A physical basis for non-Newtonian power-law viscosity

Many non-Newtonian materials have viscosities proportional to a power of shear rate. Although, generally, such relationships are regarded as being empirical, it is shown that power-law behavior arises when structure change with shear rate tends to saturation and results in flow activation energy being inversely proportional to shear rate. A temperature-dependent power-law index is predicted as is observed. Viscosity measurements are presented for a 10% sodium carboxymethylcellulose solution. It was found that both activation entropy and enthalpy decreased with shear rate, the former producing a small shear thickening and the latter a dominant shear thinning contribution to flow.