Actuation of co-flowing electrolytes in a microfluidic system by microelectrode arrays

Electric fields induce forces at the interface between liquids with different electrical properties (conductivity and/or permittivity). We explore how to use these forces for the manipulation of two coflowing streams of electrolyte in a microchannel. A microelectrode array is fabricated at the bottom of the channel and one of the two liquids is labelled with a fluorescent dye. The diffuse interface between the two liquids is deflected depending on the ac signal applied to microelectrodes and the conductivity (or permittivity) difference between the liquids. The phenomenon is found to be important over a wide frequency range and increases with conductivity ratio. An immediate application of this deflection is that electrolyte mixing time is reduced since the area of the diffusing interface is greatly increased. Remarkably, only a few volts are needed to observe the interface deflection, in contrast with other electrode configurations where hundreds of volts are applied. We present the simplest theoretical modelling capable of reproducing the experimental observations in the case of water solutions with different conductivities. Numerical computations show good agreement with the experimental observations.