Two Roles of Nonionic Surfactants on the Electrorheological Response.

The influence of three nonionic surfactants (Brij 30, GMO, and GTO) on the electrorheological response of various alumina/silicone oil suspensions is investigated. The dependence of the dynamic yield stress on such variables as surfactant type and concentration, water and ion content, and electric field strength and frequency is reported. The prevalent feature common to all formulations is that the yield stress, tau0, initially increases with surfactant concentration, passes through a maximum, and then decreases with surfactant concentration. Below the maximum, the yield stress increases quadratically with the field strength, E, while above the maximum, yield stress increases slower than E2. The increase in the yield stress with surfactant concentration is due to surfactant-enhanced interfacial polarization, which may arise from increased proton transport via neighboring hydrogen bonds. The nonlinear behavior observed at large surfactant concentrations (i.e., tau0 $\not\propto$ E2) arises from field-induced phase separation of a surfactant-rich phase as opposed to field-dependent conductivity of a homogeneous continuous phase.