Shear rheometry of fluids with a yield stress

Abstract

This work is concerned with the measurement of viscometric material functions of greases, which are fluids with a yield stress. Rheometers working under controlled stress or under kinematically controlled conditions have been used in plane and cone geometry. Viscometric functions and the problems encountered in measurements are examined simultaneously. It is shown how fracture and slippage at boundng surfaces alter the measurements of the bulk properties and how they can be controlled. Fractures limit the amplitude of deformation; hence the rate of the deformation and duration of measurements. Slippage is a limiting factor at low rates of deformation, but it can be suppressed. Within the range defined by these two limiting factors, viscometric properties have been measured: elastic properties, yield values, and start up, steady and unsteady viscometric material functions similar to those for melted polymers have been found.