zero height and more than 100%
for electrode heights comparable to the Debye length.
