Electric Field Driven Addressing of Oil-in-Water Droplets in the Presence of Gradients of Ionic and Nonionic Surfactants

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.