Impact of Ionic Surfactants on the Electrokinetic Control of Viscous Fingering: An Experimental Approach*

The displacement of a more viscous fluid by a less viscous one in a porous medium results in viscous fingering (VF), an interfacial instability where viscous and capillary forces compete. VF control is an ongoing challenge, as in some applications this instability is desirable (mixing in porous media) while in others it is detrimental (e.g. enhanced oil recovery). Control methods are divided in passive and active approaches, where in active approaches VF is controlled in-operando, while passive methods involve system alterations prior to displacement. A promising active control method is the electrokinetic method, in which an applied field induces electrokinetic thinning or thickening of the fluid, resulting in a controlled displacement. In this work we experimentally investigate the synergetic effect of an externally applied electric field and ionic surfactants on the (de)-stabilization of the interface between a more viscous perfect dielectric fluid and an immiscible, less viscous, conducting surfactant solution. We use surfactant concentrations around the critical micelle concentration for the displacement and apply the electric field in both the same and opposite direction to pressure driven flow to investigate both stabilization and destabilization situations. Our results show that in applications where instabilities are required, viscous forces should dominate capillary forces, which is achieved through a combination of an anionic surfactant and a negatively applied electric field.