High-flux magnetorheology at elevated temperatures

Commercial applications of magnetorheological (MR) fluids often require operation at elevated temperatures as a result of surrounding environmental conditions or intense localized viscous heating. Previous experimental investigations of thermal effects on MR fluids have reported significant reductions in the magnetorheological stress with increasing temperature, exceeding the predictions made by considering the thermal variations in the individual physical properties of the fluid and solid constituents of a typical MR fluid. In the low-flux regime, designers of MR fluid actuators can alleviate this thermal reduction in stress by increasing the applied magnetic field strength. However, this is not possible in the high-flux regime because of magnetic saturation, and it becomes necessary to explore and understand the intrinsic limitations of the fluid at elevated temperature. We describe a new magnetorheological fixture, which was designed as an accessory to a commercial torsional shear rheometer, capable of applying magnetic flux densities up to 1 T and controlling the sample temperature up to 150°C. During the design of the instrument, close attention was given to the uniformity of the magnetic field applied to the sample by using numerical simulations. Incorporation of a custom-built magnetic flux sensor which matches the environmental capabilities of the fixture enables in situ measurement of the local magnetic field at each temperature. The numerical results are also validated by spatially resolved measurements of the local magnetic field. Finally, we explore the ability of a shift factor between fluid magnetization and yield strength to describe the measured variation in the MR fluid response at elevated temperatures.     

A numerical method for determining the shear stress of magnetorheological fluids using the parallel-plate measuring system

This work proposes a new numerical method for determining the shear stress, which does not need any preassumption about the exact behavior of the fluid to achieve absolute data using a parallel-plate measuring system. The ability for representing different behaviors along the entire shear-rate range makes this method particularly interesting for the study of magnetorheological (MR) fluids. In this work, the conversion factors used by the rheometer for concentric-cylinder, cone-plate, and parallel-plate measuring systems are first analyzed. This analysis shows that the software used by the rheometer is not appropriate for the quantitative characterization of non-Newtonian fluids using the parallel-plate measuring system. Therefore, a new method for conversion of the parameters measured by the rheometer to the rheological parameters of the fluid is proposed; simultaneously, this new method is compared with other correction methods proposed in the literature: the Rabinowitsch-type method and the single-point method. Finally, the proposed method is applied for the quantitative characterization of an MR fluid.     

Study of magnetorheological fluids at high shear rates

The tunable rheological properties of magnetorheological (MR) materials at high shear rates are studied using a piston-driven flow-mode-type rheometer. The proposed method provides measurement of the apparent viscosity and yield stress of MR fluids for a shear rate range of 50 to 40,000 s⁻¹. The rheological properties of a commercial MR fluid, as well as a newly developed MR polymeric gel, and a ferrofluid-based MR fluid are investigated. The results for apparent viscosity and dynamic and static shear stresses under different applied magnetic fields are reported.     

Rheometry on magnetorheological (MR) fluids

Test fixtures of a commercial concentric cylinder rheometer (Physica Rheolab MC 20) were modified to enable measurements under magnetic inductions up to 0.5 Tesla in a shear rate range of 0.1 up to 1000 s ^–1 and temperatures 0° to 150°C. In the 2 x90°-cups only two 90° sectors of the stationary part of the double concentric cylinder arrangement are submitted to the magnetic field which is created outside the test tools by an electromagnet. A prototype of a 360°-cup contains the electromagnet within the cup and avoids the correction necessary for the sector geometry. Measurements are shown for a carbonyl iron MR fluid and two nano MR fluids. An encouraging comparison of the viscosity function and MR effect (shear stress changes due to the field) measured by using the various cups is presented. The detailed investigation of the magnetic field distribution in the tools yields a distinct radial field gradient and also stray fields that make the quantification of the effective field in the gap difficult. The change of the field when the gap is filled with MR fluid is addressed. MR effects up to 13 000 Pa have been found, the limited torque range of the rheometer making it necessary to use relatively small gap dimensions which introduce errors due to edge effects. Shear rates up to 40000 s^–1 as typical for the application in dampers were investigated by a piston-driven capillary rheometer making use of a thermostated rectangular slit with superimposed magnetic field. A satisfactory agreement of the magnetorheological data with the concentric cylinder results is found in the overlapping shear rate range.     

Nonlinear viscoelastic properties of MR fluids under large-amplitude-oscillatory-shear

Nonlinear viscoelastic properties of the MR fluid, MRF-132LD, under large-amplitude oscillatory shear were investigated. This was accomplished by carrying out the experiments under the amplitude sweep mode and the frequency sweep mode, using a rheometer with parallel-plate geometry. Investigations under the influence of various magnetic field strength and temperatures were also conducted. MR fluids behave as nonlinear viscoelastic or viscoplastic materials when they are subjected to large-amplitude shear, where the storage modulus decreases rapidly with increasing strain amplitude. Hence, MR fluid behaviour ranges from predominantly elastic at small strain amplitudes to viscous at high strain amplitudes. Large-amplitude oscillatory shear measurements with frequency sweep mode reveal that the storage modulus is independent of oscillation frequency and approaches plateau values at low frequencies. With increasing frequency, the storage modulus shows a decreasing trend before increasing again. This trend may be explained by micro-structural variation. In addition, the storage modulus increases gradually with increasing field strength but it shows a slightly decreasing trend with temperature.     

Viscous dissipation with fluid inertia in oscillatory shear flow

For liquids with high viscosity and low thermal conductivity, viscous dissipation can cause appreciable errors in rheological property measurements. Here, the influences of both viscous dissipation and fluid inertia on the property measurements in oscillatory sliding plate rheometry are investigated. For Newtonian fluids, Bird (1965) solved the combined problem analytically, but only for high frequencies. Here his solution is extended to any frequencies. Also, the equations of motion and energy are solved for linear viscoelastic fluids, and new analytical solutions for the velocity and temperature profiles are given. In both Newtonian and linear viscoelastic fluids, the temperature rise in the gap increases with frequency. The location of the maximum temperature shifts from the mid-plane at low frequency towards the moving wall at high frequency. The fluid inertia increases the viscous dissipation in both fluids. By solving the combined problem, this paper simplifies rheometer design by providing one unified criterion for avoiding measurement errors. Operating limits are presented graphically for minimizing the effects of both fluid inertia and viscous dissipation in oscillatory sliding plate rheometry.     

On the viscosity of magnetic fluid with low and moderate solid fraction

The design of a pressurized capillary rheometer operating at prescribed temperature is described to measure the viscosity of magnetic fluids (MFs) containing Fe3O4 magnetic nanoparticles (MNPs). The equipment constant of the rheometer was obtained using liquids with predetermined viscosities. Experimentally measured viscosities were used to evaluate different equations for suspension viscosities. Deviation of measured suspension viscosities from the Einstein equation was found to be basically due to the influence of spatial distribution and aggregation of Fe3O4 MNPs. By taking account of the coating layer on MNPs and the aggregation of MNPs in MFs, a modified Einstein equation was proposed to fit the experimental data. Moreover, the influence of external magnetic field on viscosity was also taken into account. Viscosities thus predicted are in good agreement with experimental data. Temperature effect on suspension viscosity was shown experimentally to be due to the shear-thinning behavior of the MFs.     

电流变液研究进展及最新动态──第5届国际电流变液、磁悬浮体 及相关技术研讨会评介

通过评介第５届国际电流变液、磁悬浮体以及相关技术研讨会，指出电流变液和磁流变液的应用研究有重要进展；磁流变液的剪切应力比电流变液大一个数量级，近来又受到重视；在机理研究中，要注意电流变液的表面效应、损耗对电流变效应的影响．本文还归纳了一些电流变液材料设计的思路．     

On the viscosity of magnetic fluid with low and moderate solid fraction

The design of a pressurized capillary rheometer operating at prescribed temperature is described to measure the viscosity of magnetic fluids （MFs） containing Fe3O4 magnetic nanoparticles （MNPs）. The equipment constant of the rheometer was obtained using liquids with predetermined viscosities. Experimentally measured viscosities were used to evaluate different equations for suspension viscosities. Deviation of measured suspension viscosities from the Einstein equation was found to be basically due to the influence of spatial distribution and aggregation of Fe3O4 MNPs. By taking account of the coating layer on MNPs and the aggregation of MNPs in MFs, a modified Einstein equation was proposed to fit the experimental data. Moreover, the influence of external magnetic field on viscosity was also taken into account. Viscosities thus predicted are in good agreement with experimental data. Temperature effect on suspension viscosity was shown experimentally to be due to the shear-thinning behavior of the MFs.     

磁场下2种羰基铁磁流变液的摩擦磨损特性

本文采用改进后的四球摩擦磨损试验机,考察了羰基铁磁流变液在外加磁场条件下的摩擦磨损行为,并比较了有场和无场条件下磁流变液的摩擦系数变化情况。结果表明：外加磁场能使磁流变液的摩擦系数显著增大,且磁流变液的摩擦系数随磁场强度的增加呈现出增大的趋势;钢球磨斑形貌由圆形或椭圆形变为近似矩形,磨痕变浅;无场条件下,系统的摩擦磨损形式是钢球、磁性颗粒、钢球之间的三体磨损;外加磁场时,磁流变液的摩擦磨损形式发生改变。     

A new eccentric-cylinder rheometer

A new eccentric-cylinder rheometer with guard-ring equipment was constructed as an auxiliary set-up to a Rheometrics Mechanical Spectrometer, type 7200, which enables measurements with the eccentric disk technique. Experimental tests with Newtonian fluids and polymer solutions of relatively low viscosity show that this instrument yields reliable plots of shear viscosity and first normal-stress coefficient over several decades of the effective average shear rate. These coincide very well with plots from a commercial cone-and-plate rheometer even for higher relative eccentricities (up to 0.75). However, no systematic effects of eccentricity, to be expected for higher shear rates, could be observed with these fluids, so that supplementary tests applying fluids with more pronounced viscoelastic properties are to be carried out.     

Rheometry for large-particulated fluids: analysis of the ball measuring system and comparison to debris flow rheometry

For large-particulated fluids encountered in natural debris flow, building materials, and sewage treatment, only a few rheometers exist that allow the determination of yield stress and viscosity. In the present investigation, we focus on the rheometrical analysis of the ball measuring system as a suitable tool to measure the rheology of particulated fluids up to grain sizes of 10 mm. The ball measuring system consists of a sphere that is dragged through a sample volume of approximately 0.5 l. Implemented in a standard rheometer, torques exerted on the sphere and the corresponding rotational speeds are recorded within a wide measuring range. In the second part of this investigation, six rheometric devices to determine flow curve and yield stress of fluids containing large particles with maximum grain sizes of 1 to 25 mm are compared, considering both rheological data and application in practical use. The large-scale rheometer of Coussot and Piau, the building material learning viscometer of Wallevik and Gjorv, and the ball measuring system were used for the flow curve determination and a capillary rheometer, the inclined plane test, and the slump test were used for the yield stress determination. For different coarse and concentrated sediment–water mixtures, the flow curves and the yield stresses agree well, except for the capillary rheometer, which exhibits much larger yield stress values. Differences are also noted in the measuring range of the different devices, as well as for the required sample volume that is crucial for application.     

Numerical simulation of power law and yield stress fluid flows in double concentric cylinder with slotted rotor and vane geometries

In our previous report, we showed that a rheometer equipped with a double concentric cylinder geometry with slotted rotor could effectively reduce wall slip effects and thus it could be used as an alternative to a rheometer with a vane geometry in yield stress measurements. Here, we use three-dimensional CFD simulation to compare these two geometries for rheological measurements of power law and yield stress fluids. Our results indicate that the double concentric cylinder rheometer with slotted rotor (DCCR/SR) is able to accurately measure rheological properties of a wider spectrum of test fluids than a vane rheometer because of significant reduction of the end and secondary flow effects.     

一类非线性磁流变系统局部分岔特性研究

讨论了一类基于磁流变阻尼器非线性系统的局部分岔与控制问题，建立了该系统的动力 学模型，运用中心流形定理和范式理论，得到该系统双零特征值问题的规范形及其普适开折， 进而探讨了此系统的分岔行为和稳定性；给出了分岔曲线、转迁集；并给出了此类非线性系 统的数值仿真结果.     

A modified rheometer for in-situ radial and tangential SAXS studies on shear-induced alignment

A tooth rheometer designed to investigate the influence of large amplitude oscillatory shear on the macroscopic orientation of complex fluids is described. The rheometer allows in-situ two-directional small-angle scattering measurements, making it possible to observe in particular the orientation process of hierarchically structured block copolymers in dynamics. Experiments performed with a linear poly(vinyl methyl ether) show that, despite a considerable gap/width ratio, the rheometer can be used for adequate rheological measurements. In addition we found an unexpected dependence of the dynamic parameters on the rheometer gap. The small amount of sample required favors the use of this rheometer for measuring the rheological characteristics of new compounds.     

Micro-Fourier rheometer: Inertial effects

The inertial effects in a random squeezing rheometer are examined, both theoretically and experimentally. The rheometer is based on small amplitude random squeezing between two parallel plates, where the upper plate is driven by a random displacement with a broad band spectrum. A fast Fourier transform is used to deliver the complex modulus (or viscosity) of the fluid in a single brief test, over more than two decades of frequency. The inertia of the fluid is shown to produce an error factor, which is also a function of the frequency. The correction factor can be well approximated by a first-order correction in the Reynolds number, for a very large range of Reynolds number, making the inertial correction a very simple procedure for light fluids.     

振荡剪切力作用下的电流变流体黏弹性研究

利用HAAKE RS600流变仪对电流变体进行了强制振荡实验研究,得到了流体在不同 剪切应力幅和振荡频率下,电流变体储能模量、损耗模量、损耗因子随剪切应力和 剪切频率的变化规律.     

Flow of viscoelastic fluids between plates rotating about distinct axes

We discuss the flow of BKZ fluids in an orthogonal rheometer. Some analytical results are proved, and numerical solutions are obtained for the Currie model. These solutions show a boundary layer behavior at high Reynolds numbers and the possibility of discontinuous solutions or nonexistence at high Weissenberg numbers.     

Surface tension driven jet break up of strain-hardening polymer solutions

Experimental studies attempting to ascertain the influence of viscoelasticity on the atomization of polymer solution are often hindered by the inability to decouple the effect of shear thinning from the effect of extensional hardening. Here, the influence of viscoelasticity on the jet break up of a series of non-shear-thinning viscoelastic fluids is quantified. Previous characterization using an opposed-nozzle rheometer identified the critical extensional rates for strain hardening of these model fluids. The strain hardening fluids exhibit a beads-on-string structure with reduction or elimination of satellite drops. Capillary instabilities grow on the filaments connecting the spheres and eventually break the filaments up into a string of very small drops about one order of magnitude smaller than the satellite drops formed by a Newtonian fluid with the same shear viscosity, surface tension, and density. These results confirm that strain hardening is the key rheological property in jet break up and that the critical extensional rate of a fluid is pertinent in determining the final characteristics of break up. Results suggest that the opposed-nozzle rheometer does probe extensional behavior in the range of extensional rates that are relevant to jet break up, providing a tool to roughly predict jet break up.     

Rheological properties and dispersion stability of magnetorheological (MR) suspensions

In the present article, the rheological responses and dispersion stability of magnetorheological (MR) fluids were investigated experimentally. Suspensions of magnetite and carbonyl iron particles were prepared as model MR fluids. Under an external magnetic field (H ₀) and a steady shear flow, the yield stress depends upon H ₀ ^3/2. The Yield stress depended on the volume fraction of the particle (φ) linearly only at low concentration and increased faster at high fraction. Rheological behavior of MR fluids subjected to a small-strain oscillatory shear flow was investigated as a function of the strain amplitude, frequency, and the external magnetic field. In order to improve the stability of MR fluid, ferromagnetic Co-γ-Fe₂O₃ and CrO₂ particles were added as the stabilizing and thickening agent in the carbonyl iron suspension. Such needle-like particles seem to play a role in the steric repulsion between the relatively large carbonyl iron particles, resulting in improved stability against rapid sedimentation of dense iron particles. Furthermore, the additive-containing MR suspensions exhibited larger yield stress, especially at higher magnetic field strength. Received: 4 April 2000 Accepted: 6 November 2000