Scientists and engineers are most familiar with single-crystal or polycrystalline field-responsive or “smart” materials with responses typically occurring while the materials remain in the solid state. This issue of MRS Bulletin focuses on another class of field-responsive materials that exhibits a rapid, reversible, and tunable transition from a liquidlike, free-flowing state to a solidlike state upon the application of an external field. These materials demonstrate dramatic changes in their rheological behavior in response to an externally applied electric or magnetic field and are known as electrorheological (ER) fluids or magnetorheological (MR) fluids, respectively. They are often described as Bingham plastics, and exhibit a strong field-dependent shear modulus and a yield stress that must be overcome to initiate gross material deformation or flow. Prototypical ER fluids consist of linear dielectric particles (such as silica, titania, and zeolites) dispersed in nonconductive liquids such as silicone oils. Homogeneous liquid-crystalline (LC) polymerbased ER fluids have also been recently reported. MR fluids are based on ferromagnetic or ferrimagnetic, magnetically nonlinear particles (e.g., iron, nickel, cobalt, and ceramic ferrites) dispersed in organic or “aqueous liquids. Unlike ER and MR fluids, ferrofluids (or magnetic fluids), which are stable dispersions of nanosized superparamagnetic particulates (~5–10 nm) of such materials as iron oxide, do not develop a yield stress on application of a magnetic field. Applications of ferrofluids are primarily in the area of sealing devices (see Rosensweig for more information). Since ferrofluids are well-known and have been extensively discussed elsewhere in the literature, they will not be treated in detail here.

This article focuses on the synthesis and processing of novel magnetorheological (MR) fluids. The process for preparing MR fluids typically involves introducing magnetic particles into base liquid under low shear conditions. This is followed by ball milling in the fluid with zirconia (ZrO2) grinding media for about 24 h. High-purity carbonyi iron (Fe) powders have been used for the synthesis of ironbased MR fluids while the ferrite-based MR fluids used magnetic manganesezinc ferrite and nickel-zinc ferrite powders.

Typical volume fractions of the magnetic phase that lead to MR fluids with respectable yield stresses tend to be about 0.3–0.5. Higher volume fractions, in principle, can lead to higher strength MR fluids. However, higher volume fractions tend to cause a significant, and often undesirable, increase in the “off-state” viscosity of the MR fluids. The rationale for selection and the role of different components of MR fluids are briefly discussed in the following sections.

In the absence of an applied magnetic field, magnetorheological (MR) fluids typically behave as nearly ideal Newtonian liquids. The application of a magnetic field induces magnetic dipole and multipole moments on each particle. The anisotropic magnetic forces between pairs of particles promote the head-to-tail alignment of the moments and draws the particles into proximity. These attractive interparticle forces lead to the formation of chains, columns, or more complicated networks of particles aligned with the direction of the magnetic field. When these structures are deformed mechanically, magnetic restoring forces tend to oppose the deformation. Substantial field-dependent enhancements of the rheological properties of these materials result, as demonstrated in Figure 1.

The myriad potential applications of MR and electrorheological (ER) fluids provide considerable motivation for research on these materials. The availability of fluids with yield stresses or apparent viscosities that are controllable over many orders of magnitude by applied fields enables the construction of electromechanical devices that are engaged and controlled by electrical signals and that require few or no moving parts. Potential automotive applications include electrically engaged clutches for vehicle powertrains and engine accessories as well as semiactive shock absorbers that can adapt in real time to changing road conditions. Semiactive dampers for rotorcraft control surfaces are among the potential aerospace applications. The critical need to mitigate the structural vibrations of large structures has led to the construction of large, high-force MR-fluid-based dampers. A promising application in manufacturing processes is the computer-aided polishing of precision optics in which abrasive particles are suspended in an MR fluid so that the polishing rate is determined in part by the strength of an applied magnetic field.

The phenomenon of electrorheology (ER), described in the guest editors' introduction to this issue, can largely be explained by electrostatic interactions between particles by an externally applied electric field. The purpose of this article is to review particle-polarization mechanisms active in ER suspensions and to highlight some poorly understood electrostatic phenomena that inhibit the commercialization of ER technology.

Figure 1 plots the apparent viscosity of a 20-wt% suspension of alumina particles in silicone oil as a function of shear rate for different electric field strengths. The viscosity changes are most pronounced at small deformation rates. At small shear rates the viscosity is proportional to E2 where E is the electric field strength.

Although electrorheological (ER) fluids are presently being used in some commercial devices, their full potential has yet to be realized. The maximum shear stress (~2 kPa) of commercial ER fluids is not sufficient for most pertinent applications, especially in regard to the automotive industry. However, recent theoretical studies indicate that a shear stress of at least an order of magnitude greater than 2 kPa might be attained by the proper design of the ER suspension. This article reviews the status of the understanding of ER behavior which leads to this possibility.

Most of the research to date on ER fluids has been on suspensions of either inorganic or organic particles in dielectric host oils. Hence, the present review will deal mainly with such “heterogeneous” ER fluids. However, in recent years interesting results have been reported for all-liquid ER fluids, especially those containing liquid crystals. These “homogeneous” ER fluids are discussed in the article by Inoue et al. in this issue.

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.