Comparison of methods to measure yield stress of soft solids

Many interesting industrial materials are highly viscous or paste-like, i.e., soft solids. Their complexity, proceeding from heterogeneous structures, often reveals interesting rheological properties. Their processing requires the determination of rheological parameters such as viscosity, modulus, and yield stress value. We compare three methods to measure the yield stress of one particular soft solid system, i.e., concentrated surfactant systems, models for bar soap. One method is based on orifice die extrusion and uses the Benbow-Bridgwater equation. Two methods used a rotational rheometer: in one, dynamic (small strain sinusoidal oscillation) experiments were performed as a function of increasing strain amplitude with serrated parallel plate geometry. The maximum in the elastic stress curve was used to estimate the yield stress. The other method using the rotational rheometer, which we call strand shearing, involves the use of a new fixture designed to grip these samples that were too stiff for serrated plates but too soft for traditional solids fixtures. In this method, the maximum of a plot of stress versus time at a constant shear rate is taken as the yield stress. The advantages and limitations of these techniques are discussed and applied to our particular soap model system.