Self-Oscillations Accompanying Shear Flow of Colloidal and Polymeric Systems. Reality and Instrumental Effects

Abstract

The study of different colloidal and polymeric fluids by rotational rheometry has shown that their shear deformation may be accompanied by oscillations of three types. Oscillations of the first type result from the influence of controlling or measuring systems of an instrument and are observed in a stress range of 0.01–0.1 Pa, although the elasticity of a fluid may substantially increase this threshold. Oscillations of this type arise in all regimes of measurements; however, they make an essential contribution only when the rheological properties of low-viscosity fluids or viscoelastic systems are measured near the yield stress. These instrumental oscillations represent an artifact that is not related to the properties of an object under examination. Oscillations of the second type result from the slip/stick effect upon the contact between a solid working surface of an instrument and the surface of a studied viscoelastic material. Such oscillations are observed at relatively high shear rates of moving various materials that may form solidlike structures. In particular, oscillations of this type are of special importance in the case of gels. Oscillations of the third type are due to volume effects and reflect structural bifurcations at rather high shear stresses (substantially higher than possible experiment errors). Such oscillations reflect periodic structure formation/disruption in multicomponent systems, such as, e.g., concentrated suspensions. The amplitude of these oscillations depends on the elastic properties of studied systems. “Superposition” of different reasons, which cause the oscillations observed in the course of testing multicomponent systems, is also possible.