Motion of liquid droplets with a surface electric charge can be efficiently controlled by dc electric field. Here, we show that the surface of a dielectric kerosene droplet can be charged by the addition of ionic surfactants to a surrounding aqueous electrolyte. The direction of droplet motion is determined by the polarity of the surfactant charge and the orientation of the imposed electric field. We have found that the effective electrophoretic mobility of dielectric droplets in a confined channel is directly proportional to the logarithm of the surfactant concentration even for values significantly exceeding critical micelle concentration (CMC). We attribute this finding not only to adsorption of ionic surfactants to the surface of dielectric droplets but also to the weakening of electro-osmosis at channel walls due to the increase of ionic strength in the aqueous phase. Our findings can be exploited in microfluidic reactors and separators for on request dosing, sampling, and separation of dielectric fluids.

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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