  We report that an electro-osmotic instability of concentration enrichment in
curved geometries for an aqueous electrolyte, as opposed to the well-known one,
is initiated exclusively at the enriched interface (anode), rather than at the
depleted one (cathode). For this instability, the limitation of unrealistically
high material Peclet number in planar geometry is eliminated by the strong
electric field arising from the line charge singularity. In a model setup of
concentric circular electrodes, we show by stability analysis, numerical
simulation, and experimental visualization that instability occurs at the inner
anode, below a critical radius of curvature. The stability criterion is also
formulated in terms of a critical electric field and extended to arbitrary (2d)
geometries by conformal mapping. This discovery suggests that transport may be
enhanced in processes limited by salt enrichment, such as reverse osmosis, by
triggering this instability with needle-like electrodes.
