Electrokinetic displacement of air bubbles in microchannels

Displacement of air bubbles in a circular capillary by electrokinetic flow is shown to be possible when the film flow around the bubble is less than the bulk flow behind it. In our experiments, film flow reduction is achieved by a surfactant-endowed interfacial double layer with an opposite charge from the wall double layer. Increase in the film conductivity relative to the bulk due to expansion of the double layers at low electrolyte concentrations decreases the field strength in the film and further reduces film flow. Within a large window in the total ionic concentration C-t, these mechanisms conspire to induce fast bubble motion. The speed of short bubbles (about the same length as the capillary diameter) can exceed the electro-osmotic velocity of liquid without bubble and can be achieved with a low voltage drop. Both mechanisms disappear at high C-t with thin double layers and very low values of zeta potentials. Since the capillary and interfacial zeta potentials at low concentrations scale as log C-t(-1) and log C-t(-1/3), respectively, film flow resumes and bubble velocity vanishes in that limit despite a higher relative film conductivity. The bubble velocity within the above concentration window is captured with a matched asymptotic Bretherton analysis which yields the proper scaling with respect to a large number of experimental parameters. (C) 2002 American Institute of Physics.