Electrorheological (ER) fluids are most broadly classified as either particle dispersion or homogeneous types. The history of their development has been one of a search for fluids of either type that exhibit a large ER effect—an increase in viscosity or shear stress upon exposure to an electric field—together with longterm stability under various temperatures and conditions of use.
The first particle dispersion ER fluids were invented by W.M. Winslow in the late 1940s. Many fluids of this type have been investigated since then, and a few have been developed and adopted for practical applications.
Homogeneous ER fluids have been investigated even longer. The earliest reported observations of the ER effect were for homogeneous fluids composed of glycerin and paraffin oil in the 1890s. In the 1930s, the ER effect was also observed in polar liquids such as nitrobenzene and anirin and in colloids of superfine metal powders. In all cases however the viscosity in the electric field was only two or three times the zero-field viscosity, which is too small for useful applications.
Since the advent of low-molecular-weight liquid-crystalline (LC) materials in the 1960s, investigations have been made on the viscosity changes accompanying their inherent tendency to align in an electric field, but their maximum viscosity generally is only a few times their zero-field viscosity, and their shear stress is in any case small.