mercialization of these devices has yet to be realized. The The influence of three nonionic surfactants (Brij 30, GMO, and main limitation of ER technology development is a lack of GTO) on the electrorheological response of various alumina/silieffective fluids . Most applications require fluids that cone oil suspensions is investigated. The dependence of the dypossess a large field-induced yield stress, are stable to setnamic yield stress on such variables as surfactant type and concentling and irreversible aggregation, are environmentally betration, water and ion content, and electric field strength and nign, and draw limited current. Our inability to design such frequency is reported. The prevalent feature common to all formuacceptable fluids stems largely from a lack of a fundamental lations is that the yield stress, t 0 , initially increases with surfactant understanding of the mechanisms that control ER behavior.
concentration, passes through a maximum, and then decreases with surfactant concentration. Below the maximum, the yield Surfactants are added to ER suspensions for a variety of stress increases quadratically with the field strength, E, while reasons (1, 4-7, 10-15), and can be used to tailor suspenabove the maximum, yield stress increases slower than E 2 . The sion properties. They are often used to promote colloidal increase in the yield stress with surfactant concentration is due to stability, which is necessary to keep particles from irreverssurfactant-enhanced interfacial polarization, which may arise ibly flocculating, and to control rheological properties in the from increased proton transport via neighboring hydrogen bonds. absence of the electric field. Surfactants can also be used to
The nonlinear behavior observed at large surfactant concentra-''activate'' suspensions. Some suspensions display little or tions (i.e., t 0 Ο° / E 2 ) arises from field-induced phase separation of no ER activity unless a small amount of water or surfactant a surfactant-rich phase as opposed to field-dependent conductivity is added, while other suspensions exhibit a significantly enof a homogeneous continuous phase.