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This research details a novel method of increasing the shear yield stress of magnetorheological (MR) fluids by combining shear and squeeze modes of operation to manipulate particle chain structures, to achieve so-called compression-assisted aggregation. The contribution of both active gap separation and particle concentration are experimentally measured using a custom-built Searle cell magnetorheometer, which is a model device emulating a rotary Magnetorheological Energy Absorber (MREA). Characterization data from large (1 mm) and small (250 μm) gap geometries are compared to investigate the effect of the gap on yield stress by compression enhancement. Two MR fluids having different particle concentrations (32 vol% and 40 vol%) are also characterized to demonstrate the effect of solids loading on compression-assisted chain aggregation. Details of the experimental setup and method are presented, and a chain microstructure model is used to explain experimental trends. The torque resisted by practical rotary MREAs is directly related to the strength of the MR fluid used, as measured by the shear yield stress. This study demonstrates that it is feasible, utilizing the compression-enhanced shear yield stress, to either (1) design a rotary MREA of a given volume to achieve higher energy absorption density (energy absorbed normalize by device volume), or (2) reduce the volume of a given rotary MREA to achieve the same energy absorption density.

Elastic percolation transition in nanowire-based magnetorheological fluids Appl. Phys. Lett. 95, 014102 (2009); 10.1063/1.3167815 Dynamic yield stress enhancement in bidisperse magnetorheological fluids J. Rheol. 49, 1521 (2005); 10.1122/1.2085175Study on the mechanism of the squeeze-strengthen effect in magnetorheological fluids

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Articles you may be interested inRheological characterization of a magnetorheological ferrofluid using iron nitride nanoparticles Magnetorheological behavior of magnetite covered clay particles in aqueous suspensions J. Appl. Phys. 112, 043917 (2012); 10.1063/1.4748878Magnetorheology and sedimentation behavior of an aqueous suspension of surface modified carbonyl iron particles Magnetorheological (MR) fluids are suspensions of micron-scale magnetizable particles suspended in a carrier fluid. When field is applied, MR fluids develop a field controllable yield stress and a field independent post-yield viscosity. However, this viscosity has substantial temperature dependence, varying by up to an order of magnitude over the operating temperature range of MR fluid devices. We apply non-Brownian suspension theory to explain this result and find that the majority of this effect should be caused by the temperature dependent behavior of the carrier fluid. Thus, if two fluids share the same carrier fluid, then their fluid properties should scale in temperature similarly. This result is first validated by measuring viscosity across temperature for custom model fluids designed to conform to theory, showing temperature scaling within 5% for both the MR fluids and their carrier fluid. Then, on a series of related commercially available fluids with unknown additive content, we show that the MR fluids exhibit common scaling to within 4%. We also investigate the effects of magnetic hysteresis and find that it induces a negligible increase in yield stress and no measurable change in viscosity. We conclude that our non-dimensional analysis enables the temperature dependence of novel MR fluids to be characterized with fewer experiments. V C 2015 AIP Publishing LLC. [http://dx.

Magnetorheological energy absorbers (MREAs) have been successfully deployed in occupant protection systems to protect against potentially injurious shock, crash and blast loads. These MREAs operate at shear rates upwards of 25 000 s , but magnetorheological fluids (MRFs) are typically characterized for shear rates up to 1000 s in commercially available parallel counter-rotating disk rheometers. Because of the lack of availability of data at the required high shear rates, a Searle-type magnetorheometer (essentially a concentric cylinder rotating in a cup) was designed and fabricated at the University of Maryland. Using this magnetorheometer, two commercial MRFs were characterized over the shear rate range of 0-25 000 s . It is shown that the rheometer was successful in replicating available characterization data at low shear rate, as well as quantifying high shear rate behavior as a function of applied field. In addition, it was shown that the Herschel-Bulkley constitutive model is appropriate and successfully characterized the apparent viscosity vs. shear rate behavior of the MRFs over this shear rate range. Experimental data demonstrate that an increase in field dependent yield stress can be realized over this entire shear rate range, so that MREAs can be designed using data taken with the magnetorheometer. Finally, the Mason number, which has been shown to be a useful non-dimensional number at low shear rates, also provides a useful physical interpretation at high shear rates.

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In the present work we investigated the behaviour of magnetorheological fluids (MRFs) by means of a custom Searle magnetorheometer. The Searle measuring system is composed of an outer stationary cylinder and an inner cylinder driven by a motor. The MR fluid is placed in between the two concentric cylinders. While the bob is rotating, the fluid transmits a torque to the outer cylinder, which is related to the velocity profile through the gap and the magnetic field applied. In particular, we wanted to assess the influence of different bob shapes on the shear stress-shear rate curves. Indeed, a non-circular cross section causes cyclic variations on the gap thickness and consequently can squeeze the active MR fluids causing the formation of thicker columns and higher values of shear stresses. This phenomenon known as squeeze-strengthen effect can be exploited to achieve higher energy absorption on magnetorheological devices like brakes and clutches. Two different cross sections were taken into account and then compared to the cylindrical one: elliptical and quadrilobate. In both cases, the gap size varies from 0.25 mm to 0.75 mm. The experimental results showed that with the new bob shapes, the squeeze-strengthen effect does not occur. However, using the elliptical and quadrilobate bobs changes in the slope of the flow curves has been measured. This may suggest that the post yield viscosity can be influenced by more complex factors like non-uniform magnetic field or hydrodynamic phenomena. Further studies such as 3D magnetic simulation and CFD analysis will be taken into account to explain the new phenomena brought up in this work