Electrorheology at small strains and strain rates of suspensions of silica particles in silicone oil

Abstract

The electrorheological behavior of suspensions in silicone oil of ultrafine silica particles (0–3.16 wt.%) coated with a surfactant was investigated at room temperature with electric fields up to 2 kV mm −1. The studies consisted of in situ observations with an optical microscope and determination of the rheological properties for shears in the range 10 −2−2.8 and shear rates of 10 −3 -10 1 s −1. The microscopy observations revealed that a fibrated structure formed immediately on application of the field, which ruptured near the center of the gap when the suspension was sheared. Swirling of the fluid was occasionally noted, which occurred more frequently the higher the field strength or shear rate. Strain hardening and a yield point-type phenomenon occurred during the rheological tests. The shear stress at a given strain was a complex function of the shear rate. For a given strain and strain rate, the shear stress was proportional to the electric field and to the two-thirds power of the concentration of silica particles. The electrorheological behavior was attributed to the polarization of molecules adsorbed on the silica particles and their alignment into a fibrated structure by the external field. The complex effects of strain and strain rate on the shear stress were concluded to reflect their influence on the continuing rupture and re-formation of the fibrated structure.