Electro-osmotic flow in porous media using magnetic resonance imaging

Magnetic resonance imaging (MRI) has been used to study electro-osmotic flow in a porous medium consisting of spherical glass particles ranging from 2.0 to 4.0 min in diameter. Electrodes placed at each end of a 1-cm glass tube containing the spheres and solution were used to generate flow. Pulsed field gradient MRI at 500 and 600 MHz was used to measure the fluid velocity fields and provided in-plane image resolution of approximately 78 x 78 mum(2) for 100 mum thick slices. Velocity fields as functions of applied voltage were measured for four cases: (1) empty tube with open ends, (2) empty tube with closed ends, (3) tube with packing and closed ends, and (4) tube with packing and open ends. Both closed-tube systems showed recirculation; in the closed tube with particles, recirculation occurred over the length scale of a single particle. The standard deviation or spread of the velocity distribution for the closed-tube experiments scaled approximately linearly with increasing applied voltage, while the velocity mean remained approximately zero. In the open-tube system, both the mean velocity and the standard deviation of the velocity distribution scaled linearly with the applied voltage. Detailed analyses of the velocity distribution histograms show discrepancies with predictions of the bundled-capillary tube, model of electro-osmotic flow in porous media.