<title>Particle size effects in the electrorheological behavior of silica-poly (dimethylsiloxane) mixtures</title>

Abstract

Electrorheological (ER) fluids are typically nonaqueous suspensions that show a reversible viscosity increase upon application of an external electric field. There has been some important basic work on the stress-transfer mechanisms in ER fluids but little work on actual particle size effects. Several ER fluids were characterized with a modified Brookfield viscometer; field strengths up to about 0.5 kV/mm may be employed with this setup. The ER fluids consisted of various synthetic precipitated and fumed silicas suspended in a 50-cStk. or 200- cStk. silicone oil at levels up to 9% by weight. Apparent viscosity and shear stress were measured as a function of shear rate and field strength employing several silicas with different primary particle size and composition. The data show a dependence of primary and agglomerated particle sizes on the electrorheological behavior. However, all of the data for these three systems could be combined onto one scaling plot of relative viscosity versus Mason number, Mn. The crossover from electrostatic to hydrodynamic control for the stress transfer in these systems occurred at a Mn of 1.5. A fumed silica with a smaller primary particle size, and no significant agglomeration was also examined. It showed similar behavior to the previous silicas.