Electrorheological Fluids: Mechanisms, Dynamics, and Microfluidics Applications

Electrorheological (ER) fluids, consisting of solid particles dispersed in an insulating liquid, display the special characteristic of electric-field-induced rheological variations. Nearly six decades after their discovery, ER fluids have emerged as materials of increasing scientific fascination and practical importance. This review traces the mechanisms responsible for these fluids' ER response and their attendant theoretical underpinnings. In particular, ER fluids are divided into two different types, dielectric electrorheological (DER) and giant electrorheological (GER), which reflect the underlying electric susceptibility arising from the induced dielectric polarization and the orientational polarization of molecular dipoles, respectively. The formulation of a continuum ER hydrodynamics is described in some detail. As an electric-mechanical interface, ER fluids have broad application potential in electrifying the control of mechanical devices. This review focuses on their applications in microfluidic chips, in which GER fluids have enabled a variety of digitally controlled functionalities.