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.

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

Viscosity, modulus, and yield stress for 0–6 wt% aqueous solutions of Carbopol 941 were investigated using constant shear rate, constant shear stress, and dynamic oscillatory experiments. The microgel character of the polymer was evident from the solid-like behavior of the solutions above 1 wt%. Yield stress increased with concentration, but yield occurred at a critical shear strain of 40%, independent of concentration. The static stress-strain relationship became non-linear at ~ 25% strain, in fair agreement with the onset of non-linear response in the storage modulus at ~ 10% strain. Small strain moduli from static and low frequency measurements agreed rather well; modulus values obtained from the recoverable strain after yielding were 30–40% smaller. Solutions flowed at near-constant stress in the low shear rate regime; at higher rates the stress increases with shear rate more rapidly. The viscosity did not obey the Cox-Merz rule. Steady-state viscosity scaled with polymer concentration to the 3/4 power. Results were interpreted using a cellular, deformable sphere model for the polymer, in analogy to emulsions and foams.

Rheology of concentrated microgel solutions

Dynamic rheological measurements and analysis of starch gels

The use of concentrated colloidal suspensions is common in several industries such as paints, foodstuffs and pulp and paper. These suspensions are generally composed of strongly interactive particles. If the attractive forces dominate the repulsion and Brownian forces, the particles aggregate to form a three-dimensional network yielding a gel structure. Under flow, the micro-structure of suspensions can be drastically modified and the rheological properties are then governed by structure breakdown and build-up. In this work, we propose a structural network model based on a modified upper convected Jeffreys model with a single relaxation time and a kinetic equation to describe the flow-induced micro-structure evolution. Three distinct kinetic equations are tested for this purpose. The proposed model describes yield and thixotropic phenomena, nonlinear viscoelastic behavior and output signal distortions observed for relatively small strain amplitude during oscillatory measurements, and overshoots observed in stress growth experiments. A comparison of model predictions and experimental data for fumed silica and coating colors is also presented. However, different model parameters must be used to correctly predict the different flow properties indicating that a more versatile or generalized kinetic equation must be proposed.

Rheological modeling of concentrated colloidal suspensions

The transport properties and, specifically, the rheological behavior of real and complex materials such as polysaccharide systems can be significantly affected by several factors, mainly related to molecular and supermolecular features. Most of these factors are common to all polymeric systems, as they have universal character; others are peculiar to carbohydrate polymers. If we bear in mind the concepts discussed in §1.4, we can easily imagine for polysaccharides a variety of structural conditions much wider than that generally observed for synthetic polymers. On both molecular and supermolecular scales, polysaccharides possess special characteristics that reflect specific behavior so that different classes of materials can be identified.

Rheology of polysaccharide systems

Wheat starch granules dispersed in water and heated to 70°C swell extensively to form dispersions of irregularly shaped, deformable particles. Because the granules are deformable under shear, the rheological properties of the dispersions can be examined at volume fractions up to unity, which corresponds to a starch concentration of 14%. At concentrations of 10–13%, yield stresses are observed in couette flow at 23°. At concentrations of 14% and above, the dispersions show essentially solidlike behavior in shear. Shear stress‐shear strain diagrams for these gels were obtained in a parallel plate rheometer. The major experimental problem in this testing mode is slip between the gel and the parallel plates. This problem was resolved by using cyanoacrylate ester adhesive to attach the gel to the parallel plates. With slip eliminated, the shear stress‐shear strain diagrams show an initial linear region from which a small deformation modulus is obtained. As shear strain increases, a yield point is observed where the stress‐strain curve shows an inflection point. At higher strains, the curve rises again, and forces normal to the direction of shear are generated. Failure finally occurs at strains of 25–50%.

https://sor.scitation.org/doi/pdf/10.1122/1.549717

E. B. Bagley

Papers

Yield Stresses in Concentrated Dispersions of Closely Packed, Deformable Gel Particles