CREEP-BEHAVIOR OF ELECTRORHEOLOGICAL FLUIDS

Composite particles consisting of a polymer core and an inorganic shel l were formed by suspension polymerization. For suspensions in a silic one oil, the steady-shear viscosity and creep behavior were measured i n electric fields up to 2.0 kV mm(-1). Although the polymer core is no t ER active, the suspensions of composite particles show a striking in crease in the steady-shear viscosity and the flow curve changes from N ewtonian to Bingham profiles. The ER effects can be attributed to the shell layers on the polymer surfaces. The creep curves at low stresses are composed of instantaneous elastic, retarded elastic, and viscous regions. With increasing stress the retarded elastic and viscous compo nents decrease. At some critical stress the strain almost instantaneou sly increases and reaches the equilibrium without viscous flow. After the removal of the critical stress, the suspensions show no elastic re covery. Therefore the creep and recovery behavior is purely plastic an d the critical stress corresponds to the static yield value. The appli cation of stresses above the static yield value causes the suspensions to flow. The development of yield stress (plateau value) in steady sh ear can be derived from the ideal chain model in which the particles a ll align into chains of single particle width and equal spacing. Howev er, the model cannot predict the instantaneous deformation without rec overy below the yield stress. The thick column formed by several chain s may be responsible for purely plastic responses.