ELECTRORHEOLOGICAL PROPERTIES OF SILICA SUSPENSIONS

Monodisperse silica particles were formed by hydrolyzing tetraethylorthosilicate in an ethanol solution. For silica suspensions in a silicone oil, the steady-shear viscosity and dynamic viscoelasticity were measured under electric fields up to 3.0 kV mm-1. At low shear rates and high field strength, the flow curve shows a plateau, showing development of a yield stress. The yield stress is proportional to the volume fraction of particles (PHI) and to the square of the electric field (E) for PHI < 0.3. The number of chains linearly increases with volume fraction. At higher volume fractions it varies with (PHI-E)2.4. The larger value of the exponent may be attributed to the crosslinking of chains. The scaling on Mason number is not applicable to the viscosity behavior in steady shear. This implies that the electric polarization forces are much stronger than predicted by the bulk polarization theory. The interparticle forces due to polarization are closely related to conditions at the surfaces of the particles, rather than the particle material itself. The storage and loss moduli at very low strains are approximated by a single relaxation model. The relaxation may be due to the hydrodynamic interactions with free chains, and chains attached at only one end to an electrode. At strains above 0.05, both moduli have a plateau which is associated with the yielding of chain structure. Because of the polarization effect of adsorbed substances, the critical strain for chain rupture is low.