Measuring the yield behaviour of structured fluids

Structured fluids or semi-solids, shear-thin dramatically at a critical shear stress and include household consumer products such as skin creams and lotions, sauces, pastes, spreads, mayonnaise, and ice cream. Such fluids also tend to be highly thixotropic so that theological measurements are often irreproducible due to the disruption of the microstructure during sample loading and dependence on measurement procedure. The use of a vane, consisting of four blades that form a cross shape at the end of a central shaft, inserted into the fluid is an easy and convenient method of performing accurate rheology measurements on such samples. The vane enables the flow curves to be measured for structured fluids, which would otherwise be difficult or impossible to determine accurately using conventional techniques. Example studies have shown its particular usefulness in distinguishing crucial textural attributes of mayonnaise using a simple rapid test on the sample in the jar and enabled the yielding and flow behaviour of ice-slurries to be investigated at sub-zero temperatures. Accurate theological measurements would, in principle, allow the examination of structured fluids in terms of their underlying microstructure. However, due to the thixotropic nature of structured fluids, material properties such as the apparent yield stress are typically dependent on the test procedure. The effect of various test methodologies on the material properties of a lamellar gel network used to structure skin creams are compared, including test duration time for shear stress-sweep tests and the effect of shear rate in step-shear tests used for measuring a dynamic yield stress. The influence on the results using different test methodologies is discussed and how they can be related to one another. The results highlight the difficulty in choosing the most appropriate test procedure to give 'true' and accurate material properties for highly structured fluids. (C) 2004 Published by Elsevier B.V.