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

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 polari zed particles in the electric field, which increases viscosity of the syste m, is strongly affected by the hydrophilic character of the particle modifi er. Thus, while in suspensions of non-treated silica particles or particles treated with hydrophilic 3-aminopropyl(triethoxy)silane or 2-cyanoethyl(tr iethoxy)silane a strong enhancement of the apparent viscosity at low shear rates upon application of an electric field set in, in suspension of the pa rticles modified with hydrophobic octadecyl(trichloro)silane, the ER effect was completely suppressed. According to the bulk polarization theory, pola rizability of suspension particles in the electric field is controlled by p article dipole coefficient related to particle permittivity. This seems to be true for silica particles, where the dependence of viscosity of ER struc ture on particle dipole coefficient continuously increases. The change in t he ER behaviour of silica and cristobalite on drying considerably differed. If the samples were heated at 100 degrees C, the ER effect depended on a r esidual amount of water firmly adhering to the particles. An additional hea ting at 300 degrees C, causing a complete removal of physically adsorbed wa ter, leads to a dramatic decrease in ER viscosity of suspension of compact cristobalite particles while for silica, only a much smaller decrease in th is quantity appeared, probably due to higher polarizable silanol groups pre sent in the perfectly dry porous material. (C) 1999 Elsevier Science B.V. A ll rights reserved.