Yield stress measurements with the vane

Yield stress measurements were performed on a TiO2 pigment suspension with the vane in both a rate controlled and a stress controlled mode. In the rate controlled mode, a constant rotational speed is applied to the vane immersed in the suspension, and the resulting stress is measured as a function of time. In the stress controlled mode, a constant stress or a constant stress-rate is applied to the vane immersed in the suspension, and the resulting creep angle is measured as a function of time. In both modes the yield stress is determined as the minimum stress required for continuous rotation of the vane. A Haake Rheometer and a modified Weissenberg Rheogoniometer facilitated the controlled rate measurements. The significant difference between these instruments is that the Weissenberg torsion bar is 550 times stiffer than the Haake spring. To ensure a comparable time frame of measurement with the Haake, it was necessary to use lower rotational speeds in the operation of the Weissenberg. Although agreement in the measured yield stress within 5% was established between the Haake at a rotational speed of 0.021 rad s(-1) and the Weissenberg at a rotational speed of 0.0063 rad s(-1), the shapes of the respective stress-time profiles were strikingly different. The Haake exhibited a largely elastic response, whereas the Weissenberg exhibited a largely viscoelastic response. The yield stress measured by both the Haake and the Weissenberg corresponds to the transition stress between viscoelastic and fully viscous flow. A Bohlin Rheometer facilitated the stress controlled measurements. The yield stress measured by the Bohlin was up to 13% lower than that measured by the Haake and the Weissenberg, and seemed to correspond more closely to the transition stress between fully elastic and viscoelastic flow.