Abstract
The paper presents the results of research on the rheological properties of magnetorheological (MR) fluids with different physical and chemical compositions. The research concerned complete compositions of MR fluids as well as base fluids (samples containing no particles with ferromagnetic properties). Magnetorheological fluids consisted mainly of a carrier liquid in the form of glycerin or OKS 352 silicone oil, with magnetite or iron pentacarbonyl particles. The paper demonstrates the effect of the selection of individual components of the MR fluids on their rheological properties, paying attention to the interaction of the base liquid with the particles of the solid phase of the MR fluid. Under the conditions of exposure to a magnetic field, the type of ferromagnetic particles (their magnetic properties) have a decisive impact on the magnetorheological response of the fluid (change in rheological properties due to the effect of the magnetic field on the fluid). The magnetite-based samples showed a tangential stress of 1000 Pa, while the use of iron carbonyl allowed to obtain approximately 8000 Pa (with respect to the case where B = 500 mT,
Keywords
1. Introduction
In the last few decades, numerous research studies on the development of groups of substances whose physical properties can be changed by external stimuli such as magnetic or electric fields, mechanical stress, temperature, heat, and light have been carried out in the world (Zareie and Zabihollah, 2020). These are the so-called functional “intelligent” or “smart” materials, the main advantage of which is that changes in their properties can be controlled according to a predetermined plan. As reported by Carlson and Jolly (2000), the most famous magnetorheological materials include primarily magnetorheological liquids, elastomers, as well as gels and foams. These materials, due to their specific properties, enable them to perform certain functions, therefore, over the years, they have become objects of interest not only in terms of application, but also from the cognitive point of view, and the devices taking advantage of their specific features have been continuously an object of research and development work all over the world (Kang et al., 2020).
As reported by Fonseca et al. (2017), two types of magnetic liquids can be distinguished depending on the particle size: ferrofluids, produced on the basis of particles with a diameter of several nanometers, and magnetorheological liquids, which contain particles of several micrometers in size. Molecules of ferromagnetic liquids contain single magnetic domains, therefore, their behavior is significantly different from that of magnetorheological liquids, and this is directly related to their use in various technical applications. In mechanical devices, ferrofluids are used primarily in the construction of sealing structures, but as described by Aruna et al. (2020) and Devol and Kumar (2019), magnetorheological fluids are used in vibration dampers, brakes, and clutches. The range of changes in the properties of magnetorheological liquids is dependent on many parameters, including the volumetric share of magnetic particles in the liquid, the size, shape, and magnetic properties of the particles, the type and quantity of stabilizers and additives used, as well as the value of the used magnetic field. In their work, Virpura et al. (2014) prepared a liquid using magnetic nanoparticles (∼10 nm) and micron-sized magnetic particles (∼10 μm). The authors observed a significant difference in the formation of magnetically induced structures in both fluids, which they consider to be mainly due to magnetically induced dipolar interaction, field-induced viscosity, and surface tension in both fluids.
According to Rudrajit and Utkarsh (2019) and Xie et al. (2020), typical magnetorheological liquids are suspensions of micron-sized, magnetizable particles, among others, of iron, iron oxide, carbonyl iron, nickel, cobalt, and a combination thereof, suspended in a suitable carrier fluid such as mineral oil, synthetic oil, ethylene glycol, or water. As presented in their work by Kumar et al. (2019) and Rudrajit and Utkarsh (2019), these particles can have different shapes: from irregular to spherical or close to spherical, as well as different sizes from 0.1 to 10 μm. An additional component of magnetorheological liquids are the properly selected stabilizers that counteract the processes of gravitational sedimentation of magnetic particles and their agglomeration. In the paper by Unuh and Muhamad (2020), among the well-known stabilizers of magnetorheological liquids, the authors mention thixotropic agents, including colloidal silica, inorganic clays, carbon fibers, as well as surfactants. Narwade et al. (2020) and Ronzova et al. (2021) in their publications draw attention to the fact that in order to meet the requirements of many different engineering and biomedical applications, the structure and properties of magnetically controlled liquids, both ferrofluids and magnetorheological liquids, can be precisely adjusted, through appropriate selection of components and applied synthesis procedures. The role of appropriate additives and stabilizers in the composition of magnetorheological liquids was emphasized by López-López et al. (2006), who investigated the effect of three different additives, oleic acid, aluminum stearate, and silica nanoparticles on aggregation and sedimentation and found that silica behaves as a gel-forming agent that can prevent particles sedimentation in the liquid.
Currently, there is no universal theory describing the phenomena occurring in magnetorheological liquids. However, as stated by Raju and Varma (2017) in their publication, there is a general view that the change in mechanical properties is caused by the orientation of magnetic particles along the force lines of the magnetic field and the formation of an anisotropic structure of particle aggregates in the form of paths or columns. Such orientation of the structure of the mixture in the magnetic field induces an increase in viscosity and shear strength.
An explanation for the action of a constant magnetic field on liquids before polymerization of a material based on polylactide (PLA) and epoxy resin (Epidian 5) is described in the paper by Miękoś et al. (2021). The composites contained admixtures in the form of magnetite (Fe3O4) and crystalline cellulose (Avicel PH-1010) in the amount of 10%, 20%, and 30% by mass. Changes in physicochemical properties under the influence of a constant magnetic field were observed during the tests of tensile strength, bending, impact strength, water absorption, frost resistance, chemical resistance to acids and bases as well as in microscopic investigations using the SEM method.
The properties of magnetorheological fluids allow for a wide range of their applications in various fields of science, technology, transport, and in other industries, as well as in medicine. The special features of magnetorheological liquids make it possible to use them primarily in various applications designed to control and dampen vibrations. Examples of their practical use are described in the works by Aruna et al. (2020), Lampaert et al. (2020), Salunkhe and Thikane (2017), and Utami et al. (2018) and include shock absorbers controlling vibrations in vehicles, rotary brakes, shock absorbers used to control seismic movements in buildings and bridges, magnetic polishers, and special purpose devices such as prostheses and other devices supporting the rehabilitation process. Depending on the design of the device, the magnetic liquid can work in one of several modes of operation, that is, shearing or pressure mode (Walke et al., 2019), compression mode (Horak, 2018), and the so-called pinch mode (Gołdasz and Sapiński, 2017). Depending on the method of flow of the MR liquid, different rheological parameters of the liquid are expected in the particular modes of operation. The development of MR liquids with strictly dedicated properties is still a scientific challenge.
The aim of the research presented in this paper was to investigate the rheological properties of samples of base liquids (without magnetic particles) and magnetorheological liquids containing magnetic particles, both in the magnetic field and without the participation of a magnetic field.
2. Materials and methods
2.1. Composition and method of preparation of magnetorheological fluids samples
The samples for tests were prepared as so-called base fluids, devoid of magnetic particles and samples of magnetorheological fluids of various composition, type of magnetic particles, viscosity of the carrier fluid, type of the used stabilizer, and additives modifying their properties. The materials selected to prepare the samples were purchased from commercial sources and used without further purification or chemical treatment. The manufacturers of chemical reagents were: glycerin (Chempur, Piekary Śląskie, Poland), oil OKS 352 (OKS Spezialschmierstoffe GmbH, Maisach-Gernlinden, Germany), Fe3O4 (magnetite) (Alfa Aesar GmbH, Kandel, Germany), Fe(CO)5 (iron carbonyl) (Libra, Trzebinia, Poland), oleic acid (Chempur, Piekary Śląskie, Poland), SiO2 (Aerosil 200) (Evonik Resource Efficiency GmbH, Hanau, Germany), Al2O3 (Wolem GmbH, Eschwege, Germany), Graphite EG 290 (3D Nano, Krakow, Poland), Mg(OH)2 (Acros Organics, New Jersey, USA), activated carbon (Chempur, Piekary Śląskie, Poland), yellow dextrin (Biomus, Lublin, Poland).
The fluid production procedure involved: measuring the appropriate amounts of the components, introducing magnetic particles into the carrier fluid and mixing thoroughly. A stabilizer and additives were added in small portions to the substance obtained in this way. The samples were stirred for about 30 min until a homogeneous consistency was obtained.
Eight samples of MR fluids were produced, for which CM1 to CM8 symbols were adopted. The composition of the samples is presented in detail in Table 1. Five samples of base fluids were also produced (Table 2), which were marked as follows: CM01-the base for fluids CM1 and CM2, CM03-the base for fluids CM3 and CM4, CM05-the base for fluids CM5 and CM6 and CM07 and CM08, which constituted the base fluids for CM7 and CM8.
Composition of magnetorheological fluids samples with the content of magnetic particles.
Composition of so-called base fluid samples, without the content of magnetic particles.
Some of the chemical components that are part of MR fluids can be characterized as follows: OKS 352 oil—is a synthetic hydrocarbon oil, resistant to high operating temperatures. SiO2 (Aerosil 200)—is a matted colloidal silica, with hydrophilic properties, synthetic, amorphous silicon dioxide with a very fine structure, produced by burning silicon tetrachloride in hydrogen furnaces. EG 290 expanded graphite (also referred to as swelling graphite)—is a graphite modified for the needs of industry, produced from graphite ore in the process of enrichment and refining. One of the many advantages of expanded graphite is its chemical resistance and self-lubrication (it is used to produce lubricants).
The so-called base fluids as well as magnetorheological fluids have been selected so as to contain two different carrier fluids, with different viscosities, two different types of magnetic particles, with different structures, two different types of stabilizers and various additives to change the properties of these fluids, for example graphite (EG 290) for thermal conductivity, Mg(OH)2 for non-flammable properties, activated carbon to CO2 and other impurities capture properties, or yellow dextrin resulting in biodegradable properties of the fluids.
2.2. Apparatus and test methods
Rheological studies were carried out in the MCR 301 rotational rheometer. The rheometer was equipped with a test chamber (MRD-180) enabling measurements in a magnetic field. The research was carried out using measuring geometry of the two parallel plates type. The plate diameter 20 mm, measuring gap height h = 0.5 mm, sample volume of the sample to be tested v = 175 μl.
As part of the conducted rheological research, two types of tests were carried out:
oscillation tests (DMA)—test parameters: deformation 0.001%–1000%, ramp log+5 p/decade, angular velocity 10 1/s, value of the electromagnet supply current: I = 0, 0.3, 0.6, 1.2, 2.4, 3.6 A;
rotational tests with controlled shear rate (CSR)—test parameters: shear rate 0.01–800 1/s, 50 MP, 2s/MP, electromagnet supply current value: I = 0, 0.3, 0.6, 1.2, 2.4, 3.6 A.
The applied current values correspond to magnetic field induction B = 0, 50, 100, 200, 400, or 500 mT. The magnetic field induction threshold values were arbitrarily selected based on preliminary Magneto Sweep tests (linear increase of magnetic field induction in the measuring cell). The experiments were conducted on two test specimens, one of which was prepared using carbonyl iron and the other magnetite particles. This study allowed to determine the range of variation of fluid parameters over a wide range of magnetic induction changes. Figure 1 shows the test result, the green lines indicate the magnetic field induction thresholds as selected to perform the detailed tests. For magnetite-based fluid, almost full saturation of the fluid is achieved already at about 200 mT. However, for MR fluids with carbonyl iron, a continuous increase in shear stress due to an increase in the applied magnetic field induction is observed.
Magneto sweep test.
Prior to each measurement, the test chamber was demagnetized twice in order to remove residual magnetization of the measuring system elements.
The samples that did not contain magnetic particles (so-called base fluids) were tested only under the conditions of the lack of magnetic field. For each of the test samples, at least two repetitions of the test were performed.
The research was divided into three parts. The first part concerned the study of rheological properties of the base samples (CM0x). These were zero samples, to which any further changes in properties were referred. The second part is the study of rheological properties of the samples of magnetorheological fluids containing magnetic particles (CMx) without the exposure to a magnetic field. The third part of the research is the study of rheological properties of the same magnetorheological fluids in a constant magnetic field, with different magnetic induction values.
3. Results and discussion
3.1. Studies of rheological properties of base fluids, not containing magnetic particles
The first part of the research concerned the study of rheological properties of samples not containing ferromagnetic particles. The diagram (Figure 2) shows the flow curves of the tested base fluids.
Flow curves of CM0x base fluids, rotational tests.
For all base fluids, except CM08, the nature of the shear-forced response indicates the Newtonian behavior of the fluids studied. However, for the CM08 sample containing dextrin and activated carbon, significant fluctuations in the recorded tangential stress are visible, probably related to the properties of yellow dextrin.
On the other hand, the graph (Figure 3) shows the viscosity curves of the tested base fluids and in Table 3 the average values of dynamic viscosity for the 10–800 s−1 shear rate range are summarized.
Viscosity curves of base fluids CM0x, rotational tests.
Average dynamic viscosity of the tested CM0x base fluids.
For the CM01, CM03, CM05 fluid samples, good linearity of rheological characteristics was observed, while for the CM07 samples containing EG 290 graphite and Mg(OH)2 as additives, a trend indicating the tendency of the sample to shear dilution is visible (Figure 3). In contrast, the CM08 sample indicates the occurrence of flow disturbances.
The lowest average dynamic viscosity (Table 3) was observed for the sample with OKS 352 oil with the addition of oleic acid (CM03), then for OKS 352 oil stabilized with silica (Aerosil 200), that is, the CM05 sample. As expected, the glycerin-based sample (CM01) showed a viscosity slightly below 1 Pa s. Two or even three times higher viscosity as compared to CM03 was obtained for the CM07 and CM08 fluids, which were based on OKS 352 oil, but contained additional solid particles.
The results of oscillation tests of the studied base fluids are presented in the graph (Figure 4). Using the Cox-Merz principle, the flow curves obtained from rotational and oscillation tests were compiled. In addition, the thumbnail of the chart shows a fragment of curves in a linear coordinate system.
Flow curves of base fluids CM0x. Continuous line—oscillating tests, dashed line—rotational tests.
Over the entire shear rate range, the samples showed good linearity (visible stress fluctuations within the lower shear rate range are probably due to the accuracy of the measurement at very low deformation rates).
Among the tested base fluids, CM07 and CM08 samples are distinguishable, for which the use of two different test methods resulted in significant differences in the obtained results. The above is visible within the range of low shear rates (up to approximately 10 s−1). In this respect, in rotational measurements, significantly higher stress values were obtained for these two base fluids. Such differences do not occur for the fluids containing no additives. This may indicate that the flow of samples is facilitated in the event that the forcing is of the nature of oscillatory movements with increasing amplitude.
The results of oscillatory testing of the base fluids are shown in Figure 5. For all samples, the loss modulus (G″) is approximately constant and has little strain dependence. This is the expected behavior for typical viscous samples.
Loss (G″) and elasticity (G′) modulus of CM0x samples.
For three samples CM07, CM08, and a limited range of, CM05 it was possible to register the presence of an elastic modulus (G′). This is most likely due to the addition of silica, which may have resulted in the formation of a fixed structure in the base fluid. However, the value of this parameter decreased rapidly with increasing strain. In the analyzed range of forcing, the occurrence of LVE region was not observed which indicates the low stability of the structure of the studied base fluid samples.
It should be noted that the observed values of G′ are quite low. In the case of the CM05 base fluid, with G′ values on the order of 0.2 Pa, it was difficult to capture the value of this parameter over the entire range of strains. Only for sample CM08 relatively high values of elastic modulus were obtained, especially in the low strain range, which can be interpreted as an effect of the dextrin particles addition.
3.2. Studies of rheological properties of magnetorheological fluids containing magnetic particles, without exposure to a magnetic field
The second part of the study concerned fluids containing magnetic particles and the tests were performed in an environment without the action of a magnetic field. The flow curves of the tested MR fluids are shown in the diagram (Figures 6 and 7). Whereas the behavior typical of Newtonian fluids was observed for base fluids, for MR fluids produced on their basis the obtained flow curves indicate properties typical of shear-diluted fluids.
Flow curves of the tested MR fluids, rotational tests (B = 0 mT).
Flow curves of the MR fluids tested, oscillating tests (B = 0 mT): (a) log-log coordinate system and (b) lin-lin coordinate system.
There are very clear differences between CM1 and CM2, given that the same base fluid was used to produce them, but they differ in the magnetic particles used. Differences resulting from the physical and chemical properties of magnetic particles may be the basis for the explanation of discrepancies in the results of rheological tests, and also affect the form and consistency of the CM 1 and CM2 samples. In this case, a number of factors must be taken into account. The carrier liquid in the form of glycerin is very hygroscopic, that is, absorbing water. This is shown in the diagram. Through mild oxidation of glycerin, even with oxygen from the air, glycerin aldehyde or glycerin acid is formed. The density increases from 1.26 to 1.45 g/cm3. Oxides, and therefore also magnetite, can be catalysts for the glycerin oxidation reaction. This, of course, applies to all glycerin samples, both magnetite and iron pentacarbonyl. Increasing the content of water absorbed by glycerin will be, however, important as far as magnetite is concerned. Magnetite (Fe3O4) is an inorganic chemical compound from the group of oxides, in which iron is present at +2 and +3 oxidation state. It is a compound that is prone to absorbing water (in the reaction of laboratory production it has four molecules of water in its structure), even that absorbed by glycerin. The literature premises indicate that magnetite in the glycerin environment can pass into hematite (with lower magnetization) and iron oxide (a phase transition occurs). Both of these compounds can form iron hydroxides with water at Fe(II) and Fe(III) oxidation state. Hydroxide are solids with a density of ca. 3.4 g/cm3, insoluble in water, which can further increase the density of magnetorheological fluid. In contrast, iron pentacarbonyl [Fe(CO)5] is an inorganic chemical compound from the group of carbonyls, with iron at 0 oxidation state. It is a complex characterized by a relatively low reactivity. In the 13C-NMR spectra of iron pentacarbonyl, two signals from carbonyl carbon atoms could be expected, while only one signal is visible in the spectra obtained at room temperature. This is explained by the phenomenon of very fast change of the position of axial and equatorial ligands according to the pseudorotation mechanism, as a result of which a single, averaged signal is recorded on the NMR spectrum. The phenomenon of pseudorotation further stabilizes this compound. In addition to the differences in the chemical properties of magnetite and pentacarbonyl iron, the difference in their density is of great importance: Fe3O4 (d = 5.2 g/cm3), Fe(CO)5 (d = 1.46 g/cm3) and the difference in their diameters: Fe3O4 (<45 μm), Fe(CO)5 (5–6.5 μm). Both of these parameters result in a more dense mass of the CM1 sample and a perceptible friction between the particles. The CM2 sample was less viscous and characterized by a more rapid sedimentation of magnetic particles.
It is noteworthy that the flow curve of the CM01 base fluid lies below CM07 and CM08, whereas the CM1 fluid produced on its basis shows by far the highest values of the flow limit. The CM8 sample, like the CM08 base fluid, shows significant fluctuations in tangential stresses. These changes are revealed only in the case of rotational tests, in particular for shear rates above 200 s−1. Such behavior is due to the presence of yellow dextrin in the tested samples. The diagram (Figure 7) shows the flow curves obtained as a result of oscillatory tests. Within the range up to 0.1, and in the case of the CM1 fluid up to 0.01 s−1, the graph of the stress dependence on the shear rate of the tested samples is linear.
Diagrams of the modulus of elasticity (G′) of the tested MR fluids are shown in the figure (Figure 8).
Modulus of elasticity of the tested MR fluids (B = 0 mT).
For all tested MR fluids at the zero state the significant storage modulus values are observed, although for the base fluids (without magnetic particles) this parameter was visible only for two fluids that is, CM07 and CM08 (compare with Figure 5). Thus, it can be seen that the addition of magnetic microparticles significantly changes the structure and properties of the fluids. For the sample CM1, high values of G′ (above 3800 Pa) were obtained, which is not the expected value, especially since pure glycerin does not show the occurrence of this parameter at all. Similarly, for the CM01 base fluid, no elasticity of the sample was observed. The lowest G′ values are visible in CM3 (max. 7 Pa) and CM5 (max. 14 Pa). The difference in the composition of these samples was the use of silica in the CM5, instead of oleic acid (CM3), which may be important due to the formation of the internal structure in the silica-containing base fluid.
There is a significant difference between the interaction of magnetite and carbonyl iron particles with the base fluid. For all samples with magnetite significantly lower G′ values are observed, it is 20× lower G′ sample CM5 versus CM6 and CM2 versus CM1 and even 34× lower for CM3 versus CM4. These differences are smaller if other additives besides magnetic particles are added in the MR fluid composition (for CM7 and CM8 storage modulus is only 3× lower than CM6).
The samples with the most complex composition (CM7 and CM8) are also distinguished by the least stability in terms of LVE range. Even at a strain of 0.01%, a monotonic decrease in G′ is noticeable. This indicates that the addition of graphite and dextrin caused the weakening of the MR fluid internal structure.
The diagram (Figure 9) shows the damping factors of the tested MR fluids.
The damping factor (tan δ) of the studied MR fluids (B = 0 mT).
The nature of the changes in the properties of all samples is similar. At low deformations, viscoelastic properties prevail in the performance of the tested samples, changing with increasing deformation toward “perfectly” viscous (tan δ > 1). As in the case of the measurement results of the elasticity modulus, the CM1 fluids stand out among the results of this measurement as the samples with the lowest values of the damping coefficient, in contrast to CM3 and CM5, for which the values of this parameter were the highest. To the CM8 sample, which at higher deformations shows an accelerated increase in the analyzed parameter, is also noteworthy. This may indicate a tendency of the sample to increased energy dissipation at higher shear rates. This observation is consistent with the results of previous measurements.
3.3. Studies of rheological properties of magnetorheological fluids containing magnetic particles in a constant magnetic field of different magnetic induction values
In the third part of the study, the rheological properties of the same MR fluids were checked under exposure to a constant magnetic field, with different magnetic induction values. The diagram (Figure 10) compares the flow curves of the studied magnetorheological fluids obtained in the environment of a constant magnetic field.
Flow curves of the tested MR fluids: (a) B = 0 mT, (b) B = 50 mT, (c) B = 100 mT, (d) B = 200 mT, (e) B = 400 mT, and (f) B = 500 mT.
There is a significant difference in the performance of the tested fluids due to the magnetic particle material used. The magnetite-based samples (CM1, CM4, and CM6) are saturated already at the magnetic field induction B = 200 mT. There is also an important difference between the behavior of the CM1 sample in relation to CM4 and CM6. The glycerin-based sample (CM1) shows proportionally constant and higher tangential stress values. The performance differences between the CM1 and CM2 samples visible in Figure 10 confirm the previous observation and are consistent with the previously proposed hypothesis regarding the interaction of glycerin with the particles of various types of ferromagnetic powders.
Starting from B = 400 mT, the highest tangential stress values are observed for CM2 fluids. This may indicate a higher saturation magnetization value of the iron carbonyl powder used. This hypothesis is confirmed by the results of the tests presented in the graph (Figure 11), on which higher values of tangential stress can be noted also at very low shear rates.
Flow curves of tested MR fluids—oscillating tests: (a) B = 0 mT, (b) B = 50 mT, (c) B = 100 mT, (d) B = 200 mT, (e) B = 400 mT, and (f) B = 500 mT.
Attention should also be paid to the behavior of the CM7 fluid, which stands out from other samples containing iron pentacarbonyl since lower tangential stress values are observed for it. This is particularly evident at higher magnetic field induction values (Figure 11(d)–(f)). Such behavior can be explained by the effect of the addition of graphite on the internal friction in the fluid. The addition of graphite reduced friction between iron pentacarbonyl molecules due to the physical and chemical properties of the added graphite. The graphite used for the CM7 sample is expanded graphite EG 290, with a density of 1.46 g/cm3 (bulk density 0.64 g/cm3 only) and granulation of 200–600 μm. The density is identical with that of iron pentacarbonyl (1.46 g/cm3), of 5–6.5 μm granulation. Expanded graphite (also called swelling graphite) is a graphite modified for industrial purposes. The modified graphite (EG) is produced from graphite ore in the process of enrichment and refining. The graphite purified in this way is then treated with very strong oxidants—transformed into dry graphite acid. Reheating of the graphite leads to the release of intercalate, rupture of interplant bonds, and exfoliation of graphite crystals. As a result of this transformation, graphite undergoes volumetric expansion, turning into a soft, flexible, and insulating material—the so-called expanded graphite. Expanded graphite has a fluffy consistency (specific surface of 100 m2/g) and is one of the most delicate minerals. One of the many advantages of expanded graphite is its chemical resistance and self-lubrication (it is used in the production of lubricants). Lubricant manufacturers claim that the higher the granulation of expanded graphite, the lower the friction coefficient and the better the lubrication performance. The granulation of graphite prevails over the granulation of iron pentacarbonyl, so the magnetorheological fluid takes on some features of expanded, soft, fluffy, elastic graphite, causing less friction in the fluid. The diagrams (Figure 11) compile the flow curves obtained from dynamic tests.
Within the range of low magnetic field induction values, the highest stress values were observed for the CM1 and CM2 samples. Most likely, due to the low shear rates and the oscillating nature of the forcing, the previously observed stress fluctuations for the CM8 sample and other significant differences in the behavior of individual samples were not recorded in the case of dynamic tests.
4. Conclusions
The paper discusses the results of eight MR fluids compositions and five base fluids tests. The fluids were prepared on the basis of glycerin, synthetic oil, and several stabilizing additives. The influence of individual components on the behavior of the compositions was evaluated.
Differences in the properties of glycerin-based MR fluids have been demonstrated due to the use of magnetite and iron carbonyl as a magnetically active material. The sample containing magnetite exhibits higher stresses in the zero state, whereas the MR fluid produced with carbonyl iron allows to obtain high stress values in the case of magnetic field application, while maintaining low viscosity in the zero state.
It has been demonstrated that the addition of graphite to MR fluids may result in a reduction in shear stress. The phenomenon is observed at relatively high magnetic field induction.
The addition of activated carbon and dextrin to the base sample (CM08) and the MR fluid induces the non-Newtonian behavior of the fluid. In this case, significant deviations from the linearity of the flow curves determined in the zero state were observed.
For all base fluids containing silica (CM05, CM07, and CM08), the presence of an storage modulus at zero magnetic field was observed. The small differences of this parameter was recorded between CM05 and CM07 (containing additional graphite), while the application of dextrin (CM08) in the base fluid resulted in a several dozen times increase in this parameter.
Under magnetic field conditions, the type of ferromagnetic powder (its magnetic properties) has a decisive influence on change MR fluid rheological properties. The carbonyl iron-based samples exhibited up to eight times higher tangential stresses (at a magnetic induction of 500 mT).
Footnotes
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The paper was prepared as part of the project: “InterChemMed – Interdisciplinary doctoral studies of Lodz public universities” conducted at the Lodz University of Technology, the University of Lodz and the Medical University of Lodz, implemented under the Operational Programme Knowledge Education Development 2014–2020, co-financed by the European Social Fund (No POWR.03.02.00-00-I029/16).
