EFFECT OF SURFACE MODIFICATION OF COLLOIDAL SILICA ON THE ELECTRORHEOLOGY OF SUSPENSIONS

Monodispersse silica particles are formed by hydrolyzing tetraethylorthosilicate in an ethanol solution. Silica suspensions dispersed in a silicone oil were prepared by converting the medium from an ethanol solution to silicone oil. The effects on the electrorheological (ER) behavior of surface modification of particles with silane coupling agents are studied under oscillatory shear. The magnitude of the complex viscosity, |η*|, increases on the application of electric fields. When the silica particles are treated with silane coupling agents which produce hydrogen bonding between organic groups, the ER effect is markedly improved and significant flow occurs only after some critical stress is exceeded. The suspensions change from viscous liquids to solid-like pastes. The development of high yield stress is achieved under partial coverage of silica surfaces. The time-dependent behavior on the application of electric fields in examined for suspensions in which partial coverage takes place. At high stresses, |η*| shows a steep increase, followed by a slow decrease. The thixotropic behavior is attributed to the structural breakdown induced by hydrodynamic interactions. On the other hand, |η*| gradually increases with time at low stresses. Since the dielectric polarization forces overcome the hydrodynamic forces and induce the acceleration of structural formation, this rheopectic tendency serves to produce higher yield stresses. © 1991.