Flow-driven pattern formation during coacervation of xanthan gum with a cationic surfactant

We experimentally demonstrate that the coacervation of a biopolymer can trigger a hydrodynamic instability when a coacervate is formed upon injection of a xanthan gum dispersion into a cationic surfactant (C₁₄TAB) solution. The local increase of the viscosity due to the coacervate formation induces a viscous fingering instability. Three characteristic displacement regimes were observed: a viscous fingering dominated regime, a buoyancy-controlled "volcano" regime and a "fan"-like regime determined by the coacervate membrane dynamics. The dependence of the spatial properties of the viscous fingering pattern on the Péclet and Rayleigh numbers is investigated.