Cone-and-plate rheometry of yield stress fluids. Study of an aqueous gel

TLDR: In this paper, the authors focus on the rheometric study of a physical gel exhibiting a yield stress and show how the determination of shear rheological properties can be affected by anomalous phenomena such as fracture and slip at the wall.

Abstract: This work particularly focuses on the rheometric study of a physical gel exhibiting a yield stress. The measurements were carried out in a cone—plate configuration using two different types of rheometer working under controlled torque or under controlled velocity. Shear creep, constant shear rate, and stress relaxation tests have been performed. Measurements of apparent viscometric properties were conducted at the same time as observation of the strain field in the sample. Observing the strain field enables us to confirm the reliability of the interpretation of the results and also to estimate the true shear rate in the fluid. It is shown how the determination of shear rheological properties can be affected by anomalous phenomena such as fracture and slip at the wall. The influence of roughness of the tool surfaces and of evaporation shows up. The results presented in this study show how some rheometrical measurements of the yeild stress and the microstructural interpretations given, may be erroneous. Some recommendations are made in order to improve current rheometrical tests and their interpretation. A log—log graph with typical shear stress-shear rate measurements and their corresponding strain fields is given: it should be used as a guideline in yield stress fluids rheometry. In addition it is made clear that visual observation of the sheared sample is a key technique. A protection which completely eliminates evaporation is suggested. It is shown that the measurement of residual stress in stress relaxation tests may be a convenient means of determining the value of the yield stress.