Viscous fingering in a non-Newtonian liquid radial displacement by surfactant (C14TAB)-polymer (Xanthnan Gum) interaction

We investigated a miscible displacement of a less viscous liquid by a more viscous shear-thinning liquid in a Hele-Shaw cell. Due to a coacervation
reaction between both liquids, a hydrodynamic instability appears in the form of inward viscous fingering. The liquids consisted of a solution of
the anionic biopolymer xanthan gum, as the injection liquid, which displaced a cationic surfactant (C14TAB) aqueous solution (Keshavarzi et al.,
2019). In the contact zone between the two solutions, the oppositely charged species form polymer-surfactant complexes due to electrostatic
interactions. The electrostatic and hydrophobic interactions between these complexes lead to a self-assembly process, forming a membrane
structure separated from the main solution. During the continuing radial displacement, a large variety of patterns can emerge which is attributed
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. Variation of the flow rate and gap width of the Hele-Shaw cell revealed distinct instability regimes and allowed
to identify main contributing mechanisms. These insights open the door for further investigation of fluid mechanics problems (i.e. Saffman-Taylor
instability) (Saffman & Taylor, 1958) in multiphase systems of complex rheology and its applications in engineering and technology.
References:
Keshavarzi, B., Langmuir. 2019, 35(42), 13624-13635.
Saffman, P., & Taylor, G. Proceedings of the Royal Society A: Mathematical, Physical. 1958, 245(1242), 312-329.
Acknowledgements: This work was supported by the German Aerospace Center (DLR) with funds provided by the Federal Ministry for
Economic Affairs and Energy (BMWi), Grant No. 50WM2061 (project ChemFront).