Viscosity enhancement in non-Newtonian flow of dilute aqueous polymer solutions through crystallographic and random porous media

We have studied dilute aqueous solutions of hydrolysed poly(acrylamide), in various ionic environments, in flow around single spheres and around two spheres aligned on the axis of flow. The spheres are held on flexible cantilevers while the polymer solutions, or solvent, are drawn past at controlled flow rates. We estimate the specific viscosities of the various solutions as a function of the strain rate, over strain rates encompassing both the shear thinning and extension thickening regimes. For flow of solutions without added salts around a single sphere we observe shear thinning followed by a significant increase in the non-Newtonian viscosity with increasing strain rate. The shear thinning is lost by the addition of salts. The addition of modest amounts of divalent salt also results in a loss of the maximal extensional viscosities of the solutions, which has important implications regarding the effectiveness of hydrolysed poly(acrylamide) in oilfield applications. For flow of polymer solutions around two axially aligned spheres we observe a significant reduction in the non-Newtonian forces experienced by the downstream sphere in comparison to the upstream sphere. We consider that this is of major relevance to the understanding of non-Newtonian viscosification in porous media flow.