Particle Polarization and Nonlinear Effects in Electrorheological Suspensions
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where T is the shear stress; T0 is the fielddependent yield stress, defined as the shear stress in the limit of vanishing shear rate y; and 77^ is the plastic viscosity, approximately equal to the suspension viscosity in the absence of an electric field. The plastic viscosity is different than apparent viscosity 17 = r/y. The field-induced rheological changes are accompanied by equally dramatic changes in the suspension microstructure. Upon application of the field, par30
ticles rapidly aggregate into fibrous columns that span the electrode gap. This is illustrated in Figure 2 for a 2-wt% suspension of alumina particles in silicone oil. The field-induced rheological changes are associated with the mechanical work required to break these structures in order to make the suspension flow. A wide variety of suspensions display an ER effect,' with the magnitude of the field-induced rheological changes varying from one system to another. Particles
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The Electrostatic Polarization Mechanism
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employed include starch, flour, silica, alumina, titania, barium titanate, zeolites, semiconductors, and weakly conducting polymers. Some particles require a small amount of water or some other polar molecule to display a significant ER effect. The primary requirements for the continuous fluid phase are that they have low conductivity and high dielectric breakdown strength. Electronic control of stress transfer with ER suspensions has applications in such devices as shock absorbers and engine mounts, clutches and brakes, control valves, actuators, and artificial joints. However despite much industrial and academic effort, no commercial devices are available. The main limitation to ER technology has been a lack of effective fluids. Applications require fluids with large field-induced rheological changes and low conductivities, and which are nonabrasive; environmentally benign; and stable against sedimentation, irreversible flocculation, and chemical degradation. Furthermore fluids must meet these requirements over a wide range of temperatures, particularly for automotive applications. Developing ER fluids to meet these requirements demands an understanding of the underlying mechanisms. This article reviews the underlying mechanisms of electrorheology. Electrorheology can largely be explained by the electrostatic interactions between particles polarized by the applied electric field. This understanding at the mesoscopic level can describe—at least qualitatively—a wide variety of observations. This level of understanding however does not provide much insight into limiting features of electrorheology, nor the relationships between chemical composition and fieldinduced rheological changes. Experimental data on two systems are presented in order to illustrate some of these relationships, and to investigate mechanisms at the microscopic level.
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Figure 1. Apparent viscosity as a fu
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