Tasuku Saito

The effect of blending particles with different conductivity on electrorheological properties

We have carried out a set of experiments using anhydrous carbonaceous particles to determine the effect on the electrorheological properties of blending particles of different conductivities. We find that there is a significant dip in the shear stress under an electric field as the concentration of higher conductivity particles is increased, showing that uniformity of the electrical properties among the particles is a most important factor in achieving optimum electrorheological fluid (ERF) performance. We explain this behavior by a simple model based on the strength of particle–particle interactions. Measurements of the dielectric permittivity show that the α parameter of the Cole–Cole expression provides an excellent way to express the degree of uniformity in the electrical properties. This gives a convenient method to evaluate the potential performance of an ERF.

Effect of matrix viscoelasticity on the electrorheological properties of particle suspensions

Abstract We have investigated the electrorheological properties of dispersions of semi-conducting particles in oils and elastomers. We focused on how the dynamic mechanical properties measured under oscillatory shearing change with the viscosity of the oil or the elasticity of the elastomer. The dependence on electric field and strain amplitude were also investigated. We found that the largest increment of the mechanical properties under electric fields was obtained when using oils of low viscosity and elastomers of low elasticity. The strain amplitude which produced the largest variation with electric field was found to be 0.1% for the elastomer systems, but significantly larger (1%) for the oil systems. These results are interpreted in terms of a model based on the competition between the dipole–dipole electrostatic interaction (which acts to maintain neighbouring particles together) and the shearing force due to the deformation of the matrix (which acts to separate the particles). We find that there are parallels between the electrorheological behaviour of particles dispersed in elastomers and the behaviour of particles dispersed in oils. These results should find application in the selection of suitable matrix materials for electrorheological suspensions.

Layered model of electrorheological fluid under flow

We present a theoretical model of the behavior of a concentrated electrorheological fluid (ERF) which explicitly takes into account the effects of conductivity. The increase in shear viscosity under an electric field is due to a layered structure between the electrodes, made up of the remnants of particle chains adhering to the electrodes by electrostatic image forces, and a freely flowing liquid layer where all the shear flow is concentrated. This layered model can explain the variation of electric current with shear rate, as well as the rheological response of a dynamic yield stress proportional to the square of the applied electric field.

Relationship between electric current and matrix modulus in electrorheological elastomers

Abstract We have carried out a series of tests on electrorheological elastomers, which consist of semi-conducting solid particles dispersed in a low-conductivity silicone-based elastomer, with the investigations focussing on the dependence of the DC electric current on the elastomer modulus. We find that when other conditions are held constant, the electric current under a particular electric field shows a significant decrease when an elastomer of higher modulus is used. This behaviour is explained qualitatively using a simple model based on the mechanical equilibrium of two competing forces: the electrostatic attraction force between the two adjacent particles in the field direction, and the elastic squeezing force due to the elastomer between the particles which resists their approach.

Anhydrous Electrorheological Fluid Using Carbonaceous Particulate as Dispersed Phase

An Electrorheological Fluid(ERF) is a suspension essentially composed of fine solid particulates and an electrically insulating oil. Without an electric field, ERF exhibits the rheological properties of a Newtonian fluid whose shear stress is proportional to the shear rate. When an electric field is applied to ERF, the rheological properties of the fluid are instantaneously changed to a Bingham plastic which has the yield stress given by the intercept of the shear stress and the same constant of the proportionality between shear stress and shear rate as the Newtonian viscosity without an electric field. The yield stress rises with increasing voltage and no shear occurs until the shear stress exceeds the yield stress. This phenomenon is called the ER effect which gives rise to the instantaneous change of the apparent viscosity of the fluid.