Microstructural studies of electro-rheological fluids under shear

Abstract

The formation of particle chains, in a particulate suspension, in the presence of an electric field has been found to be responsible for electro-rheological (ER) behaviour of suspensions. Both the formation of chain-like sturctures, as a result of the induced dipole–dipole interactions among dispersed particles in a fluid, and the deformation of these chains under a steady laminar shear have been investigated by optical microscopic observations and Stokesian dynamics simulations. Computed transient, or ‘snapshot’, images and the deduced coordination number (the average number of contacts of the dispersed particles) have been used in the static simulation to monitor the formation of those particle chains after the model ER suspension was subjected to an external dc electric field. The response time to the applied dc electric field was defined as the time of forming an equilibrium chain-like structure within the model ER fluid. In our static simulation, the response time of the ER fluid with a particle volume fraction of 0.2618 was computed as 2000 computer time-steps which can be used to predict the real response time of a silica particles–corn oil suspension at 1 kV mm–1 as 4 ms. Using a similar simulation technique, the dynamic microstructure of the model ER fluid under a steady shear was investigated. The coordination number, a measure of the conveyed structure, was found to decrease as the applied shear rate increased. This indicates that the chain structure is destroyed by the applied shear. Similar effects were also observed directly via microscopic observations of an ER fluid under shear. This study suggests that the shear-thinning phenomenon of actual ER fluids (the dynamic viscosity of the model ER fluid decreases as the applied shear rate increases) is due to the degradation of the particle chains inside the ER fluid. Thus, the macro-rheological behaviour of an ER fluid can clearly be described in terms of its micro-behaviour when the fluid undergoes a shearing process.