Lyophobic colloidal dispersions, aggregated surfactant systems, and polymer solutions, as well as foams and emulsions, can all be deformed by weak external forces; rheology is the study of deformation and flow of materials. Various rheological quantities arising from the response of a material to shear are defined. For liquids the stress, τ, applied is related to the rate of deformation, that is, the shear strain rate, γ̇. For Newtonian fluids τ and γ̇ are linearly related and τ / γ̇ is the viscosity, η. Other nonlinear relationships correspond to shear thinning and shear thickening fluids and to plastic behaviour in which there is a yield stress. Viscoelastic systems exhibit both viscous and elastic properties; such behaviour is often treated using the simple Maxwell model. Some illustrative experimentally observed rheological behaviour is presented.
Pre- and post-transition behavior of shear-thickening fluids in oscillating shear
