  In this paper we report a novel inertial instability that occurs in
electro-osmotically driven channel flows. We assume that the charge motion
under the influence of an externally applied electric field is confined to a
small vicinity of the channel walls that, effectively, drives a bulk flow
through a prescribed slip velocity at the boundaries. Here, we study
spatially-periodic wall velocity modulations in a two-dimensional straight
channel numerically. At low slip velocities, the bulk flow consists of a set of
vortices along each wall that are left-right symmetric, while at sufficiently
high slip velocities, this flow loses its stability though a supercritical
bifurcation. Surprisingly, the new flow state that bifurcates from a left-right
symmetric base flow has a rather strong mean component along the channel, which
is similar to pressure-driven velocity profiles. The instability sets in at
rather small Reynolds numbers of about 20-30, and we discuss its potential
applications in microfluidic devices.
