Yield stress measurement of a thixotropic colloid

Yield stress measurements have long been considered to be inconsistent and difficult, especially for a thixotropic colloid, since the thixotropic behavior gives rise to time-dependent rheological properties. In this paper, we attempted measuring the yield stress of aqueous xanthan gum (XG) solutions in different XG concentration. Our rheological measurements showed that XG in aqueous solutions, which can be characterized as weak gels, were thixotropic for 0.4, 0.6, 0.8, and 1 wt% XG concentrations. To ensure the accuracy of our rheological measurement data, detailed wall slip and stress relaxation time were first investigated prior to the actual yield stress measurement. A standard sample loading procedure was used to ensure the consistency of the loaded sample. Various methods were employed such as steady shear, oscillatory shear, rheological model fitting, and creep test to determine the yield stress of aqueous XG solutions. A sigmoidal model was also proposed to obtain a yield stress from our steady shear measurement unambiguously. In our steady shear measurements, a resistance to an avalanche of flow is observed and it is likely due to a conformational change of XG polymers under shear. As compared to other measurement methods, it was found that the most reliable way to obtain a yield stress is via small amplitude oscillatory shear measurements, in which a yield stress can be obtained conveniently at the maximum storage modulus, and it is well matched with our steady shear and creep test results.