We experimentally observed oscillatory motion of water droplets in microfluidic systems with coplanar microelectrodes under imposed DC electric fields. Two-electrode arrangement with no bipolar electrode and eight-electrode arrangement with six bipolar microelectrodes were investigated. Kerosene was used as the continuous phase. We studied the dependences of the oscillation frequency on the electric field intensity and ionic strength of the water phase. We found that the electric field dependence is strongly nonlinear and discussed possible reasons of this phenomenon, e.g., the droplet deformation at electrode edges that affects the charge transfer between the electrode and droplet or the interplay between the Coulomb force on free charge and the dielectrophoretic force. Our experiments further revealed that the oscillation frequency decreases with growing salt concentration in the two-electrode arrangement, but increases in the eight-electrode arrangement, which was attributed to surface tension related processes and electrochemical processes on the bipolar electrodes. Finally, we analyzed the effects of the electric field on the oscillatory motion by means of a simplified mathematical model. It was shown that the electric force imposed on the droplet charge is the key factor to induce the oscillations and the dielectrophoretic force significantly contributes to the momentum transfer at the electrode edges. For the same electric field strength, the model is able to predict the same oscillation frequency as that observed in the experiments.

A new method for slug-flow (segmented flow) characterization by means of ac electric sensing is proposed. Water segments regularly dispersed in kerosene are used as an experimental system. The sensing procedure is carried out in a plexiglass microchip with embedded gold microelectrodes. The presence of passing slugs over a measurement spot is determined from impedance variations. A square-shaped signal resulting from the slug flow is acquired and flow properties such as the mean velocity and length of the slugs are calculated. Complex behaviour of the corresponding electrochemical system is studied. Dependence of the impedance signal on the flow rate, ion concentration in the dispersed water slug and electric field strength are discussed and explained in detail. Advantages and disadvantages of the suggested method, in comparison with existing capacitive noncontact methods, are also clarified. Unlike the noncontact methods of electric sensing, with the insulation dielectric layer over the electrodes, our electrodes are in a direct contact with the carrier phase. The results show that the method is promising for process applications and will be further improved.

We deal with controlled transport (addressing) of dielectric kerosene droplets dispersed on water surface by DC electric field in the presence of surfactants. Experimental planar microfluidic platform with a central chamber and two electrode compartments enables formation of concentration gradients of cationic, anionic or nonionic surfactants. We show that a kerosene droplet always moves away from the surfactant reservoir due to the Marangoni phenomenon. The release of ionic surfactants can be well controlled by imposed electric field. Once surfactant molecules reach the surface of the water phase, the Marangoni phenomenon pulls the kerosene droplet in the direction from the surfactant reservoir. Thus the observed mechanism does not rely on the classical electrophoretic migration of charged macroscopic objects -droplets. As the dependence of droplet motion on the absolute value of electric field strength is linear, apparent droplet mobilities are evaluated. Finally we discuss the use of the observed phenomenon for droplet addressing in fluidic systems.

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