Apparent Shear Thickening at Low Shear Rates in Polymer Solutions Can be an Artifact of Non-Equilibration

While shear thickening is a well-known feature of some polymer solutions, its observation in the absence of a clear specific chemical or structural mechanism and at very low rates of shear (ỳ 0.1< s-1) in our own data, as well as in several polymer systems in the literature, has prompted further investigation. Using the rheologically-reproducible and commercially available dysphagia product Resource Thicken Up Clear (produced by Nestle Health Science) as a canonical demonstration fluid, we show that the observation of a local maximum in the steady shear viscosity at very low shear rates can be completely attributed to the sample not having reached steady state conditions, and correspondingly, to the measurement not having been performed in steady simple shear flow. We propose two criteria to ensure equilibration during steady state flow rheological measurements: a substantial increase in the measurement time allotted for each point such that the total material strain accumulated in the sample is allowed to reach y =ỳt < 5 and/or a stricter convergence criterion of 10 consecutive readings within a tolerance of 1%