New magneto-rheological fluids with high stability: Experimental study and constitutive modelling

Magneto-rheological fluids (MRF) are known as a category of smart materials because they exhibit sudden viscosity changes upon application of magnetic field. In contrast to normal fluids, MRFs can sustain shear up to a yield stress. Stability and resistance against movement are important factors which determine the extent of application of a MRF. In this work, new MRFs are developed using engine oil as carrier liquid, carbonyl iron powder as magnetic particle, stearic acid and CHRYSO® Optima100 as additives. Stability of the samples is measured over time. The samples are exposed to magneto-rheological tests with combined liquid and Peltier temperature control. Samples A, B and C are prepared with low, medium and high particle fractions respectively and tested at different temperatures (−10 °C, 5 °C, 60 °C) but for samples D, E, F and G the rheology tests are conducted in room temperature (25 °C) but at variable magnetic field and shear rate. Inherent assumption of the existing constitutive models is that the flow curve of MRF is shifted by a field-dependent yield stress. In this paper the effect of magnetic field is formulated and based on the physical properties of MRFs, a new method is introduced for identification of material parameters. This method predicts the yield stress by comparing the storage and shear moduli. Obtained results are compared with those obtained from fitting the experimental flow curves and also with those obtained from Bingham model. It is shown that, results of the proposed model are in good agreement with the experimental data. Moreover, the calculated sedimentation ratio shows that simultaneous use of stearic acid and Optima100 significantly improves stability of MRFs.     

Effect of polymerization temperature on polyaniline based electrorheological suspensions

As one of the intrinsically polarizable materials used in electrorheological (ER) fluid, polyaniline was synthesized by the chemical oxidation of aniline with ammonium peroxysulfate. ER fluids were prepared by dispersing polyaniline particles in silicone oil, and their rheological properties were measured. The effect of the polymerization temperature of polyaniline on its ER characteristics was investigated. ER fluids with polyaniline particles synthesized at −10°C (PA-10) showed the best ER performance (yield stress) compared with those synthesized at higher temperatures. A difference in the flow behavior of the ER fluids was also investigated through the dielectric spectra of ER fluids. Received: 24 March 1988 Accepted: 3 August 1998     

Flow of non‐newtonian fluids in porous media

The study of flow of non‐Newtonian fluids in porous media is very important and serves a wide variety of practical applications in processes such as enhanced oil recovery from underground reservoirs, filtration of polymer solutions and soil remediation through the removal of liquid pollutants. These fluids occur in diverse natural and synthetic forms and can be regarded as the rule rather than the exception. They show very complex strain and time dependent behavior and may have initial yield‐stress. Their common feature is that they do not obey the simple Newtonian relation of proportionality between stress and rate of deformation. Non‐Newtonian fluids are generally classified into three main categories: time‐independent whose strain rate solely depends on the instantaneous stress, time‐dependent whose strain rate is a function of both magnitude and duration of the applied stress and viscoelastic which shows partial elastic recovery on removal of the deforming stress and usually demonstrates both time and strain dependency. In this article, the key aspects of these fluids are reviewed with particular emphasis on single‐phase flow through porous media. The four main approaches for describing the flow in porous media are examined and assessed. These are: continuum models, bundle of tubes models, numerical methods and pore‐scale network modeling. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Some applications of the AMPC model of the shear yield stress of particulate fluids

The AMPC structural model of shear yield stress of single component suspensions is extended to three important rheological phenomena encountered in particulate fluids. These are shear yield stress of multi-component systems, time dependent shear yield stress of thixotropic suspensions, and compressive yield stress of particulate fluids. The shear yield stress of multi-component suspensions is modeled by summing the interparticle interactions due to all possible combinations of particle sizes and solid constituents. The time dependent shear yield stress of thixotropic muds is simulated by invoking the proportionality between yield stress and interparticle bond density across the shear plane. The compressive yield stress of particulate fluids is calculated by assuming that consolidation occurs by shear but at a narrower gap between the particles. The ability of the model to describe these diverse phenomena reasonably well seemingly validates its basic premise, namely, a 3-dimensional space-filling network of particles whose mechanical properties can be estimated from the theory of strength of particulate assemblage.     

Differences in the electrorheological response of a particle suspension under direct current and alternating current electric fields

We have observed an unusual reduction of shear stress with increasing shear rate under direct current electric fields, for an electrorheological fluid composed of sulfonated poly(styrene-co-divinylbenzene) particles dispersed in silicone oil. At all shear rates, the shear stress under the electric field is larger than that in the absence of the field, indicating that there is still some field-induced agglomeration of the particles. In contrast, the behavior under alternating current electric fields is the Bingham-fluid-type response commonly observed with electrorheological fluids. It is suggested that the conventional dipole–dipole interaction approach based on simplified microstructural models would be unable to explain these phenomena. Received: 27 November 2000 Accepted: 22 May 2001     

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.     

Self-organization in the flow of complex fluids (colloid and polymer systems). Part 2: Theoretical models

Flow induced transitions in complex fluids are usually accompanied by changes in the internal media structure and the flow symmetry. In this review paper, we discuss the theoretical models and approaches that have been used for the analysis of different types of flow instabilities and flow patterns. The main attention is focused on the basic fluid models which reveal vortex and banding flow structures at high shear rates. The Oldroyd-B fluid is one of such models. The Reynolds and the Weissenberg (or Deborah) numbers are the parameters governing its flow behavior. For this model, the secondary flow patterns arising in viscometric flows of different geometries at the bifurcation point are described. Complex fluids which are able to exist in multiple states can form coexisting bands of different structures with different rheological properties and flowing with different shear rates at the same shear stress. Shear banding is typical for fluids demonstrating non-monotonous flow curves described by such models as the diffusive Johnson-Segalman fluid model, for example. Recent progress in exploring this phenomenon is discussed.     

Atom transfer radical polymerized PMMA/magnetite nanocomposites and their magnetorheology

We synthesized core/shell-typed magnetic nanoparticle composites using poly(methyl methacrylate) (PMMA) as a shell and magnetite nanoparticle (MN) as a core, in which the PMMA shell was prepared via atomic transfer radical polymerization (ATRP) method. Chemical structure and morphology of the synthesized MN–PMMA nanocomposite were investigated using FT-IR and TEM, respectively. Magnetorheological (MR) fluid was prepared by dispersing synthesized MN–PMMA in non-magnetic medium. Both shear stress and shear viscosity of the MR fluids as a function of shear rate were measured using a rotational rheometer with a magnetic field generator, exhibiting that a yield stress increased with an external magnetic field strength.     

The extension of coordination number model by using the local composition theory: mixtures

The extension of a new coordination number model to mixture is presented in this work. Extended model agrees well with the Monte Carlo (MC) simulation results for square-well (SW) mixture fluids and shows better results compared with other models. To test our model, we compare the compressibility factors from various models for SW fluids at different λ values and for SW fluid mixtures at λ=1.5. Although our model is obtained by fitting simulation data at λ=1.5, it shows better results for the different λ values than other coordination number model. Compared with the compressibility factors of various binary mixtures of SW fluids calculated from other models, this model presents better results. Because our model considers the temperature dependency importantly by using the total site number, it predicts coordination number and compressibility factor well in the wide temperature range and enables one to derive an equation of state (EOS) through integration of the coordination number equation.     

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.     

用共混和共聚法改性聚苯胺及其电流变液的研究

通过共混和表面接枝共聚两种方法改性聚苯胺，制备聚苯胺（ＰＡｎ）复合粒子并用它们组成活性较高的电流变（ＥＲ）液．研究了ＰＡｎ复合粒子的结构与ＥＲ液性能的关系．结果表明在ＰＡｎ粒子表面涂覆聚乙烯醇或接枝聚丙烯酰胺等与ＰＡｎ能形成氢键或有化学键连接的绝缘物质可以既提高其ＥＲ液的电诱导屈服应力又降低漏电流密度．     

Prediction of environmental stress cracking in polycarbonate by molecular modeling

Environmental stress cracking (ESC) is a premature failure of a polymer exposed to a fluid, under stress which is much less than its yield stress. Many experimental works were done before in an effort to predict experimentally the ESC potential of a fluid in different polymers. None of the previous works applied molecular modeling techniques to predict this potential so this work is a pioneering work. This study's goal was to apply atomistic molecular modeling techniques to gain a better understanding of the ESC mechanism and to predict the ESC potential of different fluids in polymers. Our model experimental system was amorphous polycarbonate (PC) with water as an ESC fluid. The computational study was expanded to include a high level ESC fluid for PC–toluene and a non ESC fluid–BD, together with the moderate ESC fluid–water. A clear distinction between ESC fluids and non ESC fluids for PC was achieved by means of molecular modeling. The experimental work approved that water is an ESC fluid for PC as predicted in the computational part. Copyright © 2014 John Wiley & Sons, Ltd.     

Magnetorheology of soft magnetic carbonyl iron suspension with single-walled carbon nanotube additive and its yield stress scaling function

The serious dispersion problem of carbonyl iron (CI) based magnetorheological (MR) fluid, due to the large density mismatch between CI particles and continuous medium, has hampered its MR applications. To resolve this undesirable sedimentation, we introduced fibrous single-walled carbon nanotube (SWNT) into CI suspension as additives. The dynamic yield stress change measured as a function of magnetic field strength was examined by adopting a universal equation which was originally applied for electrorheological (ER) fluids. In addition, the viscoelastic performances of CI/SWNT suspension were compared to investigate the influence of additives on the pristine CI suspension. The sedimentation ratio was also examined to confirm the role of submicron SWNT bundles.     

Static Yield Stress of an Electro-Rheological Fluid

The static yield stress of an electro-rheological fluid (starch/gas-oil and mesophase-carbon/gas-oil, abbreviated as ERF), congealed by an electric field, has been examined by means of a tensile tester with parallel electrodes. Static yield stress showed different behavior from dynamic yield stress reported by many investigators: thus static yield stress is proportional to applied field strength, but not to the square of it, as is dynamic yield stress. The adhesive force between the particles was not Coulomb's force but Maxwell's force. The theory of McLean and Ikazakiet al.using the Johnsen–Rahbeck effect on the adhesive force of the dust pile on an electrode of the electro-static precipitator was applied and the deduced equation actually explained the phenomena. It was clarified experimentally and theoretically that the physical properties of the ERF particles had no effect on the adhesive force of the pearl chains of a congealed ERF; on the contrary, the volumetric concentration did affect it.     

沸石电流变液材料的研究

系统地研究了沸石的由离子类型、数目、骨架的孔道大小和结构特性对沸石电流变液性能的影响. 证实了阳离子在电场中的迁移所引起的晶粒极化是Y，A和M等沸石电流变特性的基础，指出了较大的孔径和具有足够多易迁移的阳离子是该类沸石具有较好电流变性的的前提．根据NaZSM－5沸石在高电场中与其它沸石表现不同，具有屈服应力大而漏电流小的特点，提出了沸石晶粒极化还可能存在的另一种新的晶格变形机理．     

New analysis of yield stress on giant electrorheological fluids

A new universal yield stress scaling equation is proposed to accurately model experimental data for giant electrorheological (GER) fluids. This new equation expressed in modified Bessel function predicts both regions of polarization effect predominant in the low electric field strength applied and polar molecule-dominating GER behavior, as well as collapses the experimental data of yield stress in a single line for a broad range of electric field strengths.     

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.     

An Experimental Study of Influence of Salt Concentration,Mixing Time,and pH on the Rheological Properties of Pre-Hydrated Bentonite Slurries Treated by Polymers

The aim of this study is to investigate the influence of pH between 8.5 and 12.5, mixing time, and of NaCl, KCl, and CaCl₂ concentration over the range of 0% to 20% on the rheological properties of pre-hydrated bentonite slurry following by treatments with different polymers. Viscometric data gathered by viscometer described well by the Herschel-Bulkley rheological model for all fluids tested. Increasing salt concentration and pH led to some significant changes in the rheological properties. Moreover, adding saturated salt to pre-hydrated slurry resulted in increasing shear stress and plastic viscosity for all fluids.