Factors Influencing the Measurement of the Yield Stress of Gels with a Sphere Rheometer

A forced vibration rheometer was developed to measure the viscoelastic properties of gelled hydrocarbons. In one form the instrument consists of a sphere which performs forced sinusoidal oscillations in the liquid to be investigated. The driving force is recorded as a function of amplitude and frequency. To evaluate the force on the sphere due to the viscous drag of the fluid, wall-, end-, and depth-effects as well as the nature of the surface of the sphere were studied. It was found that the observed force is particularly sensitive to the geometry of the instrument but relatively insensitive to the nature of the sphere. This work sheds light on the general applicability of existing sphere rheometers for the rheological characterization of non-Newtonian systems.