Magnetorheological Fluids (MRF) - Engineering Seminar

INTRODUCTION

Some materials have the ability to change shape or size simply by adding a little bit of heat, or to change from a liquid to a solid almost instantly when near a magnet; these materials are called smart materials.

Magnetorheological fluids (MRF) are a class of smart materials whose rheological properties (e.g. viscosity) may be rapidly varied by applying a magnetic field. Under influence of magnetic field the suspended magnetic particles interact to form a structure that resists shear deformation or flow. This change in the material appears as a rapid increase in apparent viscosity or in the development of a semisolid state. Advances in the application of MR fluids are parallel to the development of new, more sophisticated MR materials with better properties and stability. Many smart systems and structures would benefit from the change in viscosity or other material properties of MRF

A Magnetorheological Fluid (MRF) is a colloidal suspension that has the ability to change phase between liquid and solid in an applied magnetic field. These fluids exhibit Newtonian-like behavior in the absence of a magnetic field, but become a weak viscoelastic solid with a certain yield stress in an applied field. . Nowadays, these applicationsinclude brakes, dampers, clutches and shock absorbers systems.

MAGNETORHEOLOGICAL FLUID

Magnetizable particles, ranging from 0.3 to 10 m in diameter, suspended in a liquid will aggregate to form chains when an initially random dispersion is placed in a uniform, static magnetic field. The formation of these fibrous structures changes the bulk rheological properties of the medium and as such these suspensions are typically referred to as magnetorheologicalfluids (MR). MR fluids have been employed in a variety of damping and shock absorbing devices where the particle volume fractions for the suspensions range from 20% to 40%.

                     MR fluid is a suspension of magnetically soft micron-sized particles in a carrier liquid. MR fluids demonstrate an ability to change their viscosity in milliseconds by several orders of magnitude under the influence of an applied magnetic field. There are many factors that influence the rheological properties of controllable MR fluids such as concentration and density of particles, particle size and shape distribution, properties of the carrier fluid, additional additives, applied field and temperature.

                   Excellent MR fluids must have low viscosity and coercivity of particles without the influence of an external magnetic field and can achieve maximum yield stress in the presence of the external magnetic field.The essential characteristic of thesematerials is that they can be rapidlyand reversiblyvaried from the state of a Newtonian-like fluid to that of a stiff semisolid with the application of a moderate magnetic field. This feature, called the MR effect.

WORKING

The most common MR materials are of liquid state. The controllable rheological response of such fluids results from the polarization induced in the suspended particles by application of an external magnetic field. The interaction between the resulting induced dipoles causes the particles to form columnar structures, parallel to the applied field. These chain-like structures restrict the flow of the fluid, thereby increasing the viscous characteristics of the suspension. The mechanical energy needed to yield these chain-like structures increases as the applied magnetic field increases resulting in a field dependent yield stress. In the absence of an applied field, the controllable fluids exhibit Newtonian-like behavior.

PROPERTIES OF MR FLUIDS

               Typical magnetorheological fluids are the suspensions of micron sized, magnetizable particles (mainly iron) suspended in an appropriate carrier liquid such as mineral oil, synthetic oil, water or ethylene glycol. The carrier fluid serves as a dispersed medium and ensures the homogeneity of particles in the fluid. A variety of additives (stabilizers and surfactants) are used to prevent gravitational settling and promote stable particles suspension, enhance lubricity and change initial viscosity of the MR fluids. The stabilizers serve to keep the particles suspended in the fluid, whilst the surfactants are adsorbed on the surface of the magnetic particles to enhance the polarization induced in the suspended particles upon the application of a magnetic field.

FACTORS AFFECTING PROPERTIES

 Severalfactors contribute to the rheological properties of MR fluids. Such factors includeconcentration and density of particles, particle size and shape distribution, properties ofthe carrier fluid, additional additives, applied field, temperature, and others.

PARTICLE SIZE

The particle size should be meticulously selected, so that it can exhibit multi-domain characteristics when subjected to an external magnetic field. MR particles are typically in the range of 0.1 to 10 μm , which are about 1000 times bigger than those particles in the ferrofluids . In the MR fluids, magnetic particles within a certain size distribution can give a maximum volume fraction without causing unacceptable increasing in zero-field viscosity. For instance, fluid composition that consists of 50 % volume of carbonyl iron powder was used in the application of electromechanically controllable torque-applying device. Moreover, bigger particle sizes give more yield stress where they consume more magnetization than fine particles. A combination of small and big particles (bimodal) strongly affects the viscosity and can be controlled within a wide range by controlling the respective fractions of the small and large particles in the bimodal size distribution families.

CARRIER FLUID

The carrier liquid forms the continuous phase of the MR fluids. Examples of appropriate fluids include silicone oils, mineral oils, paraffin oils, silicone copolymers, white oils, hydraulic oils, transformer oils, halogenated organic liquids, diesters, polyoxyalkylenes, fluorinated silicones, cyanoalkylsiloxanes, glycols, water and synthetic hydrocarbon oils . A combination of these fluids may also be used as the carrier component of the MR fluids. Carrier liquids are typically chosen based upon their rheological andtribological properties and on their temperature stability. Additives are used to provide additional lubricating properties, as well as additivesthat inhibit sedimentation and agglomeration. Sedimentation is typically controlled by the use ofthixotropic agents and surfactants such as xantham gum, silica gel, stearates and carboxylic acids.

  

MAGNETIC MATERIAL FOR MR FLUIDS

In MR fluids, materials with lowest coercivity and highest saturation magnetization are preferred, because as soon as the field is taken off, the MR fluid should come to its demagnetized state in milliseconds. Due to its low coercivity and high saturation magnetization, high purity carbonyl iron powder appears to be the main magnetic phase of most practical MR fluid compositions. This is because carbonyl iron is chemically pure and the particles are mesoscale and spherical in nature in order to eliminate the shape anisotropy. The meso-scale particles are necessary since they have many magnetic domains. The high level of chemical purity (~ >99.7%) means less domain pinning defects. The spherical shape helps minimize magnetic shape anisotropy. The impurities that cause magnetic hardness in metals also cause mechanical hardness, due to resistance to dislocation motion, and make the iron particles mechanically harder. In MR fluid based devices it is preferred to have particles that are non-abrasive. This is another reason why spherical, high purity iron powders are more appropriate for applications as a dispersed phase in MR fluids. Thus, carbonyl iron is chosen because of its high saturation magnetization (~2.1 Tesla, at room temperature) and magnetic softness.

                Among other soft magnetic materials Fe-Co alloys (composition 50 wt%Fe) have a

saturation magnetization of ~2.43 T . Although some researchers reported an enhanced yield

stress for Fe-Co based fluid , the settling problem of the fluid will be aggravated due to the

higher bulk density (8.1 gr/cc) than that of Fe (7.8 gr/cc). Also the cost of these alloys makes

them undesirable for MR fluids.

               MR fluids have been prepared based ferrimagnetic materials such as manganese-zinc ferriteand nickel zinc ferrite of an average size of 2 µm. The saturation magnetization of ceramicferrites is relatively low (~0.4-0.6 T) and therefore the yield stresses also tend to be smaller.

             Iron powder magnet can be prepared by hydrogen reduction of ferric oxide or by Chemical Vapour Deposition (CVD) from iron pentacarbonylFe(CO)_5. Once the particles are magnetized, the oriented domains can grow with the magnetization persisted and simultaneously increased permeability. Saturation magnetization of the iron can be obtained when all of the domains are properly oriented.The domain walls can easily move, ideally making the magnetization a single-valued function of the magnetizing field, so that there is no hysteresis loss when the field reverses repeatedly.

The Volume Fraction and Particle Size Dependence of Viscosity

           At high volume fractions, the particles are close enough to each other that the flow field of one particle is affected by the neighbors. Thus the particles are said to experience hydrodynamic interactions. At a concentration of about 50%, a rapid increase in the viscosity is noticeable. The loose packing of uniform spheres assuming simple cubic packing, corresponds to 52% by volume. At this concentration, the friction due to particle interactions would become a significant factor and the resistance to shear seems to cause a rapid increase in viscosity.

                    For high concentration of particles, the particle size distribution (PSD) has a strong effect on viscosity as well as the particle shape and surface roughness. The packing of the particles can be affected by mixing two different size spheres or by using a broad continuous particle size distribution. The smaller particles act as ball bearings among the bigger particles which gives rise to a decrease in the viscosity.

 PARTICLE SEDIMENTION AND SUSPENSION STABILITY

Sedimentation stability refers to gravitational stability and ensures that the particles do not settle over time, while agglomerative stability prevents the particles from sticking together in the absence of the field.

In order to improve the stability of MR fluids against sedimentation various solutions were proposed concerning their composition and the characteristic size of the magnetic component.

Colloidal MR fluids were synthesized using ferrite-based particles of the order of 30 nm in diametercoated with long chain molecules. These fluids, which are very similar to ferrofluids, arereported to have excellent stability and abrasion properties. They, however, exhibit an order ofmagnitude less yield stress than the usual iron-based MR fluids resulting from inferior magneticproperties of ferrite and the predominance of thermal particle forces.

Bidisperse MR fluidswith nanosize (non-magnetic) filler particles were proposed in order tocircumvent the problem of sedimentation and also to increase the magnetic field induced yield stress. In a recent work bidisperse MR fluids having nanometer (30 nm) and micrometer (30 μm)size iron particles were investigated. The ratio between the kind of powders in thecomposition, keeping constant the mass concentration of iron particles (60% wt), influence therheological behavior and settling properties of the MR fluid.

Water based MR fluids with longterm stability were prepared by adding of soluble polymersthat modify the viscosity of the carrier and adsorb on the particles, giving rise to strong hydrophilicand steric repulsion between micrometric magnetite particles.The most promising type of MR fluid uses a ferrofluid as carrier liquid and micrometer size ironparticles dispersed in the magnetizable liquid matrix,resulting in an extremely bidisperse MRfluid. The increase of yield stress is due to the increased force between two iron particles mediatedby the carrier fluid with non-zero magnetic susceptibility. Also, sedimentation is prevented bymagnetic interactions between nanosized permanent dipoles and multi-domain ferromagneticparticles, resulting in a local alignment and network formation of nanodipoles between micrometer size iron particles.Nanosized particles addition to MR fluids with micrometer size ferro- orferromagnetic particles is an alternative strategy to improve the stability of these suspensions. Thesedimentation behavior of extremely bimodal suspensions, with micrometer (1450 nm) andnanometer (8 nm) size magnetite particles was examined recently.Magnetic nanoparticlesgreatly reduce gravitational settling by increasing the viscosity and density of the medium and bythe so called “halo” structure formation, as a consequence of gathering magnetic nanoparticlesaround the large micrometer range particles due to magnetic and van der Waals attraction.

A new type of magnetorheological fluid was developed based on a carbon nanotube andmagnetite (CNT/Fe3O4) nanocomposite. The MR fluid contains CNTs covered with a layer of softmagnetic magnetite nanoparticles. Due to very high length to diameter ratio of the magneticcomponent a three-dimensional network is formed which prevents settling. A significant furtheradvance in improving the stability and increasing the yield stress of MR fluids is the use ofmagnetic fibers instead of spherical ferromagnetic particles.Cobalt wires and iron filaments(approx. 60 μm in length and 4-16 μm in width) were synthesized and dispersed in silicon oil.

Two methods have been suggested for measuring sedimentation rates in MR fluids: (i) measuring the rate of change of magnetic permeability of the upper layer of the MR fluid to get a measure of the sedimentation velocity of particles and  (ii) by laser light transmission through a column of MR fluid.

    The time to mudline formation was also recorded for each of the fluids. A correlation was observed between the time to mudline formation and the wt% of nanoparticles.

This plot shows that increasing the wt% of nanoparticles in the MR fluid increases the time to mudline formation, showingthat the bidisperse fluids are capable of maintaining the suspension for longer periods of time. Thus, measuring the settling rate of the various bidisperse MR fluid samples led to the conclusion that even replacing only 15–20% of microparticles with nanoparticles drastically improved the homogeneity of the dispersion.

 FLUID MODES OF OPERATION

In general, MR fluid devices use one of the three basic modes of operation of MR fluids or any combination of them depending on the function of the system. These series of actions are known as valve mode, shear mode, and squeeze mode.

Valve Mode

Valve mode, shown schematically in figure 3.6, is one of the operating modes in the MR devices where the flow of the MR fluid between motionless plates or an orifice is created by a pressure drop. The magnetic field, which is applied perpendicular to the direction of the flow, is used to change the viscosity of the MR fluid in order to control the flow. Therefore, the increase in yield stress or viscosity alters the velocity profile of the fluid in the gap between two plates. 

Shear Mode

            The second working mode for controllable fluid devices is the direct shear mode. An MR fluid is situated between two surfaces, whereby only one surface slides or rotates in relation to the other, with a magnetic field applied perpendicularly to the direction of motion of these shear surfaces. Shear mode is useful due to the characteristics of the shear stress versus shear rate which can be controlled by the magnetic field strength

Squeeze Mode

The third working mode of MR fluids is the squeeze mode. This mode has not been widely investigated. Squeeze mode operates when a force is applied to the plates in the same direction of a magnetic field to reduce or expand the distance between the parallel plates causing a squeeze flow. In squeeze mode, the MR fluid is subjected to dynamic (alternate between tension and compression) or static (individual tension or compression) loadings. As the magnetic field charges the particles, the particle chains formed between the walls become rigid with rapid changes in viscosity. The displacements engaged in squeeze mode are relatively very small (few millimetres) but require large forces.

APPLICATIONS AND ADVANCEMENTS

In recent years, manufacturers have shown an increased interest in MR devices.For instance, the Lord Corporation has been developing MR fluid and manufacturing MR truck seat dampers for a number of years now. These seat dampers are retrofits that replace hydraulic seat dampers that are original equipment on many large commercial trucks. Lord Corporation’s truck seat dampers are arguably the most successful commercial MR dampers to date.In addition to truck seat dampers, other commercial MR dampers will be available in the near future. General Motors, for instance, has announced that an MR damper suspension system will be available on certain 2003 Cadillac models.MR dampers are not restricted, however, to vehicle applications. Recently, the military has shown interest in using MR dampers to control gun recoil on Naval gun turrets and field artillery. Another area of study that has incorporated MR dampers is the stabilization of buildings during earthquakes. This increase in commercial interest is largely due to the success of research projects and through

the efforts of Lord Corporation, which is a leader in the field of MR devices.

MR DAMPER

          A magnetorheological damper or magnetorheological shock absorber is a damper filled with magnetorheological fluid, which is controlled by a magnetic field, usually using an electromagnet. This allows the damping characteristics of the shock absorber to be continuously controlled by varying the power of the electromagnet. This type of shock absorber has several applications, most notably in semi-active vehicle suspensions which may adapt to road conditions, as they are monitored through sensors in the vehicle, and in prosthetic limbs.

Another class of MR applications exploits the torque transfer capabilities of these materials when placed between concentric cylinders or parallel disks. Large dampers may be utilized to reduce motion in such structures as buildings and bridges, for example, to damp vibration caused by earthquakes or wind.

WITHOUT MR SUSPENSIONWITH MR SUSPENSION 

During the past few years a number of commercially available products (or near commercialization) have been developed :

􀁸 linear MR dampers for real-time active vibrational control systems in heavy duty trucks,

􀁸 rotary brakes to provide tactile force-feedback in steer-bywire systems,

􀁸linear dampers for real-time gait control in advanced prosthetic devices,

􀁸 adjustable real-time controlled shock absorbers for automobiles,

􀁸 MR sponge dampers for washing machines,

􀁸magnetorheological fluid polishing tools,

􀁸 very large MR fluid dampers for seismic damage mitigation in civil engineering structures,

􀁸large MR fluid dampers to control wind-induced vibrations in cable-stayed bridges.

CONCLUSION

The technology of materials with field responsive rheology is currently enjoying renewed interest within the technical community in terms of fundamental and applied research. Research efforts of the past decade in field responsive materialsare beginning to pay of. There are now several commercial MR fluids available. Recently, MR fluid-based devices have enjoyed commercialization within theexercise industry and transportation industry. The emergence of new applicationsfor controllable materials and the ongoing commercialization of both materialsand devices provide an impetus for continued research in this area.