9: Review of Electrorheological Fluids (ERFs) as Smart Material

Chapter 9 reviews electrorheological fluids (ERFs) as smart materials. ERFs belong to a class of smart materials capable of changing from a liquid phase to a much more viscous liquid and then to an almost solid phase in the presence of an electric field. They are essentially colloidal suspensions of highly polarizable particles in a nonpolarizable solvent. The solid phase of an ERF typically has mechanical properties similar to a solid like a gel and can perform the phase change from liquid to thick liquid like honey and then solid or in reverse from a solid transform to a thick liquid and then a thin liquid in a matter of few milliseconds. The effect is called the “Winslow effect” after its discoverer Willis M. Winslow, who obtained a US patent on the effect in 1947 and published an article on it in 1949. Note that the change is not just a change in fluid viscosity but also the emerging solid-like properties and hence these fluids are now known as ERFs, rather than by the older term electro-viscous fluids (EVFs). The effect is better described as an electric field dependent shear yield stress such as what occurs in a Bingham plastic (a type of viscoelastic material like thick honey or wax), with a shear stress yield point dependent on the electric field strength. The ERF once in a yield shear mode behaves like a Newtonian fluid when there is no yield shear stress and stress is directly proportional to the shear rate γ.