flows. Electo-osmotic pumping is increasingly relevant as dimensions scale down and vis- coelastic effects become more pronounced for small dimensions due to the inverse scaling of the Deborah number with channel size (see equation 2). We observe a dependence in measured fluctuations on polymer concentration which rapidly plateaus well below the over- lap concentration, occurring near the threshold where hydrodynamic interactions between polymer coils become important. At higher voltages microgels were formed at the electrodes and moved downstream. Our experimental results demonstrate instabilities can be readily excited, even in microscaled devices where laminar flow normally predominates. Electro- osmotic flow can be made unstable intentionally through the addition of small amounts of chemically inert polymer allowing the possibility for enhanced mixing. Our results also sug- gest caution when electro-osmically pumping biological [48, 49], or otherwise viscoelastic, samples where instabilities may arise under conditions typically assumed to be laminar. We are grateful for support from the National Institute for Nanotechnology (NINT), the Natural Science and Engineering Council of Canada, the informatics Circle of Research Excellence, and the Canada Research Chairs program. RMB is supported in part by a National Research Council Canada graduate student supplemental scholarship held at NINT. We thank CMC Microsystems for fabricating the microdevices under a CMC fabrication grant and Dr. John Crabtree of Micralyne Inc. for exemplary equipment support. Acknowledgments [1] R.B. Bird, R.C. Armstrong, and O. Hassage, Dynamics of Polymeric Liquids , Wiley, 1987. [2] Y.-J. Juang, S. Wang, X. Hu, and L. J. Lee, Phys. Rev. Lett. , 2004, 93 , 268105. [3] G. C. Randall and P. S. Doyle, Phys. Rev. Lett. , 2004, 93 , 058102. [4] S. Ghosal, Electrophoresis , 2004, 25 , 214. [5] J.M. Ottino and S. Wiggins, Phil. Trans. R. Soc. Lond. A , 2004, 362 , 923. [6] A.P. Sudarsan and V.M. Ugaz, ”Proc. Nat. Acad. Sci. , 2006, 103 , 7228. [7] A.D. Stroock, S.K.W. Dertinger, A. Ajdari, I. Mezic, H.A. Stone, and G.M. Whitesides Sci- ence , 2002, 295 , 647. 16