Influence of particle size on the dynamic strength of electrorheological fluids

The electrical properties, rheology and structure of model ER fluids consisting of glass beads in silicone oil were investigated as a function of electric field E (0--4 kV/mm), particle size D (6--100 μm) and shear rate γ (2--1,000 s-1). The conductivity of the suspensions was 3 orders of magnitude greater than that of the host oil at E ≥ 1 kV/mm; their low-voltage d.c. permittivity was about 1.35 times larger. The flow stress τ of the suspensions was given. The linear dependence of τE on E was attributed to dipole saturation. The observed opposing effects of D and γ on τE were concluded to result from their respective influence on the strength of the columnar structure normally produced by the electric field and its fragmentation during shear. The constant C1 was in agreement with the Einstein equation for the effect of volume fraction of particles on the viscosity of suspensions. The parameter C2/D was concluded to reflect either the effect of particle surface area on viscosity or a polydispersion effect. The present results did not correlate with the Mason number as normally formulated, but did when it was appropriately modified