reduced graphene oxide composite nanoparticles and their magnetic stimuli-response

Journal of Colloid and Interface Science 481 (2016) 194–200

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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis

Regular Article

Fabrication of smart magnetite/reduced graphene oxide composite nanoparticles and their magnetic stimuli-response Cheng Hai Hong a, Min Wook Kim a, Wen Ling Zhang b, Il Jae Moon a, Hyoung Jin Choi a,⇑ a

Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea College of Chemical Science and Engineering, Laboratory of Fiber Materials and Modern Textile, the Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, China b

g r a p h i c a l a b s t r a c t A MR materials composed of Fe3O4/GO composite particles were fabricated through in situ chemical deposition. When dispersed in silicone oil, their magnetorheology properties were measured by using a rotational rheometer at various magnetic field strengths under steady flow and dynamic oscillation tests.

a r t i c l e

i n f o

Article history: Received 26 March 2016 Revised 18 June 2016 Accepted 23 July 2016 Available online 25 July 2016 Keywords: Magnetorheological fluid Fe3O4 Graphene oxide Magnetic particle

a b s t r a c t Novel Fe3O4/reduced graphene oxide (RGO) composite nanoparticles were synthesized and confirmed by FT-IR spectra as good candidates for magnetic stimuli-responsive magnetorheological (MR) materials. The morphology of Fe3O4/RGO was observed by both scanning and transmission electron microscopy and their sedimentation stability improved due to a decreased density of the synthesized composites. The MR performance of the Fe3O4/RGO-based fluid was investigated with a rotational rheometer, and the Cho-Choi-Jhon model of the rheological equation of state was adopted to explain their performances for the entire shear rate region. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction Graphene oxide (GO) possesses a layer structure containing phenolic, carboxyl and epoxide groups introduced by oxidation, contrasting with the pristine surface of graphene [1]. The presence of these functional groups, namely high amounts of hydroxyls and epoxides, allows GO to disperse well in water, favoring the

⇑ Corresponding author. E-mail address: [email protected] (H.J. Choi). http://dx.doi.org/10.1016/j.jcis.2016.07.060 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.

combination with other materials in water [2,3]. GO has attracted attention in various industrial applications due to unique mechanical properties, low molecular weight and favorable hydrophilic properties. In addition, not only the magnetic properties of GO have been theoretically investigated [4], but the application of GO-based magnetic materials as magnetorheological (MR) fluids has also been reported [5]. Recently, the potential of nanocomposites containing GO or reduced GO (RGO) with magnetite (Fe3O4) nanoparticles has been widely studied for applications in targeted drug delivery, magnetic

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resonance imaging and heavy metal ion removal from aqueous solutions [3,6–8]. In particular, the Fe3O4/GO composite particles have been also actively studied as an anode of lithium-ion batteries [9,10]. Given that RGO has a similar structure to GO, it can also be applied as a MR fluid. The large surface area and stability of RGO motivated us to synthesize Fe3O4/RGO composites [3]. Fe3O4 has been frequently used in biosensors and water treatment due to their high biocompatibility, low toxicity, easy preparation, and special magnetic properties. Methods to synthesize graphenebased nanocomposites with Fe3O4 include in situ chemical deposition, hydrothermal treatment, sol-gel procedure, reduction of iron salt precursors, and self-assembly of Fe3O4 on GO or RGO sheets [11–14]. These hybrids could not only enhance the dispersion stability of Fe3O4, but also reduce the restacking or aggregation of the GO or RGO sheets themselves [11–14]. On the other hand, it can be also noted that dual stimuli-response of both electrorheological (ER) under electric fields applied and MR characteristics from GO-coated iron oxide/silica core-shell nanoparticles has been recently reported [15]. MR fluids, as mentioned above, consist of soft magnetic micron-sized particles dispersed in nonmagnetic fluids such as hydrocarbon, silicone oil, or aqueous carrier fluids. They have been extensively investigated due to their interesting phase transition characteristics under an applied magnetic field strength, and their engineering applications in a variety of multidisciplinary areas [16–19]. Their rheological properties can vary significantly with the magnetic field strength, permitting a fine-tuning of the materials behavior [20,21]. MR fluids show characteristics of Newtonian fluids when no magnetic field is exerted. However, in the presence of a magnetic field, they exhibit a continuous, rapid, and reversible change from a fluid-like to a solid-like phase [20,21]. This is because dispersed magnetic particles can form chains, which align in the direction of the magnetic field due to the magnetic–polarization interaction, and then return to its free-flowing liquid state upon removal of the external magnetic field [22–25]. Due to their outstanding controllable mechanical characteristics with a high yield stress value, MR fluids have been adopted in various engineering devices such as shock absorbers, brakes, active dampers, and so on [26]. In addition to their high yield stress, since the magnetic field is more stable under operation than an electric one, several commercially available products based on MR fluids have been developed. Moreover, their application has been quite more extensive than that of ER fluids working under an applied electric field strength [27]. Among various magnetic materials, the excellent magnetic properties and particle sizes of soft magnetic carbonyl iron (CI) microspheres have been widely adopted for MR fluids and elastomers. However, application of the CI-based MR fluids in devices requires improvements in the fluid properties, because sedimentation of the heavy CI particles due to the significant density mismatch between the CI particles and oil medium hampers MR device operation as well as their redispersion after some time. The viscoelastic behavior of the CI-based MR fluid shows strong solid-like characteristics under an applied magnetic field strength [28]. So far, the most popular MR materials are CI particles (CD grade, BASF Germany, average particle size: 4.25 lm, density: 7.91 g cm
3) due to their high saturation magnetization and appropriate particle size [29–31]. Most CI-based magnetic materials possess severe sedimentation problems due to their large density, when used as dispersed particles in MR fluids. Thus, a significant effort such as introducing additives or polymer coating technology has been carried out to prevent contact between CI particles and decreasing CI particle density in order to improve the sedimentation stability [32–34]. Compared with the complicated process of modifying CI particles, the use of other magnetic species, such as Fe3O4 particles

195

with much lower density (4.32 g cm
3) but sufficiently good magnetic behavior gains an advantage [35,36]. In this work, aiming to reduce the density mismatch between Fe3O4 and the oil medium (0.96 g cm
3) as well as the aggregation problem of nanoscaled Fe3O4 particles, RGO was introduced by a hydrothermal method. This method was chosen to produce Fe3O4/RGO due to its advantages, like cost efficiency, synthesis of uniform Fe3O4 nanoparticles with good stability, and high magnetization. GO was firstly prepared by oxidizing graphite with acid via the Hummers method. Then, the oxidized material was exfoliated in water through an ultrasonication treatment, followed by the reduction of the exfoliated GO by hydrazine hydrate in the solution of FeCl3 and FeCl2 to produce the Fe3O4/RGO nanocomposite. Finally, the Fe3O4/RGO particle-based MR fluid was prepared. This simple and facile process of fabricating the Fe3O4/RGO particle is considered to be new and novel for MR community. The morphology of the Fe3O4/RGO nanocomposites was observed via scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images and Fourier-transform infrared (FT-IR) spectroscopy. The MR performance of the synthesized Fe3O4/RGO particle-based MR fluid was investigated as a function of magnetic field strength. The sedimentation stability was also checked by both testing the density of the Fe3O4/RGO particles and measuring its sedimentation profile.

2. Experimental section 2.1. Materials and synthesis of the Fe3O4/RGO composite The GO was fabricated by a modified Hummers method, starting from graphite (Aldrich, <45 lm). Graphite powder was added to H2SO4 (98%), followed by the gradual addition of KMnO4 and NaNO3. This solution was agitated for 2 h, and then a 30% H2O2 solution was added until the color of the solution turned to a brilliant brown, indicating a fully oxidized graphite. The as-obtained graphite oxide slurry was exfoliated to generate GO nanosheets by an ultrasonication process at 60 °C using an ultrasonic generator (28 kHz, 600 W, Kyungil Ultrasonic Co., Korea) for 1 h. Finally, the mixture was centrifuged, washed repeatedly with distilled water, and then finally dried in a vacuum oven. In order to obtain the Fe3O4/RGO composite, GO particles were then dispersed in 450 mL of water. An aqueous solution of FeCl3 and FeCl2 was prepared with a 2:1 mol ratio [3]. This solution was then slowly added to the GO solution at room temperature (GO = 0.700 g/450 mL, FeCl3 = 3.2442 g/25 mL and FeCl2 = 1.2675 g/25 mL). A 30% ammonia solution was added to the latter in order to adjust the pH to 10. The temperature of the solution was raised to 90 °C, and 10 mL of hydrazine hydrate were added under constant stirring, resulting in a black colored solution. After being rapidly stirred for 4 h, the solution was cooled down to room temperature, filtered, washed using water and ethanol several times, and finally dried in vacuum at 70 °C. For comparison purposes, Fe3O4 was also synthesized (FeCl3 = 0.4055 g/25 mL and FeCl2 = 0.1584 g/25 mL) using the same procedure, without the addition of GO.

2.2. Preparation of the MR fluid Two MR fluids with the same particle volume fraction were prepared by dispersing Fe3O4 and Fe3O4/RGO in silicone oil. To prepare these MR fluids, the concentration of Fe3O4 was fixed at 70 wt.% for both systems (with and without RGO). The MR fluids were immersed in a sonifier for a few minutes to obtain a homogeneous distribution of the samples.

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2.3. Characterization and MR measurements

3. Results and discussion

The morphology of the Fe3O4/RGO composites was observed by both SEM (S-4300, Hitachi, Japan) and TEM (Philips CM 200). FT-IR spectra of the three samples (GO, Fe3O4, and Fe3O4/RGO) were obtained using a Nicolet FT-IR spectroscopy instrument (Bruker IFS 66 v/s). Solid samples were mixed with KBr, and pellets were made to perform the scans. XRD spectra were obtained via a Rigaku DMAX 2500 (Cu Ka, k = 1.54 Å) diffractometer to confirm the successful synthesis of Fe3O4. The magnetic properties of Fe3O4 and Fe3O4/RGO were measured using a vibration sample magnetometer (VSM). The MR performance of two MR fluids was measured by a rotational rheometer (Physica MCR 300 Anton Paar, Austria), which was equipped with a magnetic field supplier and a 25 mm parallel plate measuring system. A controlled shear rate (CSR) mode, over a shear rate range of 0.01–200 s
1, was applied for all the tests under different magnetic field strengths. Finally, density of the synthesized Fe3O4 and Fe3O4/RGO was examined using a pycnometer.

As shown in Scheme 1, the Fe3O4/RGO can be prepared via a redox reaction between RGO and Fe2+. The morphology of both Fe3O4 and Fe3O4/RGO is shown in Fig. 1. In the Fe3O4/RGO composite, spherical Fe3O4 particles with an average diameter of about 50 nm exhibit a significantly irregular surface due to the deposition on GO. Compared with the smooth surface of GO, the Fe3O4 loaded GO spheres exhibit very rough appearance. Furthermore, in order to observe the structure of the Fe3O4/RGO composite particles, the inside-section view was observed via TEM. From TEM images shown in Fig. 2(b), the grey part represents the RGO, while the black part represents the aggregates of Fe3O4 nanoparticles. Additionally, while some regions were covered by Fe3O4 nanoparticles, other regions barely contained them. Therefore, we can consider that the Fe3O4 present on the surface of the RGO is not uniformly distributed. Further evidence of the Fe3O4/RGO nanocomposites was provided by the FT-IR spectra, as shown in Fig. 3. In this case, the

Scheme 1. The preparation route to Fe3O4/RGO via redox reaction between GO and Fe2+.

C.H. Hong et al. / Journal of Colloid and Interface Science 481 (2016) 194–200

197

(a)

500 nm

(b)

500nm Fig. 1. SEM image of (a) Fe3O4, (b) Fe3O4/RGO.

typical FT-IR spectrum of the GO agrees with that of a previous work [37]. The bands centered at 3426 and 1397 cm
1 are attributed to the presence of AOH bonds and COAH groups of GO, respectively, while the band centered at 1054 cm
1 is associated with stretching of CAO bonds. The stretching vibrations of carbonyl or carboxyl groups are detected as a band at 1724 cm
1. The spectrum of Fe3O4 in Fig. 3 shows stretching vibration bands attributable to the FeAOAFe at 670 cm
1. The spectrum of the Fe3O4/RGO nanocomposite shows peaks of both Fe3O4 and GO with peaks of hydroxyl and carbonyl groups from GO, and strong peaks of functional groups from Fe3O4. Therefore, based on SEM, TEM images and FT-IR spectra, we can confirm that the nanocomposite of Fe3O4/RGO was successfully fabricated. Fig. 4 provides the X-ray diffraction patterns for GO, Fe3O4/RGO, and Fe3O4. The strong peak at 11.85° for GO indicates the presence of residual stacked layers of GO, while Fe3O4/RGO does not show this peak, implying the exfoliation of RGO layers by Fe3O4 nanoparticles. On the other hand, both synthesized Fe3O4 and Fe3O4/RGO possess similar characteristic peaks at 30.1, 35.5, 43.2, 53.5, 57.1, and 62.6°, indicating that both Fe3O4 and Fe3O4/RGO were synthesized successfully. The magnetic hysteresis loops for the synthesized Fe3O4 and Fe3O4/RGO measured in a powder state are represented in Fig. 5. The saturation magnetization of Fe3O4/RGO was about 57 emu g
1, which was lower than that of Fe3O4 (68 emu g
1) due to the introduction of GO. The intrinsic hysteresis behavior for the Fe3O4 particles was maintained in the Fe3O4/RGO composite particles. This implies that the MR suspension based on Fe3O4/RGO particles will possess weaker MR performance compared with that of pure Fe3O4 particles, since saturation magnetization is a crucial factor to obtain a superior MR effect. The MR characterization was performed via a CSR mode for both Fe3O4 and Fe3O4/RGO particle-based MR fluids, using a shear

Fig. 2. TEM image of (a) Fe3O4, (b) Fe3O4/RGO.

rate test ranging between 0.01 and 200 s
1. The measurement interval was set from an initial 10 s to a final 1 s via log-log scale for each shear rate sweep test. The resulting flow curve responses were investigated as a function of the magnetic field strength ranging from 0 to 343 kA m
1, as given in Fig. 6. Both systems present similar Bingham behavior, where a plateau is observed at relatively low shear rate ranges, and a nearly linear behavior is detected at very high rates in a log-log plot. As expected, the obtained shear stress strongly depends on the applied magnetic field strength, which is similar to a previous work using an MR fluid [38], resulting from the formation of robust columns due to strong dipoledipole interactions among the adjacent magnetic particles. Based on the shear stress observed, we found that introducing GO sheets inevitably results in a negative effect when compared to pure Fe3O4 particles. However, the typical MR behavior was maintained. Shear thinning behavior was observed in both types of MR fluids at a fixed magnetic field strength, as shown in Fig. 8. The ChoChoi-Jhon (CCJ) model [39] was adopted to describe the flow

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1000

Shear stress (Pa)

Transmittance (%)

GO

Fe3O4/RGO

Fe3O4

100

10 Fe3O4

Fe 3O4/GO

0 kA/m 34 kA/m 86 kA/m 171 kA/m 257 kA/m 342 kA/m

1

0 kA/m 34 kA/m 86 kA/m 171 kA/m 257 kA/m 342 kA/m

0.1 0.1

0

500

1000

1500

2000

2500

3000

3500

4000

1

4500

10

100

Shear rate (1/s)

-1

Wavelength (cm ) Fig. 6. Shear stress as a function of shear rate for Fe3O4 (open symbol) and Fe3O4/ RGO (symbols).

Fig. 3. FT-IR spectra of GO, Fe3O4, and Fe3O4/RGO.

Fe3O4

3

10

Yield stress (Pa)

Intensity (a.u.)

Fe3O4/RGO

GO

1.5

2

10

1

Fe3O4/RGO

Fe3O4 2

10

10

20

30

40

50

60

70

80

90

Magnetic field strength (kA/m)

2θ Fig. 7. Dynamic yield stress vs. magnetic field for Fe3O4, and Fe3O4/RGO based MR fluid based MR fluid (Solid line obtained from CCJ model).

Fig. 4. XRD of GO, Fe3O4, and Fe3O4/RGO.

80

5

10 60

Fe3O4 0 kA/m 34 kA/m 86 kA/m 171 kA/m 257 kA/m 342 kA/m

4

40

Viscosity (Pa·s)

Magnetic moment (emu/g)

10

20 0 -20

3

10

Fe 3O4/GO 0 kA/m 34 kA/m 86 kA/m 171 kA/m 257 kA/m 342 kA/m

2

10

1

10

0

10 -40

-1

Fe3O4

-60

10

Fe3O4/RGO

-2

10

0.01

-80 -12

-8

-4

0

4

8

12

0.1

1

10

100

Shear rate (1/s)

Magnetic field (kOe) Fig. 5. VSM of GO, Fe3O4, and Fe3O4/RGO.

Fig. 8. Shear viscosity as a function of shear rate for Fe3O4 (open symbol) and Fe3O4/ RGO (symbols) based MR fluid.

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sy 1 þ ðt1 c_ Þ

a þ g1 1 þ

! 1 c_ b ðt2 c_ Þ

ð1Þ

where the first term in the equation contains the shear stress behavior at a low shear rate area, particularly with decreasing shear rate, and the second term describes the shear stress behavior at high shear rate. sy is the yield stress defined as the extrapolated stress in the low shear rate region; a is related to the decrease in shear stress, t1 and t2 are time constants, and g1 is the shear viscosity in the absence of a magnetic field. The exponent b varies between 0 and 1, because ds=c_ P 0. The solid lines in Fig. 6 were generated by the fitted parameters. Fitting using the CCJ model provided an accurate yield stress over the entire shear rate range (see Table 1). The dynamic yield stresses of the MR fluid which were obtained from Fig. 6 at a zero shear rate limit were plotted as a function of the applied magnetic field strength, and it is well known that the dynamic yield stress is proportional to the applied magnetic field as follows:

sy / H m

ð2Þ

The field dependence on the dynamic yield stress of the MR fluid follows a power law with an m slope. In Fig. 7, the slope was approximately 1.5 for the Fe3O4-based MR fluid, indicating that the Fe3O4 particles lead to stronger chains due to their magnetic dipole moment. For the Fe3O4/RGO-based MR fluid, the slope was approximately 1.0, confirming that the MR properties were affected by the addition of GO. The Fe3O4/RGO-based MR fluid exhibits relatively low shear stress, even with the formation of a fibrillar structure under applied magnetic field strengths. On the other hand, even though this result for the Fe3O4/RGO-based MR fluid cannot be directly compared with those from the recently reported GOcoated iron oxide/silica core-shell nanoparticle based MR fluid [15] because 70 vol% of GO-coated iron oxide/silica core-shell nanoparticle is much higher particle concentration than 70 wt%

Table 1 Parameters in CCJ model obtained from the flow curve of both Fe3O4 and Fe3O4/RGO based MR fluids at various magnetic field strength. Parameters 34 kA/m 86 kA/m 171 kA/m 257 kA/m 342 kA/m Fe3O4

sy t1

a g1 t2 b Fe3O4/RGO sy t1 Α

g1 t2 b

96 0.0041 0.3195 0.81 0.0212 0.8875

284 0.0159 0.4173 1.20 0.0134 0.8576

795 0.0039 0.6366 2.45 0.6909 0.9741

1377 0.2063 0.5784 2.75 0.0051 0.8958

1123 0.5566 0.8040 2.93 0.0060 0.8637

34 0.0045 0.3154 0.33 0.0128 0.7987

65 0.0055 0.2579 0.53 0.0126 0.8543

133 0.5668 0.0275 0.81 0.0088 0.9967

256 0.0490 0.1868 0.97 0.0155 0.9676

190 0.1969 0.1518 0.86 0.0075 0.9539

GðtÞ ffi G0 ðxÞ
0:560G00 ðx=2Þ þ 0:200G00 ðxÞ

ð3Þ

6

(a)

10

Storage Modulus (Pa)

s¼

(about 50 vol%) in this study when their particle densities are being considered, for the similar magnetic flux density applied of 0.5 T which corresponds to our 343 kA/m in its magnetic field strength, the yield stress of 260 Pa was obtained while about 400 Pa was estimated for the GO-coated iron oxide/silica core-shell nanoparticle [15]. The main source of the difference might come from the particle concentration difference and different structure. The oscillatory tests were carried out under six different magnetic field strengths between 0 and 342 kA m
1 in order to investigate the viscoelastic characteristics of the MR fluids when a magnetic field is applied. It is evident that at a fixed magnetic field strength, G0 values are independent of the frequency, where all samples exhibit a plateau behavior over the wide range of frequencies studied (Fig. 9). This indicates that the samples possess a very strong solid-like behavior rather than a liquid-like state, showing a dominant elastic property over the viscous one. This difference could be due to the higher magnetic properties of the Fe3O4 particles compared to those in the Fe3O4/RGO system. The stress relaxation measurements can be estimated from the dynamic moduli data obtained. Fig. 10 shows the stress relaxation behavior, and the stress relaxation modulus (G(t)) was estimated from the measured values of G0 (x) and G00 (x) in Fig. 9 through the Schwarzl equation, which is a numerical formula given by Eq. (3) [40].

10

5

4

10

Fe3O4 3

0 kA/m 34 kA/m 86 kA/m 171 kA/m 257 kA/m 342 kA/m

10

Fe 3O4/GO 0 kA/m 34 kA/m 86 kA/m 171 kA/m 257 kA/m 342 kA/m

2

10

1

10

Frequency (Hz)

(b) 105

Loss Modulus (Pa)

curves of the Fe3O4 and Fe3O4/RGO particle-based MR fluids, as shown in Eq. (1). This is a six-parameter model widely used to fit the experiment data for a range of MR suspensions. At each magnetic field strength applied, the shear stress demonstrated a plateau behavior in the low shear rates. The plateau shear stress region became widened when the magnetic field was increased, suggesting that the attractive magnetic force of the chains becomes strong enough to resist the hydrodynamic breaking force. On the other hand, the dynamic yield stress can be obtained from the flow curves by extrapolating the shear stress to a zero shear rate limit [20]. The yield stress is one of the critical physical parameters in magnetorheology. The Cho-Choi-Jhon (CCJ) equation has been used as a suitable rheological model for the steady shear behavior of many MR fluids as follows:

4

10

3

10

Fe3O4 0 kA/m 34 kA/m 86 kA/m 171 kA/m 257 kA/m 342 kA/m

2

10

1

Fe 3O4/GO 0 kA/m 34 kA/m 86 kA/m 171 kA/m 257 kA/m 342 kA/m

10

Frequency (Hz) Fig. 9. Frequency dependence of the (a) storage and (b) loss modulus, G0 and G0 0 for Fe3O4 (open symbol) and Fe3O4/RGO (symbols) based MR fluid.

C.H. Hong et al. / Journal of Colloid and Interface Science 481 (2016) 194–200

Relaxation modulus [G(t)]

200

5

10

Fe3O4 0 kA/m 86 kA/m 171 kA/m 257 kA/m 342 kA/m

4

Fe 3O4/GO 0 kA/m 86 kA/m 171 kA/m 257 kA/m 342 kA/m

10

of Fe3O4 particles loaded on the surface of GO was confirmed via SEM and TEM images. FT-IR allowed the verification of the chemical structure for the synthesized Fe3O4/RGO composites. Although the MR characterization exhibited lower shear stress values compared with a pure Fe3O4-based MR fluid, the typical MR behavior was well preserved. Further effort should be made on fabricating Fe3O4/RGO composite particles, which is considered a crucial role in presenting superior MR properties. Finally, the sedimentation rate was considerably improved due to the decreased density mismatch between Fe3O4/RGO composite particles (1.96 g cm
3) and oil medium (0.96 g cm
3). The CCJ model was adopted to fit the complex shear stress behavior, and was found to accurately fit the experiment data in the entire shear rate region.

3

10

Acknowledgement -1

10

0

1

10

10

Time [sec] Fig. 10. Relaxation modulus G(t) of Fe3O4 and Fe3O4/RGO based MR fluid.

This research was supported by both Ministry of Trade, Industry & Energy, Korea through Daeheung RNT (#10047791) and National Research Foundation, Korea (2016R1A2B4008438). References

Fig. 11. Sedimentation ratio of Fe3O4 and Fe3O4/RGO based MR fluid.

Finally, we studied the sedimentation stability. By using GO as a plate additive, the density of Fe3O4/RGO composites was reduced to 1.96 g cm
3, which is nearly 40% that of pristine Fe3O4 particles. Compared with pure Fe3O4-based MR fluid, the density mismatch between Fe3O4/RGO particles and continuous oil (0.96 g cm
3) was obviously reduced. Thus, the sedimentation stability was improved. Fig. 11 indicates the recorded sedimentation ratio as a function of time. The inset graph is a magnified view of the initial 5 h. It is obvious that the Fe3O4/RGO suspension exhibits improved sedimentation stability than that of pure Fe3O4 suspension in the same testing duration. Pure Fe3O4 suspension settles down rapidly within the initial 2 h, while the Fe3O4/RGO suspension provides a nearly stable dispersion. Thus, loading Fe3O4 nanoparticles on the surface of RGO can enhance the sedimentation stability due to the apparently reduced density mismatch. Furthermore, based on the previous report on the effect of the GO as an additive for the conventional CI based MR fluids [41], our new Fe3O4/RGO magnetic composite particles could not only enhance MR performance but also dispersion stability for the CI based MR fluids. 4. Conclusion In this study, novel Fe3O4/RGO nanocomposites were prepared to improve the sedimentation stability for MR fluids. The presence

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