Self-similarity in electrorheological behavior

In this work we study creep flow behavior of suspension of Polyaniline (PANI) particles in silicone oil under application of electric field. Suspension of PANI in silicone oil, a model electrorheological fluid, shows enhancement in elastic modulus and yield stress with increase in the magnitude of electric field. Under creep flow field, application of greater magnitude of electric field reduces strain induced in the material while application of greater magnitude of shear stress at any electric field enhances strain induced in the material. Remarkably, time evolution of strain in PANI suspension at different stresses, electric field strengths and concentrations show superposition after appropriate shifting of the creep curves on time and strain axes. Observed electric field - shear stress - creep time - concentration superposition demonstrates self-similarity in electrorheological behavior. We analyze the experimental data using Bingham and Klingenberg - Zukoski model and observe that the latter predicts the experimental behavior very well. We conclude by discussing remarkable similarities between observed rheological behavior of ER fluids and rheological behavior of aging soft glassy materials.