Negative Electrorheological Research Articles

Switching between negative and positive electrorheological effect of g-C3N4 by copper ions doping

Investigation of pristine g-C3N4 and copper ions doped g-C3N4 as dispersed phases in silicone-oil based electrorheological (ER) fluids is reported for the first time. Pristine g-C3N4 was prepared via standard thermal polycondensation of urea. Introduction of Cu ions into g-C3N4 polymeric network was performed by treatment of pristine powder by a solution containing tetraammoniumdiaquacopper(II) complex ions followed by annealing. The structure and properties of products were revealed with the aid of XRD, FTIR, UV–Vis and XPS, and complemented by SEM investigation of morphology, pycnometry, BET analysis, and conductivity measurements. The response of prepared ER fluids to an external electric field was examined by rheometry and the effects visualized with optical microscopy. While ER fluids based on g-C3N4 exhibited negative ER effect (decrease in viscosity when the field is switched-on), ER fluids based on a g-C3N4/Cu-doped analogue exhibited positive ER effect. Dielectric spectroscopy using Havriliak-Negami model revealed that introduction of the dopant almost doubled the dielectric relaxation strength while the relaxation time was halved. The ability of g-C3N4/Cu as a candidate for further studies necessary to increase the ER effect was demonstrated. Thereto, described modification of g-C3N4 by copper ions shall be applicable also for other metals forming ammonia complexes.

Negative electrorheological fluids

Electrorheological (ER) fluids are electroresponsive materials that have been extensively investigated for several decades. Despite negative ER fluids being one of the most promising new class of candidate materials for various potential applications, only positive ER fluids have been widely used thus far. Herein, we briefly review the negative ER fluids to encourage their application in commercial device manufacture. This short review discusses an overview of mechanisms, materials, and applications of negative ER fluids.

Negative electrorheological responses of mono-dispersed polypyrrole-SAN copolymer suspensions

The mono-dispersed polypyrrole (PPy)-poly(acrylonitrile-co-styrene) (SAN) copolymer particles were synthesized by emulsion polymerization and the electrorheological (ER) behavior of their suspensions was investigated. The suspensions of the mono-dispersed PPy-SAN copolymer particles showed negative ER effects. It was quite unexpected to negative ER effects for ER suspensions of conducting polymers or conducting polymer composites. The ER responses decreased with increasing electric field strength and particle size. A power-law dependence on the particle size and the electric field strength, τ(E)-τ0=−103.95 d−2E, showed a reasonable fit to the dynamic yield stress differences. It was observed that particles separated between the electrodes during experiments under the applied electric field. The migrations due to charge injection, by which the gap between the electrodes devoid of particles, seem to play a role in the yield stress decrease, leading to the negative ER response. Open image in new window

Electrorheological response of polyindene/colemanite conducting composite

In this study, electrorheological (ER) responses of colemanite and polyindene (PIn)/colemanite conducting composite dispersions in silicone oil (SO) were investigated. Antisedimentation ratios of the dispersions were determined. Some parameters which affect the ER properties of the dispersions such as electric field strength (E), shear rate, frequency and temperature were investigated. The rather unusual behavior known as "negative ER effect" was observed for colemanite/SO above E = 1.5 kV/mm and for PIn/colemanite/SO under all the values of electric field strength at volume fraction of 25%. This negative ER response was converted to positive with the addition of non-ionic surfactant.

Positive and Negative Electrorheological Response of Alginate Salts Dispersed Suspensions under Electric Field

Electrorheological (ER) effects of alginic acid and alginate salts (Na(+) alginate, NH(4)(+) alginate, and Ca(2+) alginate) dispersed suspensions were investigated under DC electric fields. A noteworthy result is that the Ca(2+) alginate dispersed suspension showed negative electrorheological effects under electric fields while the other suspensions exhibited positive electrorheological effects. It is the first time that the negative ER effect is obtained with the biomacromolecule. Interestingly, at the DC electric fields, the electromigration of particles to two electrodes was observed in the negative ER fluid, while the particles-bridges formed between two electrodes in the case of the positive ER fluid. In conclusion, the specific salt type of biomacromolecules could be suitable ER particles for negative ER suspension. We believe that our study can present a new way for the development of the biocompatible and eco-friendly negative ER fluids.

The negative and positive electrorheological behavior and vibration damping characteristics of colemanite and polyindene/colemanite conducting composite

In this study, the electrorheological (ER) properties of colemanite and polyindene (94.8% PIn)/colemanite (5.2%) conducting composite were investigated by dispersion in silicone oil (SO). The zeta (ζ)-potentials and antisedimentation ratios of the materials were determined. Some parameters which affect the ER properties of all the dispersions such as the volume fraction, electric field strength (E), shear rate, frequency and temperature were investigated. The rather unusual behavior known as the negative ER effect was observed for colemanite/SO above E = 1.5 kV mm−1 and for PIn/colemanite/SO under all values of the electric field strength even at high volume fraction. This negative ER response was converted to a positive one by the addition of non-ionic surfactant. Furthermore, glycerol was used as a polar promoter and observed to enhance the ER activity of the colemanite/SO system. Creep-recovery tests were applied to all the dispersions studied to investigate their behavior under sustained shear stress. Finally, 28% and 30% vibration damping capacities were achieved using an automobile shock absorber for the glycerol/colemanite/SO and non-ionic surfactant/PIn/colemanite/SO systems under the E = 0.17 kV mm−1 condition, respectively.

Effect of surface properties on the electrorheological response of hematite/silicone oil dispersions

In this work we present an investigation of the influence of particle surface characteristics on the electrorheological (ER) behavior of suspensions of either pure or modified hematite (α-Fe2O3) particles dispersed in silicone oil. The modification consisted of either dehydration or hydrophobization of the particles before preparing the suspensions. A comparison was performed between the electrorheological responses of suspensions with the same volume fraction of hematite particles having different surface properties. The effects of applied electric field strength on the viscosity, yield stress and dynamic moduli of these suspensions were examined. It was found that the usual positive ER response, that is, enhanced values of the yield stress and elastic modulus induced by the electric field were obtained for hematite and, to a lesser extent, for dried hematite suspensions. In contrast, a “negative ER effect”, i.e., the reduction of yield stress and elastic modulus upon application of electric field was observed for hydrophobically modified (oleic acid coated) hematite. This means that the field produces destruction of structures rather than their build up, above a threshold electric field strength.

Negative Electrorheological Behavior in Suspensions of Inorganic Particles

An investigation is described on the electric-field-induced structures in colloidal dispersions. Both rheological determinations and direct microscopic observations are used with that aim. The starting point of this study is the so-called electrorheological (ER) effect, consisting of the mechanical reinforcing of a fluid or suspension due to formation of chains of molecules or particles after being polarized by the action of the field. One macroscopic manifestation of this phenomenon is the transformation of the fluid from a typically Newtonian behavior to a viscoelastic material, with finite yield stress and high elastic modulus. The systems investigated were suspensions of elongated goethite (β-FeOOH) particles in silicone oils with varying amounts of silica nanoparticles. The results showed the rather unusual behavior known as "negative ER effect", which can be best described by saying that the application of an electric field reduces the yield stress and the elastic modulus, that is, produces destruction of structures rather than their build up. The negative behavior is also found for suspensions of other inorganic powders, including hematite and quartz. On the contrary, the usual positive ER response is found for suspensions of cellulose and montmorillonite clay. The same happens if goethite suspensions are prepared in high volume fractions, high-viscosity fluids, or both. All of the results found are compatible with the so-called interfacial model of electrorheology: the reduction of the yield stress of goethite suspensions when the applied field is high enough is the consequence of particle migration toward the electrodes because of charge injection and subsequent electrophoresis. The migration leaves the gap between the electrodes devoid of particles and explains the decrease in yield stress. The addition of silica nanoparticles contributes to reduce the strength of this effect by hindering the charging and making it necessary to increase the field strength to observe the negative effect. The model appears to also be applicable to cellulose, although the positive response found for such particles is explained by their large size: larger diameters bring about larger attraction forces between particles, leading to a tendency to produce strong aggregates. This is likely to occur in suspensions of colloids which, because of their relatively high electrical conductivity, tend to acquire charge even in such nonpolar liquids as silicone oils.

Dynamics simulation of electrorheological suspensions in poiseuille flow field

Based on a modified Maxwell-Wagner model, molecular dynamics is carried out to simulate the structural changes of ER (electrorheological) suspensions in a poiseuille flow field. The simulation results show that the flow assists in the collection of particles at the electrodes under a low pressure gradient, and the negative ER effect will show under a high pressure gradient. By analyzing the relationship curves of the shear stress and the pressure gradient in different relaxation time, it is found that for the same kind of ER suspensions materials, there is an optimal dielectric relaxation frequency.

An electrorheological hydrostatic journal bearing for controlling rotor vibration

The objective of this work is to study the dynamic behavior of a rotor supported by a new hydrostatic journal bearing, and fed with a negative electrorheological (NER) fluid. The hydrostatic bearing consists of four hydrostatic bearing flat pads fed by capillary restrictors. An ER fluid consists of a suspension of micron-sized particles dispersed in a dielectric liquid. A negative electrorheological (NER) fluid is a Newtonian fluid with a viscosity which decreases when an electric field is applied, and which can restore its property when the field is removed. A reversible change in viscosity occurs in milliseconds with the electric field applied. Therefore, these fluids are suitable for the real-time control of vibration and vibration damping. A linear modeling was performed using numerical methods in order to investigate the effect of negative electrorheological fluids, recess pressure and static eccentricity ratio on carrying load capacity, flow, and the equivalent dynamic characteristics (stiffness, damping, damping factor) of a new NER hydrostatic journal bearing. In a first step, the flow, equivalent stiffness and damping and damping factor is studied according to the pressure ratio, for different electric field values at the point of operation. In a second step, the variation of carrying load capacity and equivalent stiffness and damping is studied according to the static eccentric ratio, for different electric field values. In a third step, an application study of an NER hydrostatic journal bearing based on linear theory is presented in order to reduce or suppresses the imbalance-induced vibration or the force transmitted to the base. The discussion of results includes some thoughts on future trends.

Electro-Rheological Response of Liquid Crystals under Oscillatory Squeeze Flow

We have investigated the electro-rheological (ER) properties of liquid crystals in an oscillatory squeezing flow. A Micro Fourier Rheometer was utilized to estimate the complex viscosity and phase shift. A clear dip in the phase angle was observed, centred around a frequency which increased with electric field strength. It was found that this critical frequency was proportional to the inverse of the response time required for the liquid crystal molecules to align under the electric field. Similar ER effects to those observed under steady shearing were obtained in the oscillatory flow, but the ER effect at high frequency is smaller than that under no electric field. In other words, a negative ER effect was obtained under low electric fields. A rheological model for the dynamic response was obtained through the control theory and the transfer function thus derived will be useful for estimating the output response in a flow or motion control system.

Rheological Behavior of Poly(methyl methacrylate) Dispersions Stabilized by a Diblock Copolymer. 2. Positive and Negative Electrorheological Effect

Rheological Behavior of Poly(methyl methacrylate) Dispersions Stabilized by a Diblock Copolymer. 2. Positive and Negative Electrorheological Effect

Dielectric Criteria for the Electrorheological Effect

The suspended particles of an electrorheological (ER) fluid can be easily arrayed as a crystallite of a body-centered tetragonal (bct) lattice structure using an external electric field. Thus the entropy of this process, which is generally recognized as a first-order phase transition, would greatly decrease. From this fact, we theoretically derive that the dielectric loss tangent maximum value of the dispersed particles should be larger than 0.1, which agrees well with the currently observed experimental results; the negative ER effect becomes possible only when d∈ sp /dT < 0, where ∈ sp is the static dielectric constant of the dispersed particle material and T is temperature.

`Positive' and `negative' electro-rheological effect of liquid blends

The electrorheological (ER) effect of urethane-modified polypropylene glycol (UPPG)/dimethylsiloxane (DMS) blends was investigated by rheological measurement and observation of the domain structure under shear flow. It was found that: (i) the cause of an ER effect is in close association with the domain composition under no electric field; the ER effect appears when UPPG forms droplets, while when it forms a continuous phase, the ER effect does not appear; (ii) UPPG droplets elongate and bridge between the electrodes when an electric field is applied and the bridges remain even under the shear; and (iii) the ER effect is positive in the case that the viscosity of UPPG is larger than that of DMS and negative in the case that the viscosity of UPPG is smaller than that of DMS. The cause of both positive and negative ER effects for UPPG/DMS blends is the connection of UPPG between electrodes under the electric field.

Characteristics of Electrorheological Responses in an Emulsion System

The characteristics of electrorheological (ER) responses in an emulsion system have been examined for the first time. The emulsion system consists of a chlorinated paraffin oil and a silicone oil; these two liquids have similar dielectric constants, but the former liquid is 1000 times more conductive than the latter liquid. The mismatch in electric properties between the dispersed phase and the dispersing medium can be reversed in an emulsion by inverting the phase distribution. Transient ER responses in the emulsion system have been studied over a range of shear rates following step changes (on/off) of an external dc electric field. The relative response strength characterizing the ER effect increases with the field strength and with the volume fraction of silicone oil, but decreases with increasing shear rate imposed. The paraffin oil-in-silicone oil emulsion exhibits a response much stronger than that observed in the phase-reversed silicone oil-in-paraffin oil emulsion. No negative ER response is detected in the silicone oil-in-paraffin oil emulsion. With an optical microscope, very different microscopic response characteristics are observed for the phase-reversed emulsions under static conditions, and this can be explained according to the leaky dielectric liquid model of Taylor.

Investigation about Electrorheological Squeeze FilmDamper Applied to Active Control of Rotor Dynamic

Electrorheological (ER) fluids, discovered in 1947 by W. WINSLOW, are concentrated suspensions of solid particles in an oily base liquid. Exposed to a strong electric field, their resistance to flow increases very greatly and this change is progressive, reversible and occurs very rapidly. Nowadays, ER fluids, made of lithium salt and fluorosilicon got rid of their old abrasive characteristics and are able to provide a good interface between electronics and mechanical components. A bibliographical study on ER fluids and ER technology has been carried out. The aim of this study is adapting ER technology to Squeeze Film Damper. In order to provide an active control on a flexible rotating shaft so as to command the whole shaft/bearings device in case of high rotating speed or heavy load trouble. Results of numerical computation of a shaft bearing assembly with a Squeeze Film Damper using negative ER fluid are showed in order to see the possibility of avoiding critical speeds by natural frequency shifting. A technical study of ER Squeeze Film Damper design is also presented, taking into account ER fluid properties and ER technology requirements.