Mechanisms of the giant electrorheological effect

Electrorheology (ER) denotes the control of rheological characteristics through applied electric field. ER fluids constitute a class of colloids whose viscosity can increase under increasing electric field. Under very strong fields some ER fluids can turn into anisotropic solids, characterized by a yield stress. The recent discovery of the giant electrorheological (GER) effect in suspensions of nanoparticles has challenged the conventional wisdom on ER fluids, as the GER fluids can break the theoretical upper bound on the (high-field state) yield stress. Starting from experimental observations of the GER characteristics, we show that the model of aligned molecular dipole layers in the contact region of coated nanoparticles can yield predictions in excellent agreement with measured data. The statistical mechanics of the aligned dipole layers is studied through Monte Carlo simulations. We propose electrowetting between the particles and the suspending liquid, with hydrogen bonding as a contributing element, in inducing the aligned state.