Large amplitude oscillatory shear of ER suspensions

Electrorheological (ER) fluids are fascinating materials that undergo drama tic reversible changes in their rheological properties upon the application of electric fields. In many proposed applications, the fluids will be subj ected to a dynamic stimulus with finite deformation. We use a particle-leve l simulation method to investigate the dynamic behavior of monolayer ER flu ids. ER fluids are linear viscoelastic for only very small strain amplitude s. The transition to nonlinear deformation arises from very slight rearrang ements of unstable structures. At large strain amplitudes, the behavior is viscoplastic, while at large dimensionless frequencies (proportional to ome ga/E-0(2), where omega is the oscillation frequency and E-0 is the electric field strength), the response is Newtonian for all strain amplitudes. Simu lation results agree qualitatively with experiments. The dependence of the flow behavior on the strain amplitude and dimensionless frequency is summar ized in the form of a Pipkin diagram. (C) 1999 Elsevier Science B.V. All ri ghts reserved.