ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 3, pp. 567–575. © Pleiades Publishing, Ltd., 2010. Original Russian Text © A.V. Agafonov, A.G. Zakharov, 2010, published in Rossiiskii Khimicheskii Zhurnal, 2010, Vol. 53, No. 2, pp. 15–22. 567 Electrorheological Fluids A. V. Agafonov and A. G. Zakharov Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia e-mail: ava@isc-ras.ru Received January 10, 2009 Abstract—Advances in one of the most promising fields of the chemistry of smart materials, specifically, electrorheological fluids are considered. The electrorheological effect and the structure and properties of electrorheological fluids are described. Modern views on the nature of the electrorheological effect are considered. The review focuses on the application of nanomaterials as the disperse phases in electrorheological fluids. Recent avances in the sol–gel synthesis of nanostructured colloid systems and the electrorheological characteristics of their based liquid systems are considered. Certain aspects of practical application of electrorheological fluids are presented. INTRODUCTION Development of highly efficient electrorheological fluids capable of reversibly varying their viscosity and theoretical research in electrorheology is one of the most promising fields of the chemistry of smart materials. This problem relates to priority directions of materials science, such as synthesis of hybrid organic inorganic materials, sol–gel technology, nanomaterials, and electroconducting polymers. Stability of electrorheological fluids filled with nanosized electrorheologically active materials is one of the most discussed problems of electrorheology. The passage to a nanosized disperse phase much attenuates the gravitational instability of the disperse system, and the special structural organization of nanoparticles due to hybrid formation with polymers much enhances the aggregative stability of the colloid system. The disperse phase with a reduced particle size possesses a lower abrasiveness, which makes possible development of electrorheological devices with smaller interelectrode gaps and microscopic electro- mechanical systems. The use of nanomaterials as a packing in electrorheological fluids led to a qualitative breakthrough in reaching high electrorheological effects, thus opening up new possibilities for developing devices whose operating principle is based on this effect. Let us make a short excursus into the history of electrorheology. The electrorheological effect is a fast reversible change of viscosity of colloid dispersions of certain materials in dielectric liquids (Fig. 1). It was discovered by Winslow in 1947 [1, 2] who studied the effect of electric field on the viscosity on silica gel in kerosene. Later such systems were given the name electrorheological fluids. The solid–liquid transition DOI: 10.1134/S1070363210030382 Fig. 1. Demonstration of the electrorheological effect. Electrorheological fluid with the disperse phase (30%) on the basis of a TiO 2 –hydroxypropyl cellulose hybrid nano- composite: (а) fluid in an electric field of 400 V mm –1 (interelectrode gap 5 mm) and (b) no electric field is applied. (a) (b)