Giant electrorheological fluid comprising nanoparticles: Carbon nanotubecompositeJiaxing Li, Xiuqing Gong, Shuyu Chen, Weijia Wen, and Ping Sheng Citation: J. Appl. Phys. 107, 093507 (2010); doi: 10.1063/1.3407503 View online: http://dx.doi.org/10.1063/1.3407503 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v107/i9 Published by the American Institute of Physics. Related ArticlesInfluence of volume fraction on the yield behavior of giant electrorheological fluid Appl. Phys. Lett. 101, 101908 (2012) Liquid flow retardation in nanospaces due to electroviscosity: Electrical double layer overlap, hydrodynamicslippage, and ambient atmospheric CO2 dissolution Phys. Fluids 24, 072001 (2012) Effect of wall permittivity on electroviscous flow through a contraction Biomicrofluidics 5, 044102 (2011) Giant electrorheological effect in Fe2O3 nanofluids under low dc electric fields J. Appl. Phys. 108, 034306 (2010) Ensemble-averaged particle orientation and shear viscosity of single-wall-carbon-nanotube suspensions undershear and electric fields Phys. Fluids 22, 022001 (2010) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

Downloaded 21 Nov 2012 to 82.2.119.110. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

Giant electrorheological fluid comprising nanoparticles: Carbonnanotube composite

Jiaxing Li, Xiuqing Gong, Shuyu Chen, Weijia Wen,a� and Ping ShengDepartment of Physics and Institute of Nano Science and Technology, The Hong Kong University of Scienceand Technology, Clear Water Bay, Kowloon, Hong Kong

�Received 10 February 2010; accepted 12 March 2010; published online 3 May 2010�

We have fabricated suspensions exhibiting the giant electrorheological �GER� effect comprisingnanoparticles—multiwall carbon nanotubes �MCNTs� composite particles dispersed in silicone oil.This type of GER fluids display dramatically enhanced antisedimentation characteristic withoutsacrificing the yield stress. The nanoparticles-nanotubes composites were fabricated by modifyingthe coprecipitation method with MCNTs and urea-coated barium titanyl-oxylate �BTRU�nanoparticles as the components. The composite solid particles are denoted MCNT-BTRU. In thebest cases, stabilized suspensions with MCNT-BTRU particles dispersed in silicone oil have beenmaintained for several months without any appreciable sedimentation being observed. Both thesedimentary and rheological properties of the MCNT-BTRU suspension were systematically studiedand compared with their BTRU counterparts. Yield stress as high as 194 kPa was obtained in theMCNT-BTRU suspensions. The MCNT-BTRU based GER fluids, with their antisedimentationcharacteristic, may have broad engineering applications. © 2010 American Institute of Physics.�doi:10.1063/1.3407503�

I. INTRODUCTION

Electrorheological �ER� fluids1–31 are a type of colloidalsuspensions, comprising microparticles or nanoparticles dis-persed in nonconducting oil, that can display viscosity varia-tion or even solidification in response to an electric field. Forthe past two to three decades, there has been a persistentresearch effort in exploring the mechanism of the EReffect,7–18 in synthesizing new materials that can exhibitstrong ER effect,14,17–21 and in realizing ER-based activeelectrical-mechanical devices.7–13 However, the lack of highperformance materials, as well as the problem associatedwith no sedimentation, have inhibited broad engineering ap-plications. The discovery of giant ER �GER� effect,14–16,19–22

in urea-coated nanoparticles, has presented a new paradigmby utilizing permanent molecular dipoles to attain yieldstress values that break the theoretical upper bound of thetraditional ER effect, which is based on induced momentsarising from dielectric constants contrast between the fluidand the solid phases. However, GER fluids exhibit the samedrawback as other ER fluids in the sedimentation phenom-enon, owing to the density mismatch of the fluid and solidphases as well as the aggregation of the nanoparticles. Theresulting phase separation can cause a dramatic decrease inthe ER effect. Several methods have been devised to improvethe sedimentation property of the ER fluids, e.g., by addingsurfactant to the solvent phase20 or making the particle phaseless dense that can either decrease the density mismatch ormodify the surface or particle morphology,23–31 or both. TheER effect of the treated sample was generally lowered as aresult.

In this work, we modify the process of fabricating urea-coated barium titanyl-oxalate �BTRU� nanoparticles by em-

ploying the methods of preparing polymer �or oxide�-carbonnanotube �CNT� composites29,31–34 to the synthesis of par-ticles comprising urea-coated particles of multiwall CNT-BTRU �MCNT-BTRU�. The particles, when dispersed in dif-ferent type of silicone oil, are shown to have enhancedantisedimentation property. In the best cases a stabilized ERsuspension can be maintained as long as several monthswithout any appreciable observable sedimentation. The yieldstress of the MCNT-BTRU particle suspensions is tested tobe as high as 190 kPa, at most 10% lower than the BTRU-based GER fluids with similar solid concentrations.

II. EXPERIMENT

A. Materials, preparation, and characterization ofMCNT-BTRU and BTRU

MCNTs �synthesized by a thermal chemical vapor depo-sition method, �Shenzhen Nanotech Port Co. Ltd�� weretreated with the 3 M nitric acid, filtrated, rinsed with deion-ized water and then dispersed in a solution of titanium tetra-chloride. MCNT-BTRU nanoparticles were fabricated bymodifying the coprecipitation method as described in ourprevious work.21 In the presence of saturated urea solution,solutions of barium chloride, MCNTs, titanium tetrachloride,and oxalic acid were coprecipitated at 65 °C to form colloi-dal sols. The MCNT-BTRU particles were filtrated andwashed using deionized water to remove aqueous acid, andsubsequently dried in a freeze drier. Silicone oils with themethyl, hydroxyl, phenyl, or diglycidyl group termination,supplied by the Sigma-Aldrich Chemical Co., were dried by4 Å molecular sieves before the experiment to remove anyresidual moisture. GER fluids were formed by mixing theBTRU or MCNT-BTRU particles with different silicone oils.Concentration of the GER fluids can be denoted as the

a�Author to whom correspondence should be addressed. Electronic mail:phwen@ust.hk.

JOURNAL OF APPLIED PHYSICS 107, 093507 �2010�

0021-8979/2010/107�9�/093507/5/$30.00 © 2010 American Institute of Physics107, 093507-1

Downloaded 21 Nov 2012 to 82.2.119.110. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

amount of silicone oil, in unit of milliliter, mixed with eachgram of particles. Hence, 5 means 5 ml of oil mixed with1 g of particles.

The morphology of the BTRU particles was visualizedon a JEOL-6700F scanning electron microscopy �SEM� withan acceleration voltage of 15 kV. To prepare the SEMsample, 10 mg sample treated by silicone oil was dispersedinto 2 ml ethanol by ultrasonication. Then one drop of thesuspension was transferred to the surface of a p-type siliconslice. After volatilization of the ethanol, the sample wasgold-coated to increase its electric conductivity before theSEM measurement. The Fourier transform IR �FT-IR� trans-mission spectrum was recorded on a Bio-Rad FTS6000 spec-trometer with a DTGS detector. The number of scan is 32with a spectral resolution 4 cm−1.The samples of BTRU andMCNT-BTRU adsorbed silicone oil were prepared by the ourpervious method.21

B. Characterization of rheological properties

Rheological measurements were performed on acircular-plate type viscometer �Haake RS1� with an 8-mm-diameter rotating disk and a gap of 1 mm between rotor andstator. A PM 5134 �Philips� functional generator was used togenerate step signals for driving the dc high-voltage source�SPELLMAN SL300�. Software package RHEOWIN was usedto collect experimental data. The GER fluid was injected inthe 1 mm gap between the rotor and stator. A power-on timewas set to be 50 s with a square pulse applied to the sample.For reproducibility and repeatability of the measurements,GER fluids were redispersed before each measurement, andeach measurement was repeated at least three times. Shearstress as a function of time was measured at very low shearrates �0.1 s−1�. Yield point was reached when a stress-timecurve changed its slope to be flat after an abrupt increase atthe beginning of turning on the field. The yield stress at agiven field was taken to be the maximum of the shear stressin the corresponding time span. All the samples were mea-sured at 25 °C.

III. RESULTS AND DISCUSSION

A. Sedimentation property of MCNT-BTRU

Sedimentation occurs in colloidal systems because of thedensity mismatch between the solid and fluid phases, accen-tuated by particles aggregation through van der Walls inter-action between the particles as well as the nonfavorableparticle-solvent interactions. We have measured the sedimen-tation ratio of six GER fluid samples prepared with differentsolid particle concentrations. Sample 1 comprises just theBTRU nanoparticles dispersed in 20 cSt methyl-terminatedsilicone oil; samples 2, 3, 4, 5, and 6 were prepared by usingthe same silicone oil but with MCNT-BTRU nanoparticles inwhich the weight fraction of MCNTs are 0.22%, 0.49%,1.0%, 2.0%, 4.0%, respectively. All the samples were pre-pared to have the same concentration of 5. The sedimentationratio is defined as a / �a+b� where a is the height of the GERfluid, characterized by its opaque appearance, and b denotesthe height of clear oil phase. Figure 1�a� shows the sedimen-tation ratio as a function of time �hour� for the six samples. It

can be seen that the sedimentation ratio for each of thesamples decreases initially, for several tens of hours, andthen approaches a stable asymptotic value. It is seen thatwith 0.22% addition of MCNTs, the sedimentation ratio isalready enhanced to over 60%, from the original �40%, andwith 4% addition of MCNTs, the sedimentation ratio is over90%, i.e., sedimentation is essentially negligible in the lattercase. This is especially notable because the concentration of5 is fairly dilute for the GER fluid, and in the dilute cases thesedimentation is generally a serious problem.

The antisedimentation characteristic exhibited by theMCNT-BTRU particles is attributable to the fact that thecomposite material has a lower density, accompanied by achange in the morphology that can be seen from the SEMimages shown in Fig. 2�a�. In the left panel of Fig. 2�a� areshown the BTRU particles. This should be contrasted withthe image of the MCNT-BTRU particles shown in the rightpanel. Both BTRU and MCNT-BTRU particles are roughlyspherical in shape and less than 500 nm in size. MCNTs canbe visualized with one or both ends piercing some MCNT-BTRU particles, in effect forming a skeletal structure inwhich the MCNTs are the beams. Such structure is formeddue to the nitric acid treatment of the MCNTs that leads tothe presence of many carboxyl and hydroxyl groups on thesurface and �even more� at both ends.35,36 Those groups canreact with titanium to form a structure similar to carboxyl-titanium in BaTiO�C2O4�2. The same mechanism can also befound in the formation of CNT/iron oxide magneticcomposite.32–34 Aggregation of BTRU nanoparticles isclearly seen from the left panel of Fig. 2�a�. The presence ofMCNTs effectively prevents direct contact between the

FIG. 1. �Color online� �a� Sedimentation ratio of BTRU and MCNT-BTRUparticles suspended in methyl terminated silicone oil �20 cSt� with a con-centration of 5.0. The MCNTs mass fraction contained in the compositematerial �from which the particles are fabricated� is given in the text. �b�Images of samples 1–6 taken at 12.5 days after preparation.

093507-2 Li et al. J. Appl. Phys. 107, 093507 �2010�

Downloaded 21 Nov 2012 to 82.2.119.110. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

BTRU nanoparticles, thus minimizing their aggregation andsedimentation. The addition of MCNTs, therefore, introducesan effective short range repulsive interaction between theBTRU nanoparticles. However, such repulsive interactionmay also serve as an energy barrier that has to be overcomein order to achieve the ER effect of forming chains/columnsunder an electric field. A lower yield stress may result. Weshow below that while the yield stress of the composite par-ticles is indeed lowered by �10%, the measured value canstill be as high as 200 kPa.

Maintaining good dispersion and avoiding aggregationare important for both the antisedimentation characteristic aswell as the ER effect. In this context the wetting of theMCNT-BTRU composite particles by silicone oil is a verycrucial element. We show that the adsorption of silicone oilon MCNT-BTRU particles is indeed present as measured byFTIR spectroscopy.

We have measured both BTRU and MCNT-BTRUsamples treated by silicone oil solution,21 shown in Fig. 3.For reference, results of pure MCNT-BTRU and BTRUsamples were also recorded. The small �absorption� peaks at1050 and 1150 cm−1, attributable to the asymmetric stretch-ing vibration frequency of Si–O–Si �Ref. 21� of the attachedsilicone oils, are observed to be stronger for oil-treatedMCNT-BTRU sample �line d� than oil-treated BTRU sample�line c�. Without the oil-treatment, neither samples displaythese two peaks. This indicates that those peaks come solelyfrom the adsorption of silicone oil, and the signal for theMCNT-BTRU sample is higher simply because the compos-ite materials particles present a larger internal surface area asthe MCNTs separated the BTRU nanoparticles.

B. Yield stress measurements

When designing an ER device, the viscosity of the ma-terial without applied field is always an important factor to

be considered. We have measured the zero-field viscosity ofsamples 7–12 with the mass fraction of MCNTs varyingfrom 0% to 0.22%, 0.49%, 1.0%, 2.0%, 4.0%, respectively.The concentration of these samples is 1.0, implying a highersolid particle fraction than those in samples 1–6. In samples7–12, 20 cSt silicone oil were used. Results are shown inFig. 4. It is found that the zero field viscosity generally in-creases with increasing mass fraction of MCNTs. The in-crease is small up to 0.5% but becomes large above 1%. It isseen that the zero field viscosity decreases with increasingshear rate.

We have measured the yield stress for samples 7–12 andplotted the results in Fig. 5. The GER effect is seen to de-crease with increasing MCNTs content. This has been antici-pated. However, for the 0.22% case the decrease in the yieldstress is 10% at most, although the antisedimentation effectis dramatic. This is seen in the insets to Fig. 5 which showthe images of samples 7–9 taken two months after they wereprepared. With a concentration of 1.0, even the sample withthe 0.22% addition of MCNTs shows no observable sedi-mentation.

FIG. 2. �Color online� �a� SEM images of BTRU particles �left� and MCNT-BTRU particles �right� treated by silicone oil and ethanol. The red arrowspoint to the MCNTs that can be clearly visualized as the “beam” that sepa-rate the BTRU particles. �b� Schematic illustration of the microstructuresstructures in the two cases. The left panel shows the BTRU particles aggre-gated together, whereas the right panel shows MCNT-BTRU particle tocomprise MCNTs “beams” separating the BTRU particles.

FIG. 3. �Color online� FT-IR spectra showing MCNT-BTRU particlestreated by silicone oil �Ref. 21� �curve a�, bare MCNT-BTRU particles�curve b�, and their counterparts of BTRU particles �curves c and d�.

FIG. 4. �Color online� Zero-field viscosity plotted as a function of shearrate, for samples 7–12 whose compositions are described in the text. Theconcentration for all the samples shown is 1.0.

093507-3 Li et al. J. Appl. Phys. 107, 093507 �2010�

Downloaded 21 Nov 2012 to 82.2.119.110. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

We have also investigated the influence of the liquidphase on the GER effect, by using silicone oils with threedifferent types of termination, i.e., with the methyl group, thehydroxyl group, or the diglycidyl group. In this case we usethe MCNT-BTRU particles with a MCNT mass fraction of0.49% and at a concentration of 1.0. These samples are de-noted samples 13–15. As shown in Fig. 6, the yield stress ofsample 13, with the hydroxyl group terminated silicone oil,is the highest, followed by that of sample 14. This is com-pletely consistent with the results shown in Ref. 21. Theliquid phase effect on sedimentation property was tested forsamples 13–15 and compared with their counterparts pre-pared with BTRU particles �denoted samples 16–18�. It wasfound that MCNT-BTRU particles can always form a stable

suspension regardless of the type of silicone oil used,whereas the samples prepared by using the BTRU particlesalways exhibit sedimentation.

How large a yield stress can be attained by the new typeof GER fluid? In order to answer this question, we show inFig. 7 that the yield stress of the sample, prepared by dis-persing MCNT-BTRU particles �with a MCNTs mass frac-tion of 0.49%� in hydroxyl-terminated silicone oil at a con-centration of 0.25, can reach 194 kPa at 5 kV/mm. This isonly slightly smaller than that for similar sample comprisingBTRU nanoparticles �215 kPa at the same conditions�.21

IV. CONCLUSIONS

By using a coprecipitation method, we have fabricatedMCNT-BTRU particles that exhibit the GER effect. Whendispersed in silicone oil, the antisedimentation property isenhanced dramatically, as compared to previously fabricatedBTRU nanoparticles. The electrorehological effect can re-main nearly the same.

ACKNOWLEDGMENTS

The authors would like to acknowledge Hong KongRGC under Grant Nos. HKUST 602207, 621006, and603608 for the financial support of this project. The workwas also partially supported by the Nanoscience and Nano-technology Program at HKUST.

1W. M. Winslow, J. Appl. Phys. 20, 1137 �1949�.2H. Block and J. P. Kelly, J. Phys. D 21, 1661 �1988�.3R. Tao and J. M. Sun, Phys. Rev. Lett. 67, 398 �1991�.4M. Whittle and W. Bullough, Nature �London� 358, 373 �1992�.5T. Halsey, Science 258, 761 �1992�.6T. Chen, R. N. Zitter, and R. Tao, Phys. Rev. Lett. 68, 2555 �1992�.7L. C. Davis, Appl. Phys. Lett. 60, 319 �1992�.8L. C. Davis, J. Appl. Phys. 72, 1334 �1992�.9H. Ma, W. Wen, W. Y. Tam, and P. Sheng, Phys. Rev. Lett. 77, 2499�1996�.

10W. Y. Tam, G. H. Yi, W. Wen, H. Ma, M. M. T. Loy, and P. Sheng, Phys.Rev. Lett. 78, 2987 �1997�.

11W. Wen, N. Wang, H. Ma, Z. Liu, W. Y. Tam, C. T. Chan, and P. Sheng,Phys. Rev. Lett. 82, 4248 �1999�.

12H. Ma, W. Wen, W. Y. Tam, and P. Sheng, Adv. Phys. 52, 343 �2003�.13P. Tan, W. J. Tian, X. F. Wu, J. Y. Huang, L. W. Zhou, and J. P. Huang, J.

Phys. Chem. B 113, 9092 �2009�.

FIG. 5. �Color online� Measured GER effect for samples 7–12. Shear stressis plotted as a function of applied electrical field �square pulse with apower-on time of t=50 s, shown in the inset�. The horizontal axis is markedby the corresponding field values instead of time. The upper inset shows theimages of samples 7–9 taken two months after their preparation. It is seenthat whereas sample 7, comprising BTRU particles dispersed in silicone oil,shows clear sign of sedimentation, samples 8 and 9 do not exhibit anyobservable sedimentation.

FIG. 6. �Color online� Shear stress plotted as a function of applied �squarepulse� electric field for a sample comprising MCNT-BTRU particles �withMCNTs mass fraction of 0.49%� suspended in diglycidyl-, methyl-, andhydroxyl-terminated silicone oils at the concentration of 1.0. The insetshows the images of MCNT-BTRU samples and their BTRU counterpartstaken two months after they were prepared. The antisedimentation effect isindependent of the oil used for dispersion.

FIG. 7. Shear stress of MCNT-BTRU nanoparticles �MCNTs fraction0.49%� dispersed in hydroxyl-group-terminated silicone oil �25 cSt�, Theconcentration is 0.25.

093507-4 Li et al. J. Appl. Phys. 107, 093507 �2010�

Downloaded 21 Nov 2012 to 82.2.119.110. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions

14W. Wen, X. Huang, S. Yang, K. Lu, and P. Sheng, Nat. Mater. 2, 727�2003�.

15X. Huang, W. Wen, S. Yang, and P. Sheng, Solid State Commun. 139, 581�2006�.

16W. Wen, X. Huang, and P. Sheng, Soft Mater. 4, 200 �2008�.17R. Shen, X. Wang, Y. Lu, D. Wang, G. Sun, Z. Cao, and K. Lu, Adv.

Mater. 21, 4631 �2009�.18J. Y. Hong, E. Kwon, and J. Jang, Soft Mater. 5, 951 �2009�.19W. Wen, X. Huang, and P. Sheng, Appl. Phys. Lett. 85, 299 �2004�.20C. Shen, W. Wen, S. Yang, and P. Sheng, J. Appl. Phys. 99, 106104

�2006�.21X. Gong, J. Wu, X. Huang, W. Wen, and P. Sheng, Nanotechnology 19,

165602 �2008�.22X. Niu, M. Zhang, J. Wu, W. Wen, and P. Sheng, Soft Mater. 5, 576

�2009�.23Y. C. Cheng, X. H. Liu, J. J. Guo, F. H. Liu, Z. X. Li, G. H. Xu, and P. Cui,

Nanotechnology 20, 055604 �2009�.24B. X. Wang, X. P. Zhao, Y. Zhao, and C. L. Ding, Compos. Sci. Technol.

67, 3031 �2007�.

25B. X. Wang, M. Zhou, Z. Rozynek, and J. O. Fossum, J. Mater. Chem. 19,1816 �2009�.

26B. X. Wang, Y. Zhao, and X. P. Zhao, Colloids Surf., A 295, 27 �2007�.27B. X. Wang and X. P. Zhao, J. Mater. Chem. 13, 2248 �2003�.28Y. C. Cheng, J. J. Guo, G. J. Xu, P. Cui, X. H. Liu, F. H. Liu, and J. H. Wu,

Colloid Polym. Sci. 286, 1493 �2008�.29C. S. Choi, S. J. Park, and H. J. Choi, Curr. Appl. Phys. 7, 352 �2007�.30H. J. Choi and M. S. Jhon, Soft Mater. 5, 1562 �2009�.31P. Slobodian, V. Pavlinek, A. Lengalova, and P. Saha, Curr. Appl. Phys. 9,

184 �2009�.32Z. Huang, J. Li, Q. W. Chen, and H. Wang, Mater. Chem. Phys. 114, 33

�2009�.33C. L. Chen, J. Hu, D. D. Shao, J. X. Li, and X. K. Wang, J. Hazard. Mater.

164, 923 �2009�.34C. L. Chen, X. K. Wang, and M. Nagatsu, Environ. Sci. Technol. 43, 2362

�2009�.35N. W. S. Kam, T. C. Jessop, P. A. Wender, and H. J. Dai, J. Am. Chem.

Soc. 126, 6850 �2004�.36R. F. Service, Science 304, 1732 �2004�.

093507-5 Li et al. J. Appl. Phys. 107, 093507 �2010�

Downloaded 21 Nov 2012 to 82.2.119.110. Redistribution subject to AIP license or copyright; see http://jap.aip.org/about/rights_and_permissions