CREEP-BEHAVIOR OF ELECTRORHEOLOGICAL FLUIDS

Composite particles consisting of a polymer core and an inorganic shell were formed by suspension polymerization. For suspensions in a silicone oil, the steady-shear viscosity and creep behavior were measured in electric fields up to 2.0 kV mm-1. Although the polymer core is not ER active, the suspensions of composite particles show a striking increase in the steady-shear viscosity and the flow curve changes from Newtonian to Bingham profiles. The ER effects can be attributed to the shell layers on the polymer surfaces. The creep curves at tow stresses are composed of instantaneous elastic, retarded elastic, and viscous regions. With increasing stress the retarded elastic and viscous components decrease. At some critical stress the strain almost instantaneously increases and reaches the equilibrium without viscous flow. After the removal of the critical stress, the suspensions show no elastic recovery. Therefore the creep and recovery behavior is purely plastic and the critical stress corresponds to the static yield value. The application of stresses above the static yield value causes the suspensions to flow. The development of yield stress (plateau value) in steady shear can be derived from the ideal chain model in which the particles all align into chains of single particle width and equal spacing. However, the model cannot predict the instantaneous deformation without recovery below the yield stress. The thick column formed by several chains may be responsible for purely plastic responses. © 1985, The Society of Rheology. All rights reserved.