Conventional Electrorheological Fluids Research Articles

On the absence of collective motion in a bulk suspension of spontaneously rotating dielectric particles.

A suspension of dielectric particles rotating spontaneously when subjected to a DC electric field in two dimensions next to a no-slip electrode has proven to be an ideal model for active matter [Bricard et al., Nature, 2013, 503, 95-98]. In this system, an electrohydrodynamic (EHD) instability called Quincke rotation was exploited to create self-propelling particles which aligned with each other due to EHD interactions, giving rise to collective motion on large length scales. It is natural to question whether a suspension of such particles in three dimensions will also display collective motion and spontaneously flow like bacterial suspensions do. Using molecular dynamics type simulations, we show that dielectrophoretic forces responsible for chaining in the direction of the applied electric field in conventional electrorheological fluids and the counter-rotation of neighboring particles in these chains prevent collective motion in suspensions undergoing spontaneous particle rotations. Our simulations discover that the fundamental microstructural unit of a suspension under Quincke rotation is a pair of counter-rotating spheres aligned in the direction of the electric field. We perform a linear stability analysis that explains this observation.

Frequency response of giant electrorheological fluids in AC electric field

It is known that in traditional dielectric electrorheological (ER) fluid the shear stress decreases or increases with the frequency of the AC electric field. However, for giant ER fluid, which is based on the principle of the polar molecule dominated electrorheological (PM-ER) effect, how shear stress varies with the frequency of the AC electric field is still not clear. In the present paper, we systematically studied the frequency dependence of PM-ER fluid by measuring the shear stresses of PM-ER fluids consisting of CTO nanoparticles under a sinusoidal electric field in a wide frequency range. It is found that the shear stress decreases with the increase of frequency when an AC electric field is applied. In PM-ER fluids there are nonuniform phase shifts appearing in shear stresses with respect to the sinusoidal electric field. These phase shifts become more pronounced as the frequency increases, which leads to the decrease of the amplitudes of the shear stresses. When the frequency is high enough, the oscillations of shear stresses smear out and the amplitudes of shear stresses reduce to a single low value. This behavior in PM-ER fluids is different from that in conventional ER fluids. For a comparison with PM-ER fluid, the response of a traditional ER fluid to frequency in an AC field is also studied, in which the particles possess high dielectric constant. The origin of different frequency responses in two types of ER fluids has been discussed and explained.

Synthesis and Electrorheological Properties of SiO<sub>2</sub>/Polyaniline Nanocomposites Prepared by <i>In Situ</i> Polymerization

SiO2/polyaniline (PANI) nanocomposites were obtained by chemical oxidative polymerization using ammonium peroxydisulfate as oxidizing agent. The FE-SEM and TEM images showed that nanocomposites presented the core-shell structure with raspberry morphology. The electrorheological (ER) effect of those materials was enhanced by using the ammonium hydroxide to deprotonate the corresponding PANI base. It was found that the yield stress of those ER fluids and its near linear dependence on the electric field were different from the conventional ER fluids.

Electrorheological Phenomena in Polyhedral Silsesquioxane Cage Structure/PDMS Systems

It is shown, for the first time, that mixtures of sulfonated polyhedral silsesquioxane cage structures (sPOSS) and poly(dimethyl siloxane) (PDMS) with silicone oil exhibit significant electrorheological (ER) activity. At low sPOSS concentrations, less than 10 wt %, the viscosity is enhanced by approximately 100, which is comparable to the viscosity enhancements exhibited by conventional ER fluids, under the influence of comparable applied electric fields, E = 2 kV/mm. Measurements of the shear stress, sigma, dependencies on E, the conductivities, and relative permittivities reveal that the properties of these POSS/PDMS systems cannot be reconciled with theory developed to explain the behavior of conventional ER fluids.

Electrorheological Complex Plasmas

Conventional electrorheological (ER) fluids consist of suspensions of microparticles in usually nonconducting fluids with a different dielectric constant. The interparticle interaction, and hence, the rheology of ER fluids, is determined by an external electric field, which polarizes grains and thus induces additional dipole-dipole coupling. The electric field plays a role of a new degree of freedom that allows us to ¿tune¿ the interaction between particles. This makes the phase diagram of ER fluids remarkably diversified. Here, we report on the experimental investigations of ¿ER plasmas,¿ where the control of the interparticle interaction by an externally applied electric field is due to distortion of the Debye spheres that surround microparticles in a plasma. Interactions in ER plasmas under weak ac fields are mathematically equivalent to those in conventional ER fluids. Microgravity experiments as well as molecular dynamics simulations show a phase transition from an isotropic to an anisotropic plasma states as the electric field is increased. The variational Gibbs-Bogoliubov approach allows us to recover the phase diagram of ER plasmas, which includes string fluids and anisotropic solid phases.

First Observation of Electrorheological Plasmas

We report the experimental discovery of "electrorheological (ER) complex plasmas," where the control of the interparticle interaction by an externally applied electric field is due to distortion of the Debye spheres that surround microparticles (dust) in a plasma. We show that interactions in ER plasmas under weak ac fields are mathematically equivalent to those in conventional ER fluids. Microgravity experiments, as well as molecular dynamics simulations, show a phase transition from an isotropic to an anisotropic (string) plasma state as the electric field is increased.

Electrically tunable viscosity of dilute suspensions of carbon nanotubes

The apparent viscosity of a dilute (volume fraction ϕ=1.5×10−5), single-wall-carbon-nanotube(SWNT)/α-terpineol suspension was experimentally found to more than double at moderate shear rates under an external electric field of strength 160V∕mm. The electrorheological response is interpreted in terms of an electrostatic-polarization model, where the governing parameter is a modified Mason number giving the ratio of viscous to dipole-dipole forces. Analysis of the hydrodynamic and electrostatic forces suggests that the magnitude of the electrorheological response in the dilute SWNT suspension, which is much higher than conventional electrorheological fluids at comparable volume fractions, is due to the high aspect ratio of the nanotubes. Comparison is made to a suspension of glassy carbon spheres, in which a three-order-of-magnitude-higher volume fraction is required to achieve similar increases in the apparent viscosity under the same conditions.

POLAR MOLECULE TYPE ELECTRORHEOLOGICAL FLUIDS

The static and dynamic shear stress of newly developed electrorheological (ER) fluids can reach more than 100 kPa and over 60 kPa at 3 kV/mm, respectively. The high yield stress of those ER fluids and its near linear dependence on the electric field are different from the conventional ER fluids and can not be explained with traditional dielectric theory. Experiment demonstrates that the polar molecules adsorbed on the particles play crucial role in those ER fluids, which can be named as polar molecule type electrorheological (PM-ER) fluids. To explain PM-ER effect a model is proposed based on the interaction of polar molecule-charge in between the particles, where the local electric field is much higher than the external one and can cause the polar molecules aligning. The main effective factors for achieving high-performance PM-ER fluids are discussed.

The methods for measuring shear stress of polar molecule dominated electrorheological fluids

Recently, a series of electrorheological (ER) fluids with high yield stress up to hundreds of kilopascals, which is named as polar molecule dominated electrorheological (PM-ER) fluids, has been developed. The mechanism of PM-ER fluids is quite different from that of conventional ER fluids. The normal rheometer cannot be used anymore to measure the shear stress of PM-ER fluid because a slide occurs at the interface between PM-ER fluids and metallic electrodes. In this paper, a proper technique for measuring the yield stress of PM-ER fluids is presented. By using this method, the intrinsic yield stress and the shear modulus of PM-ER fluids can be obtained conveniently, and the boundary effect at electrodes can be eliminated.

Development and Basic Analysis of Gel-structured Electrorheological Fluids

Electrorheological fluids (ERF) are functional fluids whose viscoelastic properties vary with the intensity of the applied electric field. The property of ERF has been expected to control the performances of machine elements, and the ERF has been applied to machine elements such as variable dampers and clutches. However, the disadvantages of ERF are the sedimentation of ER particles and the requirement of seal mechanism. The sedimentation of ER particles reduces the ER effects, results in low stability of ER devices. To solve these problems, gel structured elecrorheological fluids (ERG) were developed in this study. The separation and sedimentation of ER particles from the base oil are successfully suppressed because the frames of gel prevent the particles from sedimentation. The basic characteristics of ERG were experimentally analyzed in this study. The ERG sheet was sandwiched between a pair of plate electrodes and the variation of shear stress between the sheet and the upper electrode was measured when an electric field is applied to the ERG. Comparing with a conventional ERF, the developed ERG provides extremely higher shear stress and the long durability of ER performance. The dynamic response of shear stress of ERG film was also evaluated.