Generation of Electrokinetic Flow in a Doped Non-Polar Liquid

The presence of considerable amounts of free charge dispersed in a liquid is the basis for electrokinetic phenomena which are related to the existence of an electrical double layer (EDL). In polar liquids, the dissociation of electrolytes into ionic species is well understood and numerous electrokinetic phenomena are known; a good overview is given by e.g. Delgado et al. [1]. In nonpolar liquids it is known that electrical charges can exist as well. The presence of these electrical charges is utilized, for example, in colloid science to stabilize particle suspensions [2]. For this purpose, surfactants are added which enhance the zeta potential of the particles in order to prevent their agglomeration. Additionally to the manipulation of surface charges, it is reported that the electrical conductivity of nonpolar liquids essentially increases when surfactant is added and traces of water are present [3]. Such ternary solutions of nonpolar liquid-water-surfactant are known to contain surfactant agglomerations, so-called inverted micelles with a size of several nanometers, detectable for instance by quasielastic lightscattering measurements. Figure 1 sketches the generation and structure of an inverted micelle. In general, surfactants are macromolecules consisting of different functional groups, e.g. a polar “head” and a nonpolar “tail”. Above the critical micelle concentration (cmc), surfactant molecules attach with their polar head at a water droplet forming the inverted micelle. It is assumed that electrical charges are dissolved in the polar core of the inversed micelles enabling opposite charges to be held sufficiently far apart and preventing an agglomeration of different micelles [4].