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An experimental study on the effect of fatty acid chain length on the magnetorheological fluid stabilization and rheological properties
Colloids and Surfaces A: Physicochem. Eng. Aspects 469 (2015) 29–35
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
An experimental study on the effect of fatty acid chain length on the magnetorheological fluid stabilization and rheological properties M. Ashtiani, S.H. Hashemabadi ∗ CFD Research Laboratory, School of Chemical Engineering, Iran University of Science and Technology, 16846 Tehran, Iran
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Improving MRF stability and MR effect by using stearic acid additive.
• Fatty acids with longer carbon chain as additive creates stronger gel structure. • Stronger gel structure improves stability and yield stress, consequently more efficient MRF.
a r t i c l e
i n f o
Article history: Received 9 November 2014 Received in revised form 21 December 2014 Accepted 22 December 2014 Available online 3 January 2015 Keywords: Carbonyl iron Fatty acid Herschel–Bulkley model Magnetorheological fluid Stabilization
a b s t r a c t An experimental study was conducted on the effect of acid additives on the stability and rheological properties of a suspension of carbonyl iron (CI) microparticles dispersed in silicone oil. A series of acids with the same carboxyl group but different carbon chain lengths (C12, C14, C16 and C18) were added to magnetorheological fluids (MRFs) to investigate their effect on the stability and rheological behavior. MR effect was measured with the aid of a magnetorheometer and stability measurements were made by simple determining the height of the transparent layer in the MRF over a 6-month period. Experimental results showed that by increasing carbon chain length of acids, yield stress and stability increased up to 22 times (at H = 362 kA/m) and 7 times, respectively, in comparison to the additive-free MRF. Model fitting confirmed that all of the acid-based MRFs showed a yield stress with shear thinning behavior and followed Herschel–Bulkley model. Further investigations suggested that 3 wt% stearic acid was the most promising additive in increasing MR effect and stability. The results of stability and rheological tests showed that further increase in stearic acid fraction improved stability slightly and, at the same time, increased the off-state viscosity to a huge extent undesirably. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Magnetorheological fluids (MRFs) are a class of smart materials whose rheological properties vary considerably in the presence of an external magnetic field [1]. The viscosity of these materials can increase to a huge extent when they are exposed to a magnetic field. MRFs consist of three main parts; a continuous phase, magnetizable
∗ Corresponding author. Tel.: +98 21 7724 0376; fax: +98 21 7724 0495. E-mail address: [email protected] (S.H. Hashemabadi). http://dx.doi.org/10.1016/j.colsurfa.2014.12.046 0927-7757/© 2014 Elsevier B.V. All rights reserved.
particles, and stabilizer additives [2]. When an external magnetic field is applied to an MRF, magnetizable particles build a chain-like structure in the direction of magnetic field lines and restrict the fluid flow. This behavior is recognized by a sudden increase in the fluid viscosity and a yield stress [3]. In the literature, this change in the MRF state, shear stress and viscosity is mostly interpreted as MR effect which plays an important role in MR technologies [4,5]. Magnetorheological fluids are used in a broad range of industrial applications from MR valves in chemical engineering [6,7] to seismic dampers in civil engineering [8,9]. They have attracted great attention in some industries such as automobile shock absorbers
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Nomenclature k n ˛ ˇ ˙ y
Consistency parameter, Pa sn Power law index Field-constant parameter, Pa Field-consistency parameter, Pa sn Shear rate tensor, s−1 Stress tensor, Pa Yield stress, Pa
[10,11]. Two important challenges in MRF-based technologies are their low MR effect and the fluid instability. Like any other dispersed systems, MR suspensions face sedimentation problems. Many studies have attempted to resolve the problem of MRFs instability by using proper additives [12,13], reducing particle size [14] or density [15], synthesizing more elongated particles [16], coating magnetizable particles [17–19], using ionic [20,21] or polymeric liquids [22] as carrier fluids and so on. Unfortunately, most of these methods reduce the MR effect or increase the off-state viscosity of fluid undesirably [23]. In any application, a specific MR effect and stability are needed; so the composition of MRFs for any application must be chosen carefully and consciously [4]. In MR dampers, for example, improving stability is the first and the most important problem and MR effect is of secondary importance. Also in most applications, some limitations restrict the MRF composition; in polishing applications, considering the cooling characteristics of surface and some other parameters, water is the best candidate to be used as the carrier phase in MR suspension preparation [17]. Some researchers like López-López et al. [24] focused on the effect of dispersion on the magnetorheological behavior. They used different surfactants and observed that when particles were dispersed well in the suspension, the change in the viscosity was more significant. They also showed the quality of dispersion had no important effect on the yield stress. However, in the presence of a magnetic field, the change from liquid-state to semi solid-state is more rapid when the particle dispersion is poor. Fang et al. [12] added single-walled carbon nanotube (SWNT) to a suspension of carbonyl iron (CI) particles in lubricant oil. They showed that adding SWNT could improve both MRF stability and the MR effect. They attributed this behavior to the more robust chain-like structure of the SWNT-containing MRF in the presence of a magnetic field. Wang and Gordaninejad [14] compared the rheological properties and apparent viscosity of three suspensions of a commercial MRF, a polymeric gel-based MRF and a ferrofluidbased MRF at high shear rates. They showed that the gel-based MRF enhances yield stress more in comparison to the commercial MRF by somehow increasing the off-state viscosity. The ferrofluidbased MRF showed an MR effect enhancement similar to that of the gel-based ones. However, it resulted in a sharp increase in the off-state viscosity which hampers its usage. Jiang et al. [16] used stearic acid (3 wt% of the mass of CI) as the stabilizer additive in a suspension of iron nanowires and CI spherical microparticles in silicone oil. Cheng et al. [17] coated CI particles with a hydrophilic acid (N-glucose ethylenediamine triacetic acid) and claimed that by dispersing these composite particles in water, stability of the suspension increases up to 25%. In an attempt to synthesize an efficient MRF with long lifetime, Premalatha et al. [13] used grease as the stabilizer additive and observed that by increasing the fraction of this agent up to 0.5 wt%, MRF sedimentation decreased. However, MR effect decreased with the increase in grease weight percentage. Based on the best author’s knowledge, the effect of carbon chain length of fatty acids on the strength of the network formed as a
Table 1 Samples composition. Sample no.
CI particles (wt%)
Silicone oil (wt%)
Additives (wt%)
MRF1 MRF2 MRF3 MRF4 MRF5 MRF6 MRF7 MRF8 MRF9 MRF10 MRF11 MRF12
62 62 62 62 62 72 52 42 62 62 62 62
38 35 35 35 35 28 48 58 36 37 33 28
– Lauric acid (3) Myristic acid (3) Palmitic acid (3) Stearic acid (3) – – – Stearic acid (2) Stearic acid (1) Stearic acid (5) Stearic acid (10)
result of adding additives to MRF has not been investigated thoroughly, yet. There are few researches on the relation between carbon chain length and kinetics of some specific reactions [25]. Aramaki et al. [26] reported that by increasing carbon chain length of alcohols, the viscosity and shear stress of the micellar solution increase. Yu et al. [27] found that by decreasing carbon content of magnetizable particles, MR effect is enhanced. Morrow et al. [28] examined the influence of acyl carbon chain length on the mean orientational order parameter of liquid to gel transition and found a linear relationship between them. The effect of acid carbon chain length on the rate of crystallization [29] and on some other structures [30–33] have also been investigated. In this study with the aim of synthesizing a stable MRF with a promising MR effect, four hydrophobic acids with the same functional group but different numbers of carbon atoms were added to the suspension of carbonyl iron and silicone oil. The rheological behavior of the prepared MRFs was monitored using a well-known rheological model. 2. Materials and methods Spherical carbonyl iron particles (average particle size less than 5 m, density: 7.86 × 103 kg m−3 , CS grade, BASF, Germany) were used as the dispersed phase without further purification. In all samples, magnetizable carbonyl iron particles were dispersed in polydimethylsiloxane (silicone oil, viscosity: 3.50 × 10−4 m2 s−1 , KCC, Korea). Four fatty acids with different carbon chain lengths were added to the MRFs to improve their stability. The additives employed in this study were stearic acid (MIT, Malaysia), myristic, palmitic and lauric acid (MERCK, Germany). In an attempt to determine a composition of MRF which has an efficient MR effect as well as promising stability, four samples with different weight fractions of CI particles were synthesized and examined. For stabilizing the MRFs, different acids were added to the MRF suspensions. The sample compositions with and without additives are presented in Table 1. For synthesizing samples, each acid was added to the base fluid (silicone oil). Each mixture was then stirred and heated at 100 ◦ C for half an hour until a homogeneous solution was obtained. All of the acids used in this study produced a gel-like structure in silicone oil. This structure increases the base fluid density and the MRF stability. Afterwards, magnetizable CI particles were added to the sample which was then stirred at 1000 rpm with the aid of an overhead stirrer (Heidolph, RZR 2102, Germany) for more than half an hour to ensure suspension uniformity. 3. Results and discussion The main focus of this study is on the synthesis of a stable MR suspension. However, if an MRF is stable for more than several months with no efficient MR effect, it is worthless; rheometry
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Fig. 2. Magnetization curves of CI particles.
particles (about 165 emu/g in 8 kOe magnetic field strength) made them the first choice in MRF preparation. 3.2. Stability tests
Fig. 1. SEM image of carbonyl iron particles.
analyses were done for each sample to ensure MRFs efficiency. Yield stress and MR effect can be obtained by investigating the changes in the fluid viscosity and shear stress as a function of shear rate. Lauric acid (dodecanoic acid) which has a 12-carbon atom chain is a white and powdery solid and forms a weak gel-like structure in silicone oil. However, since lauric acid does not increase the off-state viscosity of the fluid greatly, it is one of the promising candidates for being used as an additive in MRF preparation. Myristic acid (tetradecanoic acid) is a common saturated fatty acid with the chemical formula C14 H28 O2 which constructs a stronger gel in silicone oil in comparison to lauric acid. This behavior is due to the longer carbon chain of myristic acid which leads to the involvement of a greater number of carbon atoms in silicone oil. Palmitic acid (hexadecanoic acid) is a fatty acid with white crystals and 16 carbon atoms in its chemical formula. Stearic acid (octadecanoic acid) with the chemical formula C18 H36 O2 is a waxy acid that forms a strong gel-like structure in silicone oil. This acid increases the base fluid density. It, therefore, decreases the density mismatch between the dispersed particles and continuous phase and improves stability. The longer the carbon chain is, the stronger the gel-like structure is [28]. Unfortunately, this strong gel increases the off-state viscosity undesirably. As mentioned above, for analyzing a promising MRF, both stability and MR effect must be considered which will be described in detail in the following sections.
When MRFs remain stationary for a while, heavy magnetizable particles tend to settle and a transparent layer (continuous phase) forms on the top of each sample. Stability measurements were made by detecting the interface between the two phases. For this purpose, each sample was put in a motionless place for more than three months and was photographed at specific intervals. Sedimentation ratio is defined as [17]: Sedimentation ratio (%) =
Volume of supernatant liquid × 100 (1) Volume of total suspension
The height of supernatant liquid and total suspension was determined using an image processing software. Suspension and supernatant volumes were then calculated by multiplying their heights by the cross section area. Fig. 3 shows sedimentation ratio for samples containing different percentages of CI particles. As can be seen in this figure, all of the synthesized MRFs were unstable without additives. Samples with weight fractions of more than 0.62 carbonyl iron particles were more stable. This behavior may be due to the fact that magnetic hysteresis is more in higher CI particles fractions. As a result, particles maintain their chain-like structure even after removing the magnetic field and remain stable for a longer period. According to Fig. 3, no significant effect on the suspensions stability was observed at weight fractions higher
3.1. Characterization of magnetic particles Carbonyl iron (CI) microparticles, which are obtained from thermal decomposition of penta carbonyl iron, have the highest magnetization and are widely used in magnetorheological fluid preparation. Structure and average size of CI particles were measured using a scanning electronic microscope (SEM, MV2300, Tescan, Ltd., Czech Republic) with 15 kV operating voltage. Fig. 1 shows the spherical CI microparticles. The results of image processing showed that the average particle size was 3 m with 1–6 m particle size distribution. Fig. 2 illustrates the magnetorheological behavior of CI particles determined with the aid of a vibrating sample magnetometer (VSM, MDKFD, Magnetic Danesh Pajoh Co. Ltd, Iran). As can be seen in the figure, the magnetic hysteresis loop is very small and the high magnetic saturation of CI
Fig. 3. Stability measurements; samples with different CI weight fractions.
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Fig. 4. Stability measurements; samples containing 62 wt% CI, with and without acid additives.
Fig. 6. Stability measurements; samples with different stearic acid weight fractions.
3.3. Rheometry analysis
than 0.62. In addition, more increase in CI weight fraction leads to undesirable increase in the off-state viscosity, therefore 62 wt% of suspended particles was chosen for the rest of experiments. Fig. 4 depicts sedimentation ratios of samples containing different acids during more than 3 months. It is obvious that stearic acid made the suspension more stable (completely stable for 100 h). The reason for this observation is that stearic acid has a longer carbon chain in comparison to lauric, myristic and palmitic acids, so that it builds a stronger gel-like structure in silicone oil. Fig. 5 shows a schematic image of carbonyl iron particles trapped in the gellike structure of stearic acid-silicone oil. This strong gel traps heavy dispersed particles and restricts their precipitation. As a result, stability improves much. A closer investigation of Figs. 3 and 4 illustrates that using proper additives (stearic acid) is necessary in synthesizing stable MRFs, but the question arises as to what fraction of the additive is to be used. To determine the optimum weight fraction of stearic acid, four samples were synthesized. In all samples, weight fraction of stearic acid was changed but particles weight fraction was kept constant. Fig. 6 shows sedimentation ratio of samples containing different fractions of stearic acid (MRFs 5, and 9–12 in Table 1). As can be seen in the figure, stability improved by increasing fraction of the acid. Samples with at least 5 wt% stearic acid were completely stable over a long period of time. But unfortunately, the off-state viscosity increased to a huge extent (viscosity curves have not been shown here). It seems that 3% steraic acid is an appropriate candidate in preparing stable MRFs which is in agreement with other experiments [16]. As mentioned in the previous section, rheological measurements are needed to choose the best sample which is able to meet rheological and stability requirements of MRF industrial applications. Off-state viscosity, MR effect and yield stress will be discussed in detail in the next section.
Fig. 5. Schematic representations of dispersed particle trapping by gel structure.
The rheological characteristics were examined using a plate/plate rotational rheometer (MCR 300, Anton-Paar Physica, Germany) connected to a magnetorheological equipment (MRD 180, Physica, Germany) which generates a homogeneous magnetic field perpendicular to the fluid flow direction. In this study, a magnetic flux density up to 362 kA/m was used in a gap distance of 1 mm. All of the measurements were taken at 25 ◦ C. Fig. 7 depicts viscosity of the MRF samples containing different acids in the absence of a magnetic field. As can be seen, all of the additives increased the off-state viscosity. The viscosity increase was considerable in the case of palmitic and stearic acids (MRFs 4 and 5 in Table 1). This behavior is due to the much longer carbon chain of these acids which increases the base fluid density as well as the MRF viscosity. To ensure reproducibility of the obtained results, each test was repeated and the average value is reported. Fig. 8 shows shear stress versus shear rate for samples containing different acids in different magnetic field strengths (MRFs 1–5 in Table 1). According to the figure, all of the additives increased the shear stress. None of them, however, was capable of enhancing the shear stress as much as stearic acid did. As can be seen in this figure, stearic acid had a promising effect on improving shear stress as well as yield stress of the MRFs in high magnetic field strength. Closer investigation of Figs. 7 and 8 reveals that lauric and myristic acids had an insignificant effect on shear stress.
Fig. 7. Viscosity as a function of shear rate in the absence of a magnetic field.
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Fig. 8. The flow curves for MRFs (MRFs 1–5 in Table 1) with and without additives; (A) H = 0 kA/m, (B) H = 109 kA/m, (C) H = 218 kA/m and (D) H = 362 kA/m.
There are a lot of rheological models to investigate rheological behavior of smart fluids. In efficient models, yield stress has been considered to be a function of magnetic field strength. One of the most common models that are able to predict MRFs rheological behavior is Herschel–Bulkley model [34]. This model defines a yield stress at which the fluid flow has no velocity gradient if the applied stress does not exceed it. This model is defined as follows: = 0 + k˙ n
|| ≥ 0
˙ = 0
|| < 0
(2)
where is the stress tensor, 0 is the yield stress, ˙ is the shear rate tensor, n is the power-law index and k is the consistency parameter. Table 2 shows the power-law index n, the consistency parameter k and the yield stress 0 for samples containing different acids. As can be seen in the table, the sample with stearic acid (MRF5) had a lower n, which shows more shear-thinning behavior. It can be concluded that stearic acid not only enhances shear stress and stability but also improves shear-thinning behavior more in comparison to lauric, myristic and palmitic acids. The results showed that all samples followed the Herschel–Bulkley non-Newtonian model and showed strong shear thinning behavior which is consistent with the results of other researchers [3,4,21,35–37]. Shear thinning behavior of all of the MRFs was enhanced by increasing magnetic field strength. On the other hand, in the absence of a magnetic field, all samples behavior approached to semi-Newtonian behavior which was more significant in the case of additive-free MRF.
Table 2 Coefficient and the power-law index of the Herschel–Bulkley model and R2 . Sample
H (kA/m)
n
k
R2
MRF1
0 109 218
0.871 0.194 0.177
1.194 3017 4117
0.997 0.996 0.991
MRF2
0 109 218
0.762 0.194 0.113
3.030 3528 9502
0.989 0.996 0.990
MRF3
0 109 218
0.733 0.266 0.138
4.459 2383 7992
0.990 0.997 0.990
MRF4
0 109 218
0.833 0.355 0.119
3.731 1637 16,560
0.998 0.999 0.980
MRF5
0 109 218
0.751 0.163 0.110
3.501 2657 14,330
0.999 0.999 0.981
Obviously, yield stress is a key parameter in determining whether an MRF is appropriate or not, especially for its industrial applications. Fig. 9 shows yield stress of samples containing acids in different magnetic field strengths. As can be seen in the figure, stearic and palmitic acids enhanced the yield stress. It is also obvious that yield stress increased by growing the magnetic field strength. This observation is due to the fact that by increasing magnetic field strength, particles form a stronger chain which restricts the fluid flow more. This phenomenon leads to yield stress
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mismatch between dispersed particles and continuous phase. It also improved the MR effect due to the formation of a stronger gellike structure in the stearic acid-containing MRF. Different weight fractions of stearic acid were used and it was observed that adding this acid up to 3 wt% increased yield stress and improved stability and redispersion behavior. Lower fractions did not result in an accepted stability and higher fractions led to an unwanted viscosity increase in the absence of a magnetic field. All samples showed yielded shear thinning behavior in the presence of a magnetic field and followed Herschel–Bulkley model. Effect of adding more dense acids with shorter carbon chains (which restricts the increase in off-state viscosity) may be the subject of future researches. Acknowledgement Financial Support from the Iran National Science Foundation (INSF) is gratefully acknowledged. Fig. 9. Yield stress versus magnetic field strength for samples containing different acid additives in comparison to the additive-free MRF. Table 3 Coefficients and power of the yield stress equation and R2 . Sample
˛
ˇ
ı
R2
MRF1 MRF2 MRF3 MRF4 MRF5
1.786 7.846 30.230 49.350 80.050
0.457 5.464 192.000 1840.000 3499.000
1.142 0.834 0.447 0.128 0.118
0.950 0.986 0.999 0.997 0.996
enhancement, thereby improving MR effect. It is also evident that using stearic and palmitic acids improved yield stress up to 22 and 11 times, respectively, at 362 kA/m magnetic field strength in comparison to the additive-free MRFs which is attributed to the longer carbon chain of these acids and the stronger gel-like structure they form. On the other hand, myristic and lauric acids could not improve the yield stress efficiently (7 and 2 times at 362 kA/m magnetic field strength in comparison to the additive-free MRFs, respectively). As can be seen in Fig. 9, the field dependent yield stress of all samples followed a model in the form of: y = ˛ + ˇH ı
(3)
where y is the yield stress, ˛, ˇ and ı are the model constant parameters which depend on the sample composition and operating condition. The range of exponent ı is 0.1–1.1 and the highest is related to additive-free sample (MRF1). Table 3 shows the constants of Eq. (3) obtained for the MRFs. According to the table, it is evident that the additive-free MRF (MRF1) was more dependent on the magnetic field strength. Moreover, by increasing the carbon chain length of the fatty acid additives (MRFs 2 through 5), the constants of the yield stress equation increased. This enhancing effect may be attributed to the structures formed due to the addition of fatty acid additives to silicone oil. 4. Conclusion In this paper, sedimentation ratio and magnetorheological response of suspensions of carbonyl iron particles in silicone oil in the presence of different fatty acids (lauric, myristic, palmitic and stearic acid) were studied. Experimental results showed that a suspension of 62 wt% CI particles in silicone oil was promising with regard to both MR effect and stability. Further increase in the dispersed phase fraction led to an undesirable increase in offstate viscosity with no significant change in MR effect and stability. Stearic acid reduced sedimentation due to the decrease of density
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
An experimental study on the effect of fatty acid chain length on the magnetorheological fluid stabilization and rheological properties M. Ashtiani, S.H. Hashemabadi ∗ CFD Research Laboratory, School of Chemical Engineering, Iran University of Science and Technology, 16846 Tehran, Iran
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Improving MRF stability and MR effect by using stearic acid additive.
• Fatty acids with longer carbon chain as additive creates stronger gel structure. • Stronger gel structure improves stability and yield stress, consequently more efficient MRF.
a r t i c l e
i n f o
Article history: Received 9 November 2014 Received in revised form 21 December 2014 Accepted 22 December 2014 Available online 3 January 2015 Keywords: Carbonyl iron Fatty acid Herschel–Bulkley model Magnetorheological fluid Stabilization
a b s t r a c t An experimental study was conducted on the effect of acid additives on the stability and rheological properties of a suspension of carbonyl iron (CI) microparticles dispersed in silicone oil. A series of acids with the same carboxyl group but different carbon chain lengths (C12, C14, C16 and C18) were added to magnetorheological fluids (MRFs) to investigate their effect on the stability and rheological behavior. MR effect was measured with the aid of a magnetorheometer and stability measurements were made by simple determining the height of the transparent layer in the MRF over a 6-month period. Experimental results showed that by increasing carbon chain length of acids, yield stress and stability increased up to 22 times (at H = 362 kA/m) and 7 times, respectively, in comparison to the additive-free MRF. Model fitting confirmed that all of the acid-based MRFs showed a yield stress with shear thinning behavior and followed Herschel–Bulkley model. Further investigations suggested that 3 wt% stearic acid was the most promising additive in increasing MR effect and stability. The results of stability and rheological tests showed that further increase in stearic acid fraction improved stability slightly and, at the same time, increased the off-state viscosity to a huge extent undesirably. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Magnetorheological fluids (MRFs) are a class of smart materials whose rheological properties vary considerably in the presence of an external magnetic field [1]. The viscosity of these materials can increase to a huge extent when they are exposed to a magnetic field. MRFs consist of three main parts; a continuous phase, magnetizable
∗ Corresponding author. Tel.: +98 21 7724 0376; fax: +98 21 7724 0495. E-mail address: [email protected] (S.H. Hashemabadi). http://dx.doi.org/10.1016/j.colsurfa.2014.12.046 0927-7757/© 2014 Elsevier B.V. All rights reserved.
particles, and stabilizer additives [2]. When an external magnetic field is applied to an MRF, magnetizable particles build a chain-like structure in the direction of magnetic field lines and restrict the fluid flow. This behavior is recognized by a sudden increase in the fluid viscosity and a yield stress [3]. In the literature, this change in the MRF state, shear stress and viscosity is mostly interpreted as MR effect which plays an important role in MR technologies [4,5]. Magnetorheological fluids are used in a broad range of industrial applications from MR valves in chemical engineering [6,7] to seismic dampers in civil engineering [8,9]. They have attracted great attention in some industries such as automobile shock absorbers
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Nomenclature k n ˛ ˇ ˙ y
Consistency parameter, Pa sn Power law index Field-constant parameter, Pa Field-consistency parameter, Pa sn Shear rate tensor, s−1 Stress tensor, Pa Yield stress, Pa
[10,11]. Two important challenges in MRF-based technologies are their low MR effect and the fluid instability. Like any other dispersed systems, MR suspensions face sedimentation problems. Many studies have attempted to resolve the problem of MRFs instability by using proper additives [12,13], reducing particle size [14] or density [15], synthesizing more elongated particles [16], coating magnetizable particles [17–19], using ionic [20,21] or polymeric liquids [22] as carrier fluids and so on. Unfortunately, most of these methods reduce the MR effect or increase the off-state viscosity of fluid undesirably [23]. In any application, a specific MR effect and stability are needed; so the composition of MRFs for any application must be chosen carefully and consciously [4]. In MR dampers, for example, improving stability is the first and the most important problem and MR effect is of secondary importance. Also in most applications, some limitations restrict the MRF composition; in polishing applications, considering the cooling characteristics of surface and some other parameters, water is the best candidate to be used as the carrier phase in MR suspension preparation [17]. Some researchers like López-López et al. [24] focused on the effect of dispersion on the magnetorheological behavior. They used different surfactants and observed that when particles were dispersed well in the suspension, the change in the viscosity was more significant. They also showed the quality of dispersion had no important effect on the yield stress. However, in the presence of a magnetic field, the change from liquid-state to semi solid-state is more rapid when the particle dispersion is poor. Fang et al. [12] added single-walled carbon nanotube (SWNT) to a suspension of carbonyl iron (CI) particles in lubricant oil. They showed that adding SWNT could improve both MRF stability and the MR effect. They attributed this behavior to the more robust chain-like structure of the SWNT-containing MRF in the presence of a magnetic field. Wang and Gordaninejad [14] compared the rheological properties and apparent viscosity of three suspensions of a commercial MRF, a polymeric gel-based MRF and a ferrofluidbased MRF at high shear rates. They showed that the gel-based MRF enhances yield stress more in comparison to the commercial MRF by somehow increasing the off-state viscosity. The ferrofluidbased MRF showed an MR effect enhancement similar to that of the gel-based ones. However, it resulted in a sharp increase in the off-state viscosity which hampers its usage. Jiang et al. [16] used stearic acid (3 wt% of the mass of CI) as the stabilizer additive in a suspension of iron nanowires and CI spherical microparticles in silicone oil. Cheng et al. [17] coated CI particles with a hydrophilic acid (N-glucose ethylenediamine triacetic acid) and claimed that by dispersing these composite particles in water, stability of the suspension increases up to 25%. In an attempt to synthesize an efficient MRF with long lifetime, Premalatha et al. [13] used grease as the stabilizer additive and observed that by increasing the fraction of this agent up to 0.5 wt%, MRF sedimentation decreased. However, MR effect decreased with the increase in grease weight percentage. Based on the best author’s knowledge, the effect of carbon chain length of fatty acids on the strength of the network formed as a
Table 1 Samples composition. Sample no.
CI particles (wt%)
Silicone oil (wt%)
Additives (wt%)
MRF1 MRF2 MRF3 MRF4 MRF5 MRF6 MRF7 MRF8 MRF9 MRF10 MRF11 MRF12
62 62 62 62 62 72 52 42 62 62 62 62
38 35 35 35 35 28 48 58 36 37 33 28
– Lauric acid (3) Myristic acid (3) Palmitic acid (3) Stearic acid (3) – – – Stearic acid (2) Stearic acid (1) Stearic acid (5) Stearic acid (10)
result of adding additives to MRF has not been investigated thoroughly, yet. There are few researches on the relation between carbon chain length and kinetics of some specific reactions [25]. Aramaki et al. [26] reported that by increasing carbon chain length of alcohols, the viscosity and shear stress of the micellar solution increase. Yu et al. [27] found that by decreasing carbon content of magnetizable particles, MR effect is enhanced. Morrow et al. [28] examined the influence of acyl carbon chain length on the mean orientational order parameter of liquid to gel transition and found a linear relationship between them. The effect of acid carbon chain length on the rate of crystallization [29] and on some other structures [30–33] have also been investigated. In this study with the aim of synthesizing a stable MRF with a promising MR effect, four hydrophobic acids with the same functional group but different numbers of carbon atoms were added to the suspension of carbonyl iron and silicone oil. The rheological behavior of the prepared MRFs was monitored using a well-known rheological model. 2. Materials and methods Spherical carbonyl iron particles (average particle size less than 5 m, density: 7.86 × 103 kg m−3 , CS grade, BASF, Germany) were used as the dispersed phase without further purification. In all samples, magnetizable carbonyl iron particles were dispersed in polydimethylsiloxane (silicone oil, viscosity: 3.50 × 10−4 m2 s−1 , KCC, Korea). Four fatty acids with different carbon chain lengths were added to the MRFs to improve their stability. The additives employed in this study were stearic acid (MIT, Malaysia), myristic, palmitic and lauric acid (MERCK, Germany). In an attempt to determine a composition of MRF which has an efficient MR effect as well as promising stability, four samples with different weight fractions of CI particles were synthesized and examined. For stabilizing the MRFs, different acids were added to the MRF suspensions. The sample compositions with and without additives are presented in Table 1. For synthesizing samples, each acid was added to the base fluid (silicone oil). Each mixture was then stirred and heated at 100 ◦ C for half an hour until a homogeneous solution was obtained. All of the acids used in this study produced a gel-like structure in silicone oil. This structure increases the base fluid density and the MRF stability. Afterwards, magnetizable CI particles were added to the sample which was then stirred at 1000 rpm with the aid of an overhead stirrer (Heidolph, RZR 2102, Germany) for more than half an hour to ensure suspension uniformity. 3. Results and discussion The main focus of this study is on the synthesis of a stable MR suspension. However, if an MRF is stable for more than several months with no efficient MR effect, it is worthless; rheometry
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Fig. 2. Magnetization curves of CI particles.
particles (about 165 emu/g in 8 kOe magnetic field strength) made them the first choice in MRF preparation. 3.2. Stability tests
Fig. 1. SEM image of carbonyl iron particles.
analyses were done for each sample to ensure MRFs efficiency. Yield stress and MR effect can be obtained by investigating the changes in the fluid viscosity and shear stress as a function of shear rate. Lauric acid (dodecanoic acid) which has a 12-carbon atom chain is a white and powdery solid and forms a weak gel-like structure in silicone oil. However, since lauric acid does not increase the off-state viscosity of the fluid greatly, it is one of the promising candidates for being used as an additive in MRF preparation. Myristic acid (tetradecanoic acid) is a common saturated fatty acid with the chemical formula C14 H28 O2 which constructs a stronger gel in silicone oil in comparison to lauric acid. This behavior is due to the longer carbon chain of myristic acid which leads to the involvement of a greater number of carbon atoms in silicone oil. Palmitic acid (hexadecanoic acid) is a fatty acid with white crystals and 16 carbon atoms in its chemical formula. Stearic acid (octadecanoic acid) with the chemical formula C18 H36 O2 is a waxy acid that forms a strong gel-like structure in silicone oil. This acid increases the base fluid density. It, therefore, decreases the density mismatch between the dispersed particles and continuous phase and improves stability. The longer the carbon chain is, the stronger the gel-like structure is [28]. Unfortunately, this strong gel increases the off-state viscosity undesirably. As mentioned above, for analyzing a promising MRF, both stability and MR effect must be considered which will be described in detail in the following sections.
When MRFs remain stationary for a while, heavy magnetizable particles tend to settle and a transparent layer (continuous phase) forms on the top of each sample. Stability measurements were made by detecting the interface between the two phases. For this purpose, each sample was put in a motionless place for more than three months and was photographed at specific intervals. Sedimentation ratio is defined as [17]: Sedimentation ratio (%) =
Volume of supernatant liquid × 100 (1) Volume of total suspension
The height of supernatant liquid and total suspension was determined using an image processing software. Suspension and supernatant volumes were then calculated by multiplying their heights by the cross section area. Fig. 3 shows sedimentation ratio for samples containing different percentages of CI particles. As can be seen in this figure, all of the synthesized MRFs were unstable without additives. Samples with weight fractions of more than 0.62 carbonyl iron particles were more stable. This behavior may be due to the fact that magnetic hysteresis is more in higher CI particles fractions. As a result, particles maintain their chain-like structure even after removing the magnetic field and remain stable for a longer period. According to Fig. 3, no significant effect on the suspensions stability was observed at weight fractions higher
3.1. Characterization of magnetic particles Carbonyl iron (CI) microparticles, which are obtained from thermal decomposition of penta carbonyl iron, have the highest magnetization and are widely used in magnetorheological fluid preparation. Structure and average size of CI particles were measured using a scanning electronic microscope (SEM, MV2300, Tescan, Ltd., Czech Republic) with 15 kV operating voltage. Fig. 1 shows the spherical CI microparticles. The results of image processing showed that the average particle size was 3 m with 1–6 m particle size distribution. Fig. 2 illustrates the magnetorheological behavior of CI particles determined with the aid of a vibrating sample magnetometer (VSM, MDKFD, Magnetic Danesh Pajoh Co. Ltd, Iran). As can be seen in the figure, the magnetic hysteresis loop is very small and the high magnetic saturation of CI
Fig. 3. Stability measurements; samples with different CI weight fractions.
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Fig. 4. Stability measurements; samples containing 62 wt% CI, with and without acid additives.
Fig. 6. Stability measurements; samples with different stearic acid weight fractions.
3.3. Rheometry analysis
than 0.62. In addition, more increase in CI weight fraction leads to undesirable increase in the off-state viscosity, therefore 62 wt% of suspended particles was chosen for the rest of experiments. Fig. 4 depicts sedimentation ratios of samples containing different acids during more than 3 months. It is obvious that stearic acid made the suspension more stable (completely stable for 100 h). The reason for this observation is that stearic acid has a longer carbon chain in comparison to lauric, myristic and palmitic acids, so that it builds a stronger gel-like structure in silicone oil. Fig. 5 shows a schematic image of carbonyl iron particles trapped in the gellike structure of stearic acid-silicone oil. This strong gel traps heavy dispersed particles and restricts their precipitation. As a result, stability improves much. A closer investigation of Figs. 3 and 4 illustrates that using proper additives (stearic acid) is necessary in synthesizing stable MRFs, but the question arises as to what fraction of the additive is to be used. To determine the optimum weight fraction of stearic acid, four samples were synthesized. In all samples, weight fraction of stearic acid was changed but particles weight fraction was kept constant. Fig. 6 shows sedimentation ratio of samples containing different fractions of stearic acid (MRFs 5, and 9–12 in Table 1). As can be seen in the figure, stability improved by increasing fraction of the acid. Samples with at least 5 wt% stearic acid were completely stable over a long period of time. But unfortunately, the off-state viscosity increased to a huge extent (viscosity curves have not been shown here). It seems that 3% steraic acid is an appropriate candidate in preparing stable MRFs which is in agreement with other experiments [16]. As mentioned in the previous section, rheological measurements are needed to choose the best sample which is able to meet rheological and stability requirements of MRF industrial applications. Off-state viscosity, MR effect and yield stress will be discussed in detail in the next section.
Fig. 5. Schematic representations of dispersed particle trapping by gel structure.
The rheological characteristics were examined using a plate/plate rotational rheometer (MCR 300, Anton-Paar Physica, Germany) connected to a magnetorheological equipment (MRD 180, Physica, Germany) which generates a homogeneous magnetic field perpendicular to the fluid flow direction. In this study, a magnetic flux density up to 362 kA/m was used in a gap distance of 1 mm. All of the measurements were taken at 25 ◦ C. Fig. 7 depicts viscosity of the MRF samples containing different acids in the absence of a magnetic field. As can be seen, all of the additives increased the off-state viscosity. The viscosity increase was considerable in the case of palmitic and stearic acids (MRFs 4 and 5 in Table 1). This behavior is due to the much longer carbon chain of these acids which increases the base fluid density as well as the MRF viscosity. To ensure reproducibility of the obtained results, each test was repeated and the average value is reported. Fig. 8 shows shear stress versus shear rate for samples containing different acids in different magnetic field strengths (MRFs 1–5 in Table 1). According to the figure, all of the additives increased the shear stress. None of them, however, was capable of enhancing the shear stress as much as stearic acid did. As can be seen in this figure, stearic acid had a promising effect on improving shear stress as well as yield stress of the MRFs in high magnetic field strength. Closer investigation of Figs. 7 and 8 reveals that lauric and myristic acids had an insignificant effect on shear stress.
Fig. 7. Viscosity as a function of shear rate in the absence of a magnetic field.
M. Ashtiani, S.H. Hashemabadi / Colloids and Surfaces A: Physicochem. Eng. Aspects 469 (2015) 29–35
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Fig. 8. The flow curves for MRFs (MRFs 1–5 in Table 1) with and without additives; (A) H = 0 kA/m, (B) H = 109 kA/m, (C) H = 218 kA/m and (D) H = 362 kA/m.
There are a lot of rheological models to investigate rheological behavior of smart fluids. In efficient models, yield stress has been considered to be a function of magnetic field strength. One of the most common models that are able to predict MRFs rheological behavior is Herschel–Bulkley model [34]. This model defines a yield stress at which the fluid flow has no velocity gradient if the applied stress does not exceed it. This model is defined as follows: = 0 + k˙ n
|| ≥ 0
˙ = 0
|| < 0
(2)
where is the stress tensor, 0 is the yield stress, ˙ is the shear rate tensor, n is the power-law index and k is the consistency parameter. Table 2 shows the power-law index n, the consistency parameter k and the yield stress 0 for samples containing different acids. As can be seen in the table, the sample with stearic acid (MRF5) had a lower n, which shows more shear-thinning behavior. It can be concluded that stearic acid not only enhances shear stress and stability but also improves shear-thinning behavior more in comparison to lauric, myristic and palmitic acids. The results showed that all samples followed the Herschel–Bulkley non-Newtonian model and showed strong shear thinning behavior which is consistent with the results of other researchers [3,4,21,35–37]. Shear thinning behavior of all of the MRFs was enhanced by increasing magnetic field strength. On the other hand, in the absence of a magnetic field, all samples behavior approached to semi-Newtonian behavior which was more significant in the case of additive-free MRF.
Table 2 Coefficient and the power-law index of the Herschel–Bulkley model and R2 . Sample
H (kA/m)
n
k
R2
MRF1
0 109 218
0.871 0.194 0.177
1.194 3017 4117
0.997 0.996 0.991
MRF2
0 109 218
0.762 0.194 0.113
3.030 3528 9502
0.989 0.996 0.990
MRF3
0 109 218
0.733 0.266 0.138
4.459 2383 7992
0.990 0.997 0.990
MRF4
0 109 218
0.833 0.355 0.119
3.731 1637 16,560
0.998 0.999 0.980
MRF5
0 109 218
0.751 0.163 0.110
3.501 2657 14,330
0.999 0.999 0.981
Obviously, yield stress is a key parameter in determining whether an MRF is appropriate or not, especially for its industrial applications. Fig. 9 shows yield stress of samples containing acids in different magnetic field strengths. As can be seen in the figure, stearic and palmitic acids enhanced the yield stress. It is also obvious that yield stress increased by growing the magnetic field strength. This observation is due to the fact that by increasing magnetic field strength, particles form a stronger chain which restricts the fluid flow more. This phenomenon leads to yield stress
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mismatch between dispersed particles and continuous phase. It also improved the MR effect due to the formation of a stronger gellike structure in the stearic acid-containing MRF. Different weight fractions of stearic acid were used and it was observed that adding this acid up to 3 wt% increased yield stress and improved stability and redispersion behavior. Lower fractions did not result in an accepted stability and higher fractions led to an unwanted viscosity increase in the absence of a magnetic field. All samples showed yielded shear thinning behavior in the presence of a magnetic field and followed Herschel–Bulkley model. Effect of adding more dense acids with shorter carbon chains (which restricts the increase in off-state viscosity) may be the subject of future researches. Acknowledgement Financial Support from the Iran National Science Foundation (INSF) is gratefully acknowledged. Fig. 9. Yield stress versus magnetic field strength for samples containing different acid additives in comparison to the additive-free MRF. Table 3 Coefficients and power of the yield stress equation and R2 . Sample
˛
ˇ
ı
R2
MRF1 MRF2 MRF3 MRF4 MRF5
1.786 7.846 30.230 49.350 80.050
0.457 5.464 192.000 1840.000 3499.000
1.142 0.834 0.447 0.128 0.118
0.950 0.986 0.999 0.997 0.996
enhancement, thereby improving MR effect. It is also evident that using stearic and palmitic acids improved yield stress up to 22 and 11 times, respectively, at 362 kA/m magnetic field strength in comparison to the additive-free MRFs which is attributed to the longer carbon chain of these acids and the stronger gel-like structure they form. On the other hand, myristic and lauric acids could not improve the yield stress efficiently (7 and 2 times at 362 kA/m magnetic field strength in comparison to the additive-free MRFs, respectively). As can be seen in Fig. 9, the field dependent yield stress of all samples followed a model in the form of: y = ˛ + ˇH ı
(3)
where y is the yield stress, ˛, ˇ and ı are the model constant parameters which depend on the sample composition and operating condition. The range of exponent ı is 0.1–1.1 and the highest is related to additive-free sample (MRF1). Table 3 shows the constants of Eq. (3) obtained for the MRFs. According to the table, it is evident that the additive-free MRF (MRF1) was more dependent on the magnetic field strength. Moreover, by increasing the carbon chain length of the fatty acid additives (MRFs 2 through 5), the constants of the yield stress equation increased. This enhancing effect may be attributed to the structures formed due to the addition of fatty acid additives to silicone oil. 4. Conclusion In this paper, sedimentation ratio and magnetorheological response of suspensions of carbonyl iron particles in silicone oil in the presence of different fatty acids (lauric, myristic, palmitic and stearic acid) were studied. Experimental results showed that a suspension of 62 wt% CI particles in silicone oil was promising with regard to both MR effect and stability. Further increase in the dispersed phase fraction led to an undesirable increase in offstate viscosity with no significant change in MR effect and stability. Stearic acid reduced sedimentation due to the decrease of density
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