ABSTRACT
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.