I. INTRODUCTION It is known that the inclusion of polymers into fluids allows flow fields and instabili- ties associated with elastic effects to be excited in pressure or other mechanically-driven liquids [1]. Here we show that by adding dilute amounts of inert polymer to liquids exten- sional instabilities can be excited in microchannels under steady electro-osmotic flow (EOF) pumping. For mechanically-driven liquids there is always a significant shear component to the fluid flow, due to no-slip boundary conditions; the effective slip boundary conditions of EOF sup- presses the shear allowing elongational effects to be emphasized and studied. Considerable efforts have considered “infinitely dilute” single-chain behavior of DNA in elongation flow (see, for example, Refs. [2, 3]), making use of electro-phoresis which places a body force on the polymer coil dragging it through a stagnant liquid. We consider here finitely dilute solutions driven at the boundary and undergoing elongtional stress in order to investigate the possibility of instabilities in such flows, which we determine can be easily excited for low viscosity solvents. Placing polar liquids in contact with ionizable surfaces produces an electric double layer (EDL) where ions of opposite charge are attracted to the wall [4]. This electrically non- neutral EDL sheaths surfaces, allowing electric fields to produce flows by driving the sheath, which in turn viscously couples to the bulk fluid away from the wall [4]. Such electro- osmotic flow is desirable for use in micro- and nano-fluidics, as flow with flat velocity fronts is observed away from the EDL in uniform channels of arbitrary cross section, with shear confined to the EDL regions. As a result scalability is superior to pressure-driven flows which become difficult to drive through very narrow channels. Microfluidic devices usually display creeping laminar flows [5], for both pressure and electro-osmotic pumping, as the achievable Reynolds numbers are small when working with small volumes in microscaled (O(100) µ m typical) channels. The low Reynolds regime prevents instabilities and turbulence which enhance mixing of chemical reactants allowing timely and complete reactions. As counter examples to typical “smooth” low Reynolds number flows see, for example, Ref. [6] where high velocities result in intermediate Reynolds numbers (O(1-100)) allowing inertial effects to induce Lagrangian chaos and efficient mixing in planar 2D microchannels and Ref. [7] where bas-relief (3D microchannels, requiring more 2