Experiments in non-Newtonian fluids: Saffman-Taylor instability

Viscous fingering instability can be controlled by adding polymers to either displacing or displaced fluid as polymers alter the fluid viscosity, the main cause of viscous fingering. Most of the polymeric solutions exhibit shear-rate dependent viscosity and elasticity, simultaneously. The fluid rheology can affect the growth of fingers due to the non-uniform viscosity distribution and the presence of normal stresses. To investigate the role of rheological properties on the instability, experiments are performed using Hele-Shaw cell when one of the fluids is polymeric fluid. The aqueous solutions of polyethylene oxide (PEO) of different concentrations and molecular weights are used. The evolution of the fingers shows that displacement of PEO solutions of high concentration or high molecular weight leads to more complex and fractal-like patterns involving tip-splitting and side-branching mechanisms. The displacement when the displaced fluid is PEO solution shows more intensified patterns than the displacing PEO solution for similar viscosity contrast and rheological properties. It is observed that the shear-thinning behavior always strengthens the shielding behavior whereas the fluid elasticity always leads to tip-spitting, irrespective of the flow arrangement.