The structure-induced nonlinear dielectric properties of electrorheological ͑ER͒ fluids have been studied in anhydrous KNbO 3 -silicone oil and SrTiO 3 -silicone oil systems. We find that the permittivities of ER fluids increase linearly with the increase of the field strength as the electric field exceeds a threshold value E 1 , and tend to saturated constants beyond a high field strength E 2 . The alignment of particles in the direction of the electric field causes variation of the dielectric properties of the ER fluids. Below E 1 , when the particles are randomly dispersed, and beyond E 2 , when almost all the particles are form chains and columns, the permittivities do not vary with E and attain their lowest and highest values, respectively. Correspondingly, the current densities of ER fluids no longer follow Ohm's law between E 1 and E 2 , whereas they still show Ohmic behavior in the regions below E 1 and beyond E 2 . ͓S1063-651X͑97͒05302-6͔
The frequency dependence of the electrorheological response was studied experimentally in a suspension of barium titanate spherical particles suspending in silicone oil. In the system, only one factor, namely the frequency of the applied electric field, affects the electrorheological effect. The experimental data reflect the frequency effect more reliably and more accurately. Under the sinusoidal electric fields, the shear stress increases sharply with frequency below 500 Hz and reaches a saturated value beyond 500 Hz. The phenomena can be explained well with the permittivity mismatch theory. More experiments indicate that the electrorheological effect should be the sum of the mismatch polarization and the interfacial polarization.
A ferroelectric KNO3/silicone oil electrorheological (ER) fluid is introduced to investigate the conductivity dependence of the ER effect under dc electric fields where the ER effect is conductivity dominated. By measuring the temperature dependence of the shear stress across the Curie temperature of particles, the dependence of the ER effect on conductivity has been quantitatively obtained in experiments. There is a critical conductivity ratio Γc (or mismatch factor βc2): when Γ<Γc, the shear stress increases with Γ; when Γ>Γc, the shear stress decreases with Γ. An agreement is obtained between theory and experiment when Γ (or β2) is lower. In the higher Γ(or β2) range, the experimental data are not in agreement with the theoretical prediction and the interfacial effect should be taken into account. The experimental data are more reliable due to the same conditions, such as the chemical nature, the surfacial property of particles, and the interfacial property between particles and suspending liquid as well as the size and shape of the particles.
We have developed a new Ewald summation for a three-dimensional dipolar system with two-dimensional periodicity in a uniaxial field and a rotating field in a horizontal plane. Under a constant pressure and temperature, Monte Carlo simulation has been carried out; phase transitions are found and chainlike structure for a uniaxial field and monolayer or multilayer for rotating field are obtained, which are well consistent with experiments.
A TGS ((NH2CH2COOH)3·H2SO4)/silicone oil suspension, which is the simplest system, is employed to
investigate the complex permittivity dependence of the electrorheological (ER)
effect under ac electric fields. By measuring the temperature dependence of
the shear stress across the Curie temperature of TGS particles, the dependence
of the ER effect on dielectric constant has been reliably obtained under ac
fields. Under high-frequency fields in which range the dielectric mismatch
dominates, the shear stress increases with the particle dielectric constant.
The experimental data are in agreement qualitatively with the previous
mismatch models although deviation appears. The shear stress of the ER fluid
was also measured at intermediate frequencies.
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