Coacervation-induced viscous fingering by surfactant-polymer interaction

The dynamics of a fluid’s displacement by another fluid are always dictated by the viscosity contrast between the two due to phenomena related to the Saffman-Taylor instability. In this study, a miscible displacement of a less viscous liquid by a more viscous shear-thinning liquid in a Hele-Shaw cell was investigated. Due to a coacervation process between the two fluids, a hydrodynamic instability, in the form of inward viscous fingering, appears, in an otherwise hydrodynamically stable system. The aqueous miscible system used consisted of an anionic xanthan gum dispersion, as the injection liquid, which displaced a cationic C14TAB solution. The two oppositely charged species form polymer-surfactant complexes due to electrostatic interactions, which in turn separate from the water phase forming a gel-like membrane. In the course of the displacement, a variety of patterns was observed, the mechanism behind of them is suspected to be governed by a synergy of background contributing factors related to the rheological properties of the system involving viscosity gradients, the non-Newtonian nature of the displacing solution and the complex rheology of the coacervate phase as well as the mechanisms related to the membrane growth. The displacement flow rate and the Hele-Shaw cell gap width were found to determine the distinct displacement regimes; one viscosity-dominated, the other buoyancy-dominated. The above aspects are relevant to applications of displacement-involving coacervation systems in engineering and technology.