A Comparison of Techniques for Measuring Yield Stresses

Three techniques for measuring yield stresses are compared by performing parallel experiments on five model oil-in-water emulsions. The emulsions displayed yield stresses of 50—550 dynes/cm². The three techniques involved: (1) measurements of rotation rate and stress in a concentric-cylinder geometry, (2) measurement of torque and apparent shear rate in a parallel disk geometry at two different gap heights, and (3) measurement of rotation under constant applied stress in a vane geometry previously described by Roger. Emulsions are prone to slip at solid boundaries; therefore, methods to assess wall slip contributions for techniques (1) and (2) are developed. The vane device eliminates the possibility of slip. The measurements give comparable results. The techniques are compared with respect to: precision, ease of implementation, information obtained, and sample requirements.