Controlling the surface charge of water droplets in non-polar oils

We show that the surface charge and, hence, the electrophoretic mobility of water droplets dispersed in non-polar oils can be adjusted in magnitude and sign through the addition of ionic surfactants. The positive, native charge of water-in-oil droplets is reduced by increasing the concentration of the anionic surfactant SDS in the water. At high enough SDS concentration, the droplet charge becomes negative. This mechanism works with, both, silicone as well as paraffin oils and is limited by the critical micelle concentration of SDS in water, at which the interface is saturated by the surfactant. Direct, physical contact-charging of water droplets at biased electrodes, however, overrides any charge due to chemical species at the water–oil interface and confers an overall charge of the same sign as the electrode potential to the droplets. The possibility to easily adjust the droplet charge by chemical as well as physical means permits flexible electrophoretic manipulation in microfluidic devices.