Simple shear of materials characterized by steady non-monotonous flow curve

The non-monotonic character of the steady flow curve is a possible explanation for spurious phenomena (wall depletion, yield stress, stick-slip flow and shear banding) observed in the shear rheological tests of complex fluids with unstable internal network structure. In this work, lubricating greases, polyxiloxane and cosmetic creams, sensitive magnetic fluids and solder pastes are experimentally investigated to detect possible existence of instability region in the flow curve. The experiments are based on simple shear and dynamic tests performed both in the linear and non-linear flow regimes. The results showed the time dependence and the existence of some discrete steady preferred values of the viscosity function. The experiments are accomplished by numerical simulations of generalized Newtonian models with non-monotonous flow curve. The correlation between experiments and simulations indicate that material instability is always related with the onset of banding flow structure in the vicinity of the walls. The numerical results are qualitatively consistent with experiments and prove that shear bands location within the gap is dependent (for given material parameters) by the dynamics of the flow, respectively the values of initial/boundary conditions.