Creep and recovery behaviors of a polythiophene-based electrorheological fluid

We investigate the creep response of poly(3-thiopheneacetic acid) (PTAA) particles doped with perchloric acid. With increase in applied stress, these suspensions exhibit an evolution from a linear viscoelastic response, with three components of instantaneous elastic strain, retarded elastic strain and viscous strain, to a nonlinear viscoelastic response, where the retarded elastic and viscous strains monotonically decrease and a plastic contribution to the instantaneous strain grows, followed by a viscoplastic solid behavior, with fully plastic instantaneous strain, and finally a transition from plastic solid to a plastic liquid at the yield stress. With increase in electric field strength at fixed particle concentration and applied stress, the viscoplastic response diminishes, and more elastic behavior ensues. For highly doped samples, at high-electric field strengths, a fully elastic solid response is observed in the linear viscoelastic regime. The equilibrium compliance, JC and steady state recoverable compliance JR, were investigated as a function of electric field strength, particle concentration and particle conductivity. The results are interpreted in terms of the field-induced formation of thick fibrillar aggregates spanning the gap between the electrodes, each consisting of bundles of particle strings. Strings, which are fully connected to both electrodes generate an elastic response to the applied stress, whereas strings which are attached at only one end or are unattached generate a viscoplastic response. The net effect of an increase of the electric field strength, particle concentration, or particle conductivity is an increase in elasticity, i.e. predominantly creation of fully connected particle strings.