Electric field driven addressing of oil in water droplets in the presence of gradients of ionic and nonionic surfactants

We deal with addressing of dielectric kerosene droplets dispersed in water by DC electric field. Planar microfluidic platform with a central compartment and two electrode chambers was fabricated. The platform enables the formation of concentration gradients of cationic, anionic or nonionic surfactants. We show that a kerosene droplet always moves in the direction from a 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 water phase, the Marangoni phenomenon pulls the kerosene droplet in the direction from the surfactant reservoir. So 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 were evaluated. Finally we discuss the use of the observed phenomenon for droplet addressing in fluidic systems.