Magneto rheological Fluid to Enable Continuous Control

October 20, 2022 Post a Comment

Magnetorheological Fluid

Magnetorheological fluids (MRFs) are included in the group of smart materials, having the unique ability to change their viscosity and their yield stress in response to a magnetic field as well as the external operating power.

From: Encyclopedia of Smart Materials , 2022

Experimental Studies on Magnetorheological Fluids

Seval Genc , in Encyclopedia of Smart Materials, 2022

Abstract

Magnetorheological (MR) fluids are a class of smart materials whose yield stress increases considerably in the presence of externally applied magnetic field. These fluids are composed of soft, spherical, magnetic particles whose diameters range from 0.01 to 20 µm dispersed in an organic liquid. Magnetorheological effect and sedimentation stability are two important factors that make a good MR fluid. Experimental studies mostly concentrate on the synthesis of MR fluids with high yield stress and enhanced sedimentation stability. The focus of this study is to present a comprehensive review on the recent experimental studies, including rheological characterization and sedimentation stability of MR fluids.

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Test Methods, Nondestructive Evaluation, and Smart Materials

MARK R. JOLLY , J. DAVID CARLSON , in Comprehensive Composite Materials, 2000

5.27.5.2 Magnetorheological Foams

MR fluid foam devices contain MR fluid that is constrained by capillary action in an absorbent matrix such as a sponge, open-celled foam, felt, or fabric (Carlson, 1998a, 1998b). The absorbent matrix serves to keep the MR fluid located in the active region of the device between the poles where the magnetic field is applied. The absorbent matrix requires only a minimum volume of MR fluid that is operated in a direct shear mode without the need for seals, bearings, or precision mechanical tolerances. The absorbent matrix is normally attached to one of the poles. Application of the magnetic field causes the MR fluid in the matrix to develop a yield strength and resist shear motion. This basic arrangement may be applied in both linear or rotary devices wherever a direct shear mode would normally be used.

Because of their open structure, the shape of a MR fluid foam device is much less constrained than that of a normal controllable MR fluid device. Multiple degrees of freedom are easily accommodated. Linear devices such as dampers may be tubular, flat, or planar, while rotary brakes may take on the form of a localized magnetic "caliper" operating on a thin, unhoused disk. MR fluid foam devices are highly robust and exhibit very low off-state forces. They are particularly suitable for low to medium force applications where a high dynamic range is desired. Fluids in these devices are resistant to gravitational settling because of the wicking action of the matrix.

The basic elements of a simple, linear, MR fluid foam damper are shown in Figure 8. No seals or bearings are required and only about 3 ml of MR fluid are needed. A layer of open-celled, polyurethane foam saturated with MR fluid surrounds the steel bobbin and coil. Together, these elements form a piston on the end of the shaft that is free to move axially relative to the tubular housing. The steel tube provides the magnetic flux return path. Since MR foam dampers stress the MR fluid in a direct shear mode, maximum force is proportional to the area of active MR fluid foam. Control currents of 1A or less and corresponding operating voltages of 12 V or less are typical.

MR fluid foam dampers exhibit long life. Little wear of the foam matrix occurs as the stresses are carried by the field-induced iron particle structure in the MR fluid. Further, performance is largely unaffected by wear of the foam. The fit of the foam in the gap between the poles is not critical; successful devices have been constructed in which precompression of the foam ranges from 0% to 70%. The absence of seals, bearings, and gas accumulators found in normal fluid dampers means that the achievable stroke length is virtually unlimited.

Figure 8 shows a caliper type of brake geometry. Rather than a housing that fully encloses the rotor, the MR fluid and magnetic circuit are localized in a simple, magnetic caliper arrangement. The absorbent foam filled with MR fluid is attached to the pole faces of the steel yoke. Again, the containment of the MR fluid in the absorbent foam eliminates the need for a fluid seal. MR foam brakes of this sort can provide a very large controlled torque simply by using a large-diameter rotor. If the rotor is very thin it is not even necessary that it be made from a highly magnetically permeable material. Partial arc versions in which the rotor is a pie-shaped sector are another possibility.

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Rheological Analysis of Magnetorheological Fluids

Modesto T. Lopez-Lopez , ... Andrey Y. Zubarev , in Encyclopedia of Smart Materials, 2022

Abstract

Magnetorheological (MR) fluids are smart materials characterized by fast, tuneable and reversible changes of their rheological properties under application of magnetic fields. MR fluids consist of dispersions of micronsized particles of magnetizable materials dispersed in a liquid. Application of magnetic fields result in the magnetization of the dispersed particles, which consequently experience attractive forces, giving rise to the formation of particle structures that oppose to the flow. In this article we describe the experimental techniques used for the rheological characterization of MR fluids, the typical rheological properties of these systems, as well as the theoretical background that justified these properties.

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Modeling Behavior of Magnetorheological Fluids

Maher Y. Salloom , in Encyclopedia of Smart Materials, 2022

Abstract

A magnetorheological (MR) fluid is a fluid that has good magnetic properties. MR fluid responds to the magnetic field and changes its properties when a magnetic field is presented. The areas of application for MR fluids are MR dampers, brakes, clutches and MR valves. The working principles of the MR fluid flow rheological behavior is based on an infinite chain of particles aligned along with the direction of the magnetic field (H). That is the case when there is a gap between any two plates where a magnetic field is applied. This article investigates the modeling behavior of magnetorheological fluid. The model working principle is based on the dipole interaction of particles within a presumed structure. The bipolar model is augmented with a mechanism. The mechanism helps magnetic flux density to distribute in the iron particle network. Modeling of MR fluid is dependent on many models; e.g., the Bingham model, the Herschel-Bulkley model, the Biviscous model, Hysteretic Biviscous models and others. The Bingham Plastic model is considered the best model to describe the relation between the magnetic field and shear stress of MR fluid. The modeling of MR fluid is dependent on the type of mode and application. The mathematical model of flow in MR valve models and modeling of brakes are presented. These models can be used in application design of MR fluid devices.

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Magnetorheological Fluid Applications

Michele Dassisti , Giovanna Brunetti , in Encyclopedia of Smart Materials, 2022

Introduction

MRFs are suspensions of micron-size magnetizable particles in a viscous carrier fluid (usually oil) enriched with one (or more than one) additive. MR technology is drawing high attention of research and industrial worlds, due to the unique capability of these fluid to change the flow behavior under the action of an externally applied magnetic field, in a reversible mode and having a reaction time of few milliseconds. For this reason, MRFs are considered a unique class of smart materials (Carlson and Jolly, 2000). In the future, traditional device might be replaced by MRFs based devices, taking into account the most important principle when designing new devices endeavoring innovative technologies: sustainability (Dassisti et al., 2013). This means that without the action of the external magnetic field, the MRF behaves like a free-flowing liquid; while, when the magnetic field is applied, it behaves like a semi-solid material and can contrast a defined level of force before starting to flow again. Combining the direction of flow of the MRF, the direction of the magnetic field and the direction of the applied force, three different MR operational modes were outlined: direct shear, valve, and squeeze mode. Several applications of these modes (in particular, direct shear and valve) are already present in the marketplace, like for example clutches, dumpers and shock absorbers.

The devices based on MRF technology demonstrate highly desirable features like high controllability, fast reaction, and a simple interface between electrical power input and mechanical power output. These aspects make MRF technology attractive for many applications in different fields: civil engineering, manufacturing, biomechanics, automotive, etc.

The aim of this paper is to review different MRF applications showing the relationship between the device (and so the MR operational mode) and the chemical and physical properties of the fluids, to give a rationale to select the best fluid for some different applications.

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Application of Magnetorheological Fluids (MRF) in a Suspension System

Michele Dassisti , ... Giovanna Brunetti , in Encyclopedia of Smart Materials, 2022

Magneto-Rheological (MR) Fluid Material Performance

MR fluid-based dampers change their performance on the basis of the MR fluid used. Generally speaking, the most influencing parameters are: volume fraction of solid particles (also called "particle loading"), particle size and viscosity of the carrier fluid. A high ratio of solid particles to carrier liquid in the MR fluid is an indication of high magnetic properties (Mazlan et al., 2008). Jolly et al. (1996) concluded that the magnetic properties of MR fluids vary significantly due to particle density loading in the MR Fluid. The theoretical model results demonstrate that magnetic induction increases with field strength until the saturation boundary of the fluid is reached. This is due to the particles forming chain-like structures (MR structures) parallel to the magnetic field lines. The more the particles, the more the MR chains are formed; this increases the on-state viscosity (when the magnetic field is applied) as well as the off-state viscosity (in absence of magnetic field).

With regards to particle size, it has been demonstrated that fluids with two different distributions of particles size (one being characterized by an average diameter significantly bigger than the other) offer a higher yield stress than those fluids with a single average diameter distribution for particle size (Chand et al., 2014). The ratio is that, when the MR fluid is activated, there is a greater magnetostatic interaction between larger magnetic particles while smaller particles fill the voids between big particles, so reinforcing the MR chains at the same time. The best result is obtained when the solid fraction is constituted by 75% of big particles, and 25% of small ones (Foister, 1997; Chiriac and Stoian, 2010).

Some studies also report that adding nanoparticles ad an additive to a MR fluid can improve the yield stress because nanoparticles in the on-state reinforce the MR chains (Wang et al., 2017; Dassisti et al., 2020).

In order to better formulate MR fluids, it is important to take into account also the evolution of the materials over time with particular regard to yield stress. One of the most recent studies by Utami et al. (Utami et al., 2018) demonstrated once again the detrimental effect of In Use Thickening (IUT): during damper operations, particles are forced in contact one with the other and also with the damper walls, which causes friction and breakage of magnetic particles. This leads to an increase in the viscosity of the MR fluid either in the on-state or in the off-state. So, durability in working conditions is very relevant problem for the damper design and the fluid should be formulated with particular attention to the stabilizers adopted.

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Key Elements of Magnetorheological Fluids

Piero Mastrorilli , ... Giovanna Brunetti , in Encyclopedia of Smart Materials, 2022

Abstract

A magnetorheological fluid (MRF) is a fluid able to change its rheological behavior under magnetic field application, developing a yield stress function of the intensity of the field. A magnetorheological fluid (MRF) can be seen as a mixture made up of a continuous phase (the carrier fluid) and a dispersed phase (magnetizable particles), with the addition of additives apt to mitigate some drawbacks and enhance some properties. The aim of this article is to provide the reader with an overview on the commonest materials used for the formulation of MRFs to date.

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Introduction to Magnetorheological Fluids

Michele Dassisti , Giovanna Brunetti , in Encyclopedia of Smart Materials, 2022

Basic Functioning Principles

An MRF is typically made of a suspension of micron-sized (20–50 µm range) magnetizable particles in a carrier fluid, with a typical particle volume percentage of 30%–40%. Additives may enrich the mixture to keep the fluid stable over time, preventing it from losing its operational properties such as, for instance, through the reduction of the transmitted force due to particle sedimentation.

The main property of an MRF is the capability of changing its rheological behavior in a reversible and swift way. Without magnetic field action, the fluid behaves as a regular fluid with properties near to those of the carrier fluid of which it is made of. In presence of a magnetic field, the MRF starts behaving like a semisolid as a function of the field intensity. This change is completely reversible: when the magnetic field is removed, the MRF behaves like the carries fluid again (Olabi and Grunwald, 2007). The condition in which the MRF is subjected to the action of an external magnetic field is called "on-state", while the condition in which the MRF is not activated by the magnetic field is called "off-state".

To explain principles of MRF functioning, the concepts of 'actuation interface' and 'MRF System', can be used as in Fig. 1. A MRF System can be defined as a set of elements (magnetic particles, carrier fluid and additives) performing different roles under magnetic field control. The actuation interface is the locus of points representing the physical separation between the inner and outer part of an MRF system through which actions are exchanged with the external environment.

The magnetorheological performance of MRFs are the result of the magnetic dipole induced on each particle in the fluid by the application of an external magnetic field as well as of the interaction between these dipoles and the magnetic field itself. Each dipole is represented by its magnetic moment vector (see Fig. 3). In absence of magnetic field, particles are randomly oriented and so dispersed in the carrier fluid, because of thermal motion (see Fig. 2(a)) and the overall magnetic moment is null (Hagenbüchle and Liu, 1997). When the interaction energy between magnetic moments and the applied magnetic field exceeds the Brownian thermal energy (Olabi and Grunwald, 2007), the dipoles align head-to-tail and consequently particles are dragged in proximity. In this way, chain structures are formed (see Fig. 2(b)).

This happens because when the magnetic moments are aligned to the magnetic field lines, the potential magnetic energy of the system is minimized, being minimized the angle formed between them. The more intense is the applied magnetic field strength, the more the magnetic moment on each particle aligns to the magnetic field (Ginder, 1998). Fig. 3(a) shows the behavior of two particles not subjected to external magnetic fields; Fig. 3(b) shows particles subjected to a "weak" magnetic field, while Fig. 3(c) shows the interaction with a strong one. The most regular chains are reached when all the dipoles are aligned with the external magnetic field lines (Hagenbüchle and Liu, 1997); this happens when the threshold of magnetization saturation (M_s) is reached. In this condition, the MRF results in a "weakly solid network of particles" (see, e.g., Ginder et al., 1996; Grasselli et al., 1994; Lemaire et al., 1991; Jolly et al., 1996; Olabi and Grunwald, 2007).

The presence of induced chain structures explains the different behaviors of MRFs in the on-state and off-state conditions. In the on-state condition the MRF is subjected to the application of a magnetic field; on the contrary, the off-state condition, is associated with the absence of the externally applied magnetic field to the MRF. When the fluid is in its on-state, and it is mechanically deformed like in Fig. 4, the induced chains in the fluid oppose to the motion, thus determining the change of the flow behavior of the fluid.

Fig. 5 shows the two commonly used models describing the MRFs behavior: the Newtonian model for the off-state and Bingham-plastic model for the on-state. In the off-state the relation between shear stress and shear rate is linear and the dynamic viscosity is independent of the stress (Forte et al., 2004). The Bingham-plastic model, instead, describes the fluid flow in his on-state condition: differently from the Newtonian model, to obtain the fluid movement it is necessary to exert a certain amount of shear stress on it that causes a plastic deformation without any change in viscous behavior. The maximum shear stress that an activated MRF can resist before starting to flow is called yield stress ( $τ_{y}$ ) and it is a function of the strength of the magnetic field (Olabi and Grunwald, 2007). Once the threshold of the yield stress is overcome, the MRF behaves according the Newtonian model again.

The value of the yield stress ( $τ_{y}$ ) of the fluid increases with the intensity of the externally applied magnetic field. Once magnetic saturation M_s is reached, the yield stress no longer increases at the increase of the magnetic flux density, and remains constant (Ginder et al., 1996). The value of M_s depends on the magnetic properties of the particles: particle materials with large saturation magnetization may enhance the maximum yield stress reachable (Zhang et al., 2004; Ginder et al., 1996).

A significant contribution to the yield stress is given by the interaction between the MRF and the actuation interfaces in terms of magnetic properties of the material and roughness characteristic. For a fixed magnetic field and with fixed surface roughness, when the actuation interface material is ferromagnetic, the higher the attractive magnetic force between the particle chains in the MRF and the interfaces (Fig. 6(a)), the higher the yield stress value. On the contrary, when the material is paramagnetic, there is no magnetic force between the chains and the interface, resulting in a lower value of the yield stress. As viewed above, the other factor influencing the yield stress is the surface roughness: a low value of it is detrimental in terms of yield stress, because when the asperities in the material are smaller than the particle size, particles slip tangentially on the interfaces without any force retaining them, even in case of ferromagnetic materials (Fig. 6(b)). The worst case is that of polished surfaces made in paramagnetic material (Lemaire and Bossis, 1991).

In general, the interaction between particles results also in a friction force, which contributes to the overall shear stress in reason of one third, in large shear-strain conditions. This produce wear on the actuation interfaces. It is important then to take into account this effect when designing MRFs and MRF based devices (Li and Zhang, 2008). The friction, in fact, depends on the viscosity of the carrier fluid. In case of low-viscosity (10 mPa s), friction is constant and particles are entrapped in the asperities in the walls of the MR system. In case of intermediate viscosity, friction firstly decreases because particles slip one over the others, and then increases because particles get embedded in asperities again. At high viscosities (200 mPa s), particles accumulate around the contact zone reducing the boundary lubrication (see e.g., Bombard and Vicente, 2012; Leung et al., 2004). Particle concentration also influences friction force. In absence of magnetic field, MRFs with higher particle concentration have higher friction force due to higher number of MR particles trapped in the asperities of surfaces (Song et al., 2011). This is explained by the fact that under operation, particles in the contact zone are plastically deformed and tend to aggregate, creating a thin layer that shields the surface of the actuation interface, reducing their effective roughness and thus the possibility for particles to fill the asperities. Finally, the higher the surface roughness the higher the yield stress exerted, due to the larger number of particles mechanically entrapped in the asperities (Gordaninejad et al., 2005; Wong et al., 2001).

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Magnetorheological Devices

M. Lokander , B. Stenberg , in Elastomers and Components, 2006

2 MR- AND ER-FLUIDS

ERF and MRF are suspensions of polarizable particles in a carrier fluid. When an electric or magnetic field is applied, the particles polarize and align with the field. The interparticle forces cause the particles to form chains that results in a reversible increase in apparent viscosity by several orders of magnitude. When the field is removed the fluid immediately returns to its original state. Both the activation and the deactivation of the fluids are completed within milliseconds after the field is turned on or off (Weiss & Carlson, 1993; Jolly et al, 1999).

Although there is an increase in apparent viscosity from a macroscopic point of view, the actual plastic viscosity, defined as the change in stress per unit change in shear strain, is approximately constant as the field is varied. The fluids in the absence of the field behave approximately as Newtonian fluids. When the field is applied the behaviour is as Bingham bodies (Weiss & Carlson, 1993; Jolly et al, 1999). The yield stress (τ_y) of the Bingham bodies, which is field dependent, is the most important property of MRF as well as of ERF. The required magnetic field for activating MRF is in the order of magnitude of tenths of Tesla (Jolly et al, 1999, which may be achieved using an ordinary 12 V battery and a proper magnetic circuit). The corresponding electric field for ERF is in the order of magnitude of kV, which makes it necessary to use high voltage equipment (Weiss & Carlson, 1993).

The possible applications for ER- or MR-materials are numerous. For example clutches, brakes, dampers and shock absorbers have been suggested for both ER- and MR-fluids (Weiss & Carlson, 1993; Jolly et al, 1999). Commercial products today are a seat damper for heavy vehicles and a compact smooth acting brake for exercise bikes, both using MR-fluids. Both products are sold by Lord Corporation. Lord is also the only commercial supplier of MR-fluids today (www.mrfluid.com).

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Magnetically responsive polymer gels and elastomers: properties, synthesis and applications

M. Zrinyi , in Smart Polymers and their Applications, 2014

5.7 Polymer gels in a non-uniform electric or magnetic field

Electrorheological fluids, magnetorheological fluids and ferrofluids contain dispersed small particles in the size range from nanometres to micrometres ( Jones, 1995; Tao and Roy, 1994) These fluids respond to an applied field by rapidly changing their apparent viscosity and yield stress. Since polymer gels contain substantial amounts of liquid as swelling agent, it is possible to fabricate field sensitive gels by using a polymer network swollen by a complex fluid. The incorporated colloidal particles, characterized by strong adsorptive interactions between solid particles and polymer chains, couple the shape and physical properties of the gel to the external field. These field sensitive gels can be exploited to construct new types of soft actuators, sensors, micromachines, biomimetic energy-transducing devices and controlled delivery systems.

If a field sensitive gel is exposed to an external field, two distinct types of interactions can be identified: field–particle interaction, as well as particle–particle interaction. If the field is non-uniform, then the field–particle interactions are dominant. Particles experience a dielectrophoretic (DEP), or magnetophoretic (MAP) force, respectively. As a result the particles are attracted to regions of stronger field intensities. Because of the cross-linking bridges in the network, changes in molecular conformation due to either DEP or MAP forces can accumulate and lead to macroscopic shape changes and/or motion. The main features of the DEP and MAP forces are summarized in Table 5.1.

Table 5.1. Particle–field interactions in non-uniform fields

Electric field	Magnetic field
Dielectrophoretic force (DEP)	Magnetophoretic force (MAP)
f _DEP = 2πε ₁ R ³ K∇E ₀ ²	f _Map = 2πμ ₁ R ³ K∇H ₀ ²
Permittivities	Permeabilities
$K = \frac{ε_{2} - ε_{1}}{ε_{2} + 2 ε_{1}}$	$K = \frac{μ_{2} - μ_{1}}{μ_{2} + 2 μ_{1}}$
Field gradient	Field gradient
∇E ₀	∇H ₀
Low energy consumption	Significant energy consumption
Dangerous	Safe

Note: R = radius of solid particles, ε and μ = respective permittivity and permeability; the index 2 refers to the colloidal particle; the index 1 denotes the swelling agent.

The field sensitive gels can be made to repeatedly bend and straighten, as well as elongate and contract many times without damaging the gel (Fehér et al., 2001; Zrinyi, et al., 2001, 2002). The response time to obtain the new equilibrium shape was found to be less than a second and seems to be independent of the size of the gel. This is demonstrated for magnetic field sensitive as well as electric field responsive gels in Figs 5.15 and 5.22. This latter figure shows that TiO₂ -loaded PDMS gel cylinder, suspended into silicon oil, undergoes significant bending deformation when an external electric field is applied. One of the electrodes is a metal ball with a diameter of 5.5 mm, the other electrode is a copper plate. By applying a DC field, the gel cylinder bends toward the metal ball. It is obvious from the pictures that the non-uniform field induces bending of the gel. A large deflection has been observed due to the DEP forces. It is also important to mention that the bending is rapid and the final equilibrium shape is reached within 5 s depending on the viscosity of the silicon oil and the size of the gel.

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Magneto rheological Fluid to Enable Continuous Control