Electrorheological behaviour of suspensions of various surface-modified porous silica particles

Abstract

Electrorheological (ER) properties of suspensions in silicone oil of three silane-modified porous silicas Separon SGX and porous non-modified Separon SGX and compact cristobalite particles Sibelco (cristobalite flour M 3000) were studied. The experiments showed that the formation of chains of polarized particles in the electric field, which increases viscosity of the system, is strongly affected by the hydrophilic character of the particle modifier. Thus, while in suspensions of non-treated silica particles or particles treated with hydrophilic 3-aminopropyl(triethoxy)silane or 2-cyanoethyl(triethoxy)silane a strong enhancement of the apparent viscosity at low shear rates upon application of an electric field set in, in suspension of the particles modified with hydrophobic octadecyl(trichloro)silane, the ER effect was completely suppressed. According to the bulk polarization theory, polarizability of suspension particles in the electric field is controlled by particle dipole coefficient related to particle permittivity. This seems to be true for silica particles, where the dependence of viscosity of ER structure on particle dipole coefficient continuously increases. The change in the ER behaviour of silica and cristobalite on drying considerably differed. If the samples were heated at 100°C, the ER effect depended on a residual amount of water firmly adhering to the particles. An additional heating at 300°C, causing a complete removal of physically adsorbed water, leads to a dramatic decrease in ER viscosity of suspension of compact cristobalite particles while for silica, only a much smaller decrease in this quantity appeared, probably due to higher polarizable silanol groups present in the perfectly dry porous material.