J. Fluid Mech. (2004), vol. 506, pp. 357–367. c  2004 Cambridge University Press DOI: 10.1017/S0022112004008626 Printed in the United Kingdom 357 An experimental study of electro-osmotic flow in rectangular microchannels By R. SADR 1 , M. YODA 1 †, Z. ZHENG 2 AND A. T. CONLISK 3 1 G. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405, USA 2 Biomedical Engineering Center, The Ohio State University, Columbus, OH 43210, USA 3 Department of Mechanical Engineering, The Ohio State University, Columbus, OH 43210, USA (Received 27 August 2003 and in revised form 21 February 2004) Experimental studies were carried out on fully developed and steady electro-osmotic flow in a rectangular channel where the channel height h is comparable to its width and the thickness of the electric double layer characterized by the Debye length is much less than h. The nano-particle image velocimetry technique was used to measure the two components of the velocity field parallel to and within about 100 nm of the channel wall for h  25 µm. The mobility of the particle tracers was calculated from averaged velocity data for various electric field strengths. The experimentally determined mobility values are compared with analytical predictions for dilute aqueous solutions of sodium tetraborate. 1. Introduction Electro-osmotic flow (EOF) is by definition the flow of an electrolyte solution driven by an external electric field through a channel with charged walls. The surface charge is created in this case by adsorption of ions from the electrolyte solution onto the channel walls. We consider EOF through rectangular microchannels with dimensions along the (x,y,z) axes of (L, h, W ), respectively, where x is the streamwise direction, y is the direction normal to the wall, and z is the direction along the channel wall of interest normal to x . In the cases studied here, the channel height and width are comparable, or h/W = O(1), and the flow is fully developed, with h/L  1. The charged channel walls attract free ions of the opposite sign from the electrolyte solution to create an electric double layer (EDL), whose thickness is the Debye (screening) length λ, typically O(0.1 nm–10 nm) for aqueous solutions at molar concentrations C = O(1 m–10 −4 m), respectively. The difference in potential across the EDL, or between the charged wall and the bulk electrolyte solution, is known as the wall ζ -potential. Outside the EDL, the velocity profile in electro-osmotic flow is uniform. The electro-osmotic mobility µ eo is defined as the bulk electro-osmotic speed divided by the driving electric field E. Electrokinetic ‘pumping’ is the leading technology for driving flows through microchannels, especially for channels where h<O(10 µm), because EOF can achieve much higher volumetric flow rates Q than pressure gradient-driven flow. In fully developed EOF, Q ∝ h for a given E; in Poiseuille flow, Q ∝ h 3 for a given pressure gradient. Electrokinetic pumping is also the leading technology for biochemical † Author to whom correspondence should be addressed: minami.yoda@me.gatech.edu