Electrorheological Response and Structure Growth of Colloidal Silica Suspensions

The electrorheological response and structure growth of colloidal silica suspension was studied with in situ measurements of the shear stress, electric conductivity, and dielectric permittivity of the suspension. The measurements were carried out under steady and sweep shears after the application of an electric field of alternative current (100 Hz) using silica particles with a diameter of 630 nm and a water content of 4.5 wt%. The measurements of the conductivity enabled the detection of structure growth formed by particle aggregation and clarified that the development of the particle aggregation enlarged the dielectric permittivity and the shear stress. Hysteretic behavior observed in the electrorheological response was explained by considering structure growth of the particle aggregation. The correlation equation for the shear stress and the dielectric permittivity obtained in our previous work (1) was found to be applicable to the present results. Copyright 2001 Academic Press.     

聚苯胺类电流变体的稳态剪切流动分析

对聚苯胺、及聚苯胺/聚丙烯酸盐复合粒子的稳态剪切流动行为进行了综合考察.结果表明, 在交流电场下,电流变体的剪切应力和剪切速率的关系符合Bingham流体形为.并对此现象进行了分析,提出用非理想塑性体的新模型来描述交流电场下聚苯胺类电流变体的稳态剪切流动.     

Electrorheological suspensions of laponite in oil: rheometry studies

We have studied the effect of an external direct current (DC) electric field ( approximately 1 kV/mm) on the rheological properties of colloidal suspensions consisting of aggregates of laponite particles in a silicone oil. Microscopy observations show that, under application of an electric field greater than a triggering electric field Ec approximately 0.6 kV/mm, laponite aggregates assemble into chain- and/or columnlike structures in the oil. Without an applied electric field, the steady-state shear behavior of such suspensions is Newtonian-like. Under application of an electric field larger than Ec, it changes dramatically as a result of the changes in the microstructure: a significant yield stress is measured, and under continuous shear the fluid is shear-thinning. The rheological properties, in particular the dynamic and static shear stress, were studied as a function of particle volume fraction for various strengths (including null) of the applied electric field. The flow curves at constant shear rate can be scaled with respect to both the particle fraction and electric field strength onto a master curve. This scaling is consistent with simple scaling arguments. The shape of the master curve accounts for the system's complexity; it approaches a standard power-law model at high Mason numbers. Both dynamic and static yield stresses are observed to depend on the particle fraction Phi and electric field E as PhibetaEalpha, with alpha approximately 1.85 and beta approximately 1 and 1.70 for the dynamic and static yield stresses, respectively. The yield stress was also determined as the critical stress at which there occurs a bifurcation in the rheological behavior of suspensions that are submitted to a constant shear stress; a scaling law with alpha approximately 1.84 and beta approximately 1.70 was obtained. The effectiveness of the latter technique confirms that such electrorheological (ER) fluids can be studied in the framework of thixotropic fluids. The method is very reproducible; we suggest that it could be used routinely for studying ER fluids. The measured overall yield stress behavior of the suspensions may be explained in terms of standard conduction models for electrorheological systems. Interesting prospects include using such systems for guided self-assembly of clay nanoparticles.     

Transient response of an electrorheological fluid under square-wave electric field excitation

The transient process of an electrorheological (ER) fluid based on zeolite and silicone oil sheared between two parallel plates to which a square-wave electric field is applied has been experimentally studied. The transient shear stress response to the strain or time is tested. The characteristic constants of time under different applied electric fields and shear rates have been determined. The response time is found to be proportional to shear rate with an exponent of about -0.75 in the tested shear rate range, which agrees with the theoretical predictions made by others. But it only shows a small dependence on the strength of the applied electric field. The results show that the transient process of ER fluids is related to the structure formation in the shearing. When the required shear strain is reached, the shear stress rises to a stable value under constant electric field. Although the electric field strength greatly affects the yield strength, it shows little effect on the stress response time. Also, experiments showed the electric field-induced shear stress decreased with an increase of shear rate.     

A new approach of enhancing the shear stress of electrorheological fluids of montmorillonite nanocomposite by emulsion intercalation of poly-N-methaniline

Poly-N-methaniline/montmorillonite (PNMA-MMT) nanocomposite particles with high dielectric constant as well as suitable conductivity were synthesized by an emulsion intercalation method and characterized by FT-IR, XRD, and TEM spectrometry, respectively. The electrorheological (ER) properties of the suspensions of PNMA-MMT particles in silicone oil (20 wt%) were investigated under DC electric fields. It was found that the shear stress of poly-N-methaniline/montmorillonite electrorheological fluid (ERF) is 6.0 kPa in 3 kV/mm (74.5 s(-1)), which is 3.6 times that of electrorheological fluid at zero field, and also much higher than that of pure poly-N-methaniline (PNMA) and montmorillonite (MMT). In the range of 10-90 degrees C, the shear stress changes slightly with the temperature. The sedimentation ratio of PNMA-MMT ERF was about 97% after 60 days. Furthermore, the dielectric constant of PNMA-MMT nanocomposite was increased 3.74 times that of PNMA and 1.99 times that of MMT at 1000 Hz, the dielectric loss tangent also increased about 1.58 times that of PNMA. It is apparent that the notable ER effect of PNMA-MMT ER fluid was attributed to the prominent dielectric property of the poly-N-methaniline/montmorillonite nanocomposite particles.     

Facile fabrication of self-assembled PMMA/graphene oxide composite particles and their electroresponsive properties

Core–shell-structured poly(methyl methacrylate) (PMMA)/graphene oxide (GO) composite particles were prepared using a facile process, in which GO was adsorbed spontaneously onto a microspherical PMMA surface when hydrophobic microspheres were dispersed in deionized (DI) water stabilized by amphiphilic GO under ultrasonication. The fabricated composite was characterized by SEM, TEM, FT-IR, and thermogravimetric analysis. Results showed that the particle surface could be wrapped with GO without the need for surfactants. In addition, electrorheological behavior of the chain-forming process of the PMMA/GO composite particles was observed by optical microscopy under an applied electric field. Both shear stress and shear viscosity related to the strength of the applied electric field were measured using a rotational rheometer. The proposed Cho–Choi–Jhon model was used to describe their ER performances for the entire shear rate region. Moreover, the response of the shear stress to an imposed square voltage at a fixed shear rate was also examined.     

Fabrication and stimuli response of rice husk-based microcrystalline cellulose particle suspension under electric fields

We report on the electro-responsive electrorheological (ER) properties of microcrystalline cellulose (MCC) particles. It was synthesized from raw rice husk (Downes Rice) through the 3-step preparation of alkali treatment, bleaching, and hydrolysis. The MCC particles with mean particle size about 26 μm were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy and thermal gravimetric analysis. The MCC particles were then dispersed in silicone oil to create an ER fluid; its dramatic electro-responsive phase changes under an applied electric field were observed by an optical microscopy. The effect of electric field strengths on ER performances of the ER fluid were determined using a rotational rheometer equipped with a high-voltage generator from the controlled shear rate mode and dynamic oscillation measurements. The fluid showed typical ER effects of Bingham fluid behavior with yield stress and viscoelastic properties under an applied electric field.     

Quasi-static electrorheological properties of hematite/silicone oil suspensions under DC electric fields

In this work, a modified rheometer has been used to gain information on the "start-up" of the shear flow of an electrorheological (ER) fluid consisting of hematite particles dispersed in silicone oil. The results show that unelectrified suspensions behave essentially as fluids, continuously deforming upon application of shear. However, this behavior changes in the presence of an electric field. For low fields and low volume fractions of solids, a solidlike (drastic increase in shear stress after the strain is applied) behavior is observed for small deformations. If the strain is increased, the yield starts and a transition to a viscoelastic-plastic nature is observed. Finally, a plastic behavior is characteristic of the post-yield regime. If the field strength and solids content are high, a discontinuous flow profile develops. These results, together with direct structural observations, suggest that the observed behavior is compatible with the formation of layers of particles electrophoretically deposited on the electrodes; the layers turn into rings when the shear field is applied. It is the slip of the fluid between these rings that can be considered responsible for the ER effect in these suspensions.     

Structure changes of electrorheological fluids based on polyaniline particles with various hydrophilicities and time dependence of shear stress and conductivity during flow

Particles of polyaniline protonated with perfluorooctanesulfonic acid provided a material with hydrophobic surface. This property enabled its perfect dispersion in silicone oil due to its good compatibility with the hydrophobic medium. In contrast, in a suspension of hydrophilic polyaniline particles doped with sulfamic acid, strong interactions of particles prevailed, which led to the formation of entangled chains of aggregated particles in suspension. The difference in structural properties of suspensions exists already in the absence of electric field and significantly influences their electrorheological behavior after application of electric field. The formation of electrorheological structure has been monitored by recording time dependences of the shear stress and the electric current passing through the flowing suspensions.     

Nonlinear alternating current responses of electrorheological solids

When a composite containing nonlinear dielectric particles suspended in a host medium is subjected to a sinusoidal alternating current (ac) electric field, the dielectric response of the composite will generally consist of ac fields at frequencies of higher order harmonics. For an electrorheological (ER) solid under structure transformations due to external fields, we apply the Ewald-Kornfeld formulation to derive the local electric fields and induced dipole moments explicitly, and then we perform the perturbation expansion method to extract their fundamental and third-order harmonics analytically. It is shown that the degree of anisotropy of the ER solid can affect these harmonics significantly. Our results are well understood in the spectral representation theory. Thus, it seems possible to perform a real-time monitoring of the structure transformation by measuring the nonlinear ac responses of ER solids.     

Preparation and electrorheological property of rare earth modified amorphous BaxSr1-xTiO3 gel electrorheological fluid

In this paper, we present a newly developed rare earth modified amorphous barium strontium titanate (Ba(x)Sr(1-x)TiO3) gel/silicone oil electrorheological (ER) fluid. The ER behaviors of suspensions of pure and rare earth modified amorphous Ba(x)Sr(1-x)TiO3 particles in silicone oil have been investigated under a dc electric field. The shear yield stress of the rare earth modified amorphous BaTiO3 gel/silicone oil ER fluid could reach 14.9 kPa at E=3.5 kV/mm while the leaking current density was very low, about 7.64 microA/cm2. The ER fluids with a higher volume fraction had a higher current density and a higher shear yield stress under the same electric field. The ER fluid has a long-term stability against sedimentation. The problem of caking was not serious and the agglomerated particles could be easily broken up by strongly stirring.     

An investigation of electrorheological properties of calcium carbonate suspensions in silicone oil

In this study, electrorheological behavior of suspensions prepared from 0.9 and 5.0 µm calcium carbonate particulates, dispersed in insulating silicone oil medium was investigated. Sedimentation stabilities of suspensions (c = 5 wt %) prepared using these calcium carbonate powders were determined to be 6 and 4 days, respectively. Electrorheological activity of all the suspensions was observed to increase with increasing electric field strength, concentration and decreasing shear rate. Shear stress of calcium carbonate suspensions increased linearly with increasing concentrations of the particles and with the applied electric field strength. Electric field viscosity of all the suspensions was decreased sharply with increasing shear rate and particle size, showing a typical shear thinning non-Newtonian visco-elastic behavior. Effects of elevated temperature and polar promoter onto electrorheological activity of calcium carbonate/silicone oil system were also investigated.__________From Kolloidnyi Zhurnal, Vol. 67, No. 2, 2005, pp. 268–273.Original English Text Copyright © 2005 by Yilmaz, Ünal, Yavuz.This article was submitted by the authors in English.     

Effect of Electrode Pattern on the Column Structure and Yield Stress of Electrorheological Fluids

For electrorheological (ER) suspensions, the aggregate structures of particles were observed in electric fields by the use of transparent cells with different electrode patterns. Although the suspension is dispersed to noninteracting particles without electric fields, many aggregates are formed on the electrode surface in electric fields. Since the dipole–dipole interactions cause chain structures of particles and equilibrium conformations of chains are always aligned with electric field, the aggregates indicate the presence of columns spanning the electrode gap. The particle concentration in columns which are developed between parallel-plate electrodes is about 22 vol %. In striped electrodes, the particles construct striped aggregates along the electrodes and no particles remain in the insulating region. The particle concentration in striped aggregates is about 35 vol %. The nonuniformity of electric field is responsible for the high particle concentration. The increase in particle concentration of column lead to the high yield stress of electrified suspension. Therefore, the ER performance of suspension as an overall response can be improved by the electrode design.     

Influence of an electric field on the non-Newtonian response of a hybrid-aligned nematic cell under shear flow

The authors study shear flow in hybrid-aligned nematic cells under the action of an applied electric field by solving numerically a hydrodynamic model. The authors apply this model to a flow-aligning nematic liquid crystal (4'-n-pentyl-4-cyanobiphenyl) and obtain the director's configuration and the velocity profile at the stationary state. The authors calculate the local and apparent viscosities of the system and found that the competition between the shear flow and the electric field gives rise to an interesting non-Newtonian response with regions of shear thickening and thinning. The results also show an important electrorheological effect ranging from a value a bit larger than the Miesowicz viscosity etab [Nature (London) 17, 261 (1935)] for small electric fields and large shear flows to etac for large electric fields and small shear flows. The analysis of the first normal stress difference shows that for small negative shear rates, the force between the plates of the cell is attractive, while it is repulsive for all other values of shear rates. However, under the application of the electric field, one can modify the extent of the region of attraction. Finally, the authors have calculated the dragging forces on the plates of the cell and found that it is easier to shear in one direction than in the other.     

Synthesis and electrorheological characterization of emulsion polymerized SAN‐clay nanocomposite suspensions

Styrene‐acrylonitrile (SAN) copolymer‐clay nanocomposite was synthesized by emulsion polymerization, which is the easiest method of intercalation (e.g., melt or solution intercalation). Existence of the intercalated polymer was verified by Fourier transform‐infrared spectroscopy and X‐ray diffraction (XRD) analysis. From XRD, we confirmed the insertion of styrene‐acrylonitrile copolymer between the interlayers of clay, whose separation consequently becomes larger than that of the polymer‐free clay. Thermogravimetric analysis showed that the thermal stability of the organic polymers was sustained. Using electrorheological (ER) fluids composed of intercalated particles and silicone oil, we observed typical ER behavior, such as higher shear stress in the presence of an electric field and increasing yield stress with particle concentration. We further observed the critical shear rate at which the ER fluids exhibit pseudo‐Newtonian behavior.     

Monodisperse conducting colloidal dipoles with symmetric dimer structure for enhancing electrorheology properties

This study introduces an electrorheological (ER) approach that allows us to obtain remarkably enhanced ER properties by using monodisperse colloidal dimer particles. Two sets of colloidal particles, which are spheres and symmetric dimers, were synthesized employing the seeded polymerization technique. The aspect ratio of dimer particles was ~1.43. Then, the surface of the particles was coated with polyaniline by using the chemically oxidative polymerization method. After preparation of the particle suspensions having the same particle volume and concentration, their ER behavior was investigated with changing the electric field strength. At the same experimental condition, both shear stress and shear yield stress of the dimer particle suspension remarkably increased, compared with those of the spherical particle suspension. This attributes to the fact that the shape anisotropy of suspending particles effectively led to increase in the dipole moment under the electric field, thus resulting in formation of a well-structured colloidal chains between the electrodes.     

Electrorheological effect in hybrid fluids with liquid crystalline additives

Conventional electrorheological (ER) fluids consist of electrically polarizable particles dispersed in an inert insulating liquid. They are characterized by a development of a yield stress upon application of an external electric field. They resemble Bingham fluids with yield stress value depending on electric field. A viscosity increase in the presence of an electric field has been also found in homogeneous solutions of liquid crystalline polymers with no yield stress observed. In this study these two types of fluids and combined dispersions of the solid particles in the liquid crystalline matrix were investigated. A lyotropic liquid crystalline polymer—poly(n‐hexyl isocyanate) (PHIC)—dissolved in xylene was chosen as the active matrix. The dispersed solid phase was comprised of two kinds of polymers: pyrolyzed polyacrylonitryle (PAN) showing electron conductivity, and PAN doped with two salts (KSCN, NaSCN), resulting in ionic conductivity. The rheological measurements under an electric field were performed. The pristine xylene solution of PHIC was characterized first as well as the 15% m/m dispersions of PAN powders in silicone oil. Then the dispersions in the liquid crystalline matrix were investigated showing a strong ER effect whose magnitude was considerably enhanced in comparison to both ER active components measured separately. Copyright © 2006 John Wiley & Sons, Ltd.     

Effects of medium oil on electroresponsive characteristics of chitosan suspensions

Biocompatible chitosan particle suspensions in host oils of corn, soybean, and silicone were prepared and their electrorheological (ER) characteristics were examined under the imposition of electric fields. The effects of the weight concentration of particulate chitosan and the strength of the applied electric field on ER response in the various chitosan particle suspensions were investigated via measurements of rheological properties including flow curve, shear viscosity, and yield stress. The yield stresses of the three different chitosan–oil systems showed different values of slope in the electric field, but all data were found to fit well with our previously proposed universal scaling function.     

聚N-甲基苯胺/蒙脱土纳米复合材料的合成及其协同电流变效应

用插层聚合法制备了聚N 甲基苯胺 蒙脱土纳米复合材料微粒 ,通过IR、XRD及TEM对其结构进行了表征 .观察发现聚N 甲基苯胺插入蒙脱土层间后 ,蒙脱土片层间距由 0 96nm扩大至 1 34nm .将其分散在甲基硅油中 (2 0wt% )配制成无水电流变液 ,该复合材料表现出显著的协同效应 ,具有较好的电流变行为 .实验表明在电场作用下聚N 甲基苯胺 蒙脱土纳米复合材料的电流变效应比聚苯胺、蒙脱土都有显著提高 ,在 3kV mm(DC ,74 5s- 1 )时 ,剪切强度达 6 0kPa ;同时抗沉降性极好 ,静置 6 0天沉淀率小于 3% .介电性能测试表明聚N 甲基苯胺 蒙脱土纳米颗粒的介电常数和介电损耗较蒙脱土和聚N 甲基苯胺明显提高 ,电导率也达到了最佳范围 .     

Conductivity of flowing polyaniline suspensions in electric field

The formation of chain structures by polarized polyaniline (PANI) particles suspended in silicone oil in the electric field has been monitored by recording suspension conductivity in the course of time. For that purpose, three types of PANI particles differing in the conductivity (3.1 × 10⁻³, 1.7 × 10⁻¹, and 2.0 × 10⁻¹ S cm⁻¹) have been chosen out of a series of nine samples prepared by controlled protonation of PANI base in orthophosphoric acid solutions. Relaxation times reflecting this process and characterizing the rate of the response to the electric field decreased with particle conductivity, indicating a higher polarizability of particles. At the same time, the maximum conductivity of suspension increased as a consequence of the electric and shear forces acting on the particles. In the shear fields, shorter relaxation times appeared than at rest. The simultaneous measurement of the shear stress confirmed that the conductivity investigation can reliably characterize the development of electrorheological structures.