The Materials Science of Field-Responsive Fluids
- PDF / 1,342,097 Bytes
- 4 Pages / 576 x 777.6 pts Page_size
- 33 Downloads / 284 Views
Many other applications such as those for variable-resistance exercise equipment, earthquake-resistant high-rise structures, positioning devices, and optical polishing of aspherical surfaces are also emerging and are recapitulated in the articles presented here. While these materials are fascinating, the fundamental science of these materials—the relationships between synthesis and processing and the connections between microstructural evolution and the rheological properties—is very complex and is not fully understood. An improved understanding of the underlying materials science, physics, chemistry, and rheology of these materials, more closely coupled with efforts to engineer devices using them, would potentially enable the development of a broad range of novel technologies.
Magnetorheological Fluids 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 fielddependent shear modulus and a yield stress that must be overcome to initiate gross material deformation or flow.1 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 Rosensweig2'3 for more information). Since ferrofluids
MRS BULLETIN/AUGUST 1998
are well-known and have been extensively discussed elsewhere in the literature, they will not be treated in detail here. This issue of MRS Bulletin captures some of the central materials issues that underpin the science and technology of ER and MR fluids. Representative compositions and physical properties of ER, MR, and ferrofluids are compared in Table I. One of the common features of ER and MR fluids is that after an external field is applied, the material rapidly transforms from a fluid into a weak viscoelastic solid through the formation of
Data Loading...
