An experimental study of the first normal stress difference — shear stress relationship in simple shear flow for concentrated shear thickening suspensions

Abstract

For many polymeric solutions in a simple shear flow a plot of the logarithm of the first normal stress difference N ₁ against the logarithm of the shear stress σ, for a range of temperatures results in a linear relationship. For such polymer solutions these plots yield a straight line of slope very close to 2 when measured at low shear rates. This relationship is tested against a polymer solution (polyacrylamide in a 50/50 mixture of glycerol/water), a polymer melt (polyethylene), and three concentrated suspensions. These are Superclay (supplied by English China Clays, Cornwall, U.K.) in water, polyvinylchloride in dioctylphthalate and polystyrene latex in water, at volume concentrations of 40% 48%, and 60%, respectively. It was found that the log N ₁ — log σ relationship is applicable to the polymer solution and melt over a significant range of shear rates and temperatures. In the cases of concentrated suspensions the relationship holds to the point of onset of the shear thickening behaviour. Beyond this point a different relationship exists, however, flow instabilities are apparent. A comment on the contribution of N ₁ and N ₂ to the flow instabilities is made.