Rheol Acta (2011) 50:87–95DOI 10.1007/s00397-010-0516-z

ORIGINAL CONTRIBUTION

Micro/nano-structured montmorillonite/titania particleswith high electrorheological activity

Liqin Xiang · Xiaopeng Zhao · Jianbo Yin

Received: 10 March 2010 / Revised: 14 August 2010 / Accepted: 29 November 2010 / Published online: 21 December 2010© Springer-Verlag 2010

Abstract A low-cost electrorheological (ER) materialmade of micro/nano-structured montmorillonite/titaniaparticles was prepared by a one-pot solvothermalmethod. The micro/nano-structured particles werecharacterized by X-ray diffraction, Fourier transforminfrared spectra, and scanning electron microscopy.It was found that the nanorod-like titania assembledon the surface of montmorillonite, the diameters ofthe nanorods were about 30 nm, and the lengthswere about 300 nm. The electrorheological property ofthe micro/nano-structured particles in silicone oil wasmeasured under dc electric fields. It was found thatthe micro/nano-structured montmorillonite/titania ERfluid exhibited much stronger electrorheological effectcompared to pure montmorillonite and pure titaniananorod ER fluids, while its leaking current densitywas significantly lower than that of montmorilloniteER fluid. The stronger electrorheological effect mightbe attributed to the larger interfacial polarization andinterparticle friction, which originated from the uniquestructure and morphology of micro/nano-structuredparticles, compared to pure montmorillonite and puretitania nanorods.

Keywords Micro/nano-structure · Montmorillonite ·TiO2 nanorod · Electrorheology

L. Xiang · X. Zhao (B) · J. YinDepartment of Applied Physics,Institute of Electrorheological Technology,Northwestern Polytechnical University,Xi’an, 710072, People’s Republic of Chinae-mail: xpzhao@nwpu.edu.cn

Introduction

Micro/nano-structured materials have attracted largeinterests in the development of new-type functionalmaterials recently (Whitesides and Grzybowski 2002;Bartl et al. 2005; O’Dwyer et al. 2006). The importantcharacteristic of the micro/nano-structured materialsis that they possess both nano-scale and micro-scalestructures (Bartl et al. 2005). This hierarchical characteroften endows the kind of materials with advantagesfrom nano-scale and micro-scale structures. Therefore,micro/nano-structured materials promote much exper-imental activity for use in photoelectronics, catalysts,semiconductors, and so on (Bartl et al. 2005; O’Dwyeret al. 2006). Here, we present novel and low-costmicro/nano-structured montmorillonite/TiO2 particlesand their electrorheological (ER) application. ER fluid,usually referred to as “smart fluid”, is a kind of sus-pension consisting of polarizable particles dispersed inan insulating media. When an external electric field isapplied, the suspension can change from a fluid-likestate to a solid-like state and the rheological change israpid, reversible, and adjustable (Halsey 1992; Blockand Kelly 1988). This behavior of ER fluid has at-tracted much interest for uses in many applications,such as valve, damping systems, brakes, miniature ro-botic joints, and other areas such as ultra fine polishing,display, ink jet printers, human muscle stimulators,mechanical sensors, and so on (Gast and Zukoski 1989;Couter et al. 1993; Parthasarathy and Klingenberg 1996;Zhao et al. 2006). To satisfy these applications, the ERfluid with high yield stress, low current density, optimalworking temperature, suspended stability, and low-costis especially needed (Zhao and Yin 2006; Fang et al.2008).

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Montmorillonite (MMT) is a kind of versatilemineral with various applications in absorbents,medicaments, catalysis, rheological control of coat-ing and cosmetics, and so on (Dong and Feng 2005;Manikandana et al. 2007; Sudha and Sasikala 2007;Han et al. 2008). These widespread uses of MMT arisefrom its unique properties such as layered structures,large surface area, and high cation exchange capacity.Furthermore, the good environment compatibility andlow cost are other reasons of that MMT is so favorable.MMT is also potential as an ER active material (Gastand Zukoski 1989; Fang et al. 2008; Kim et al. 1999).However, the open lamellar structure of MMT easilyresults in a strong absorption to moisture and impurity,which promotes the movement of cations in interlayerso that the leaking current density increases rapidlyand even electric breakdown happens. All of these willdecrease ER performances. There are several reportsinvestigating ER properties of modified MMT by inter-calated organic (Kim et al. 1999; Lim et al. 2002; Lu andZhao 2002, 2004). However, the ER performance of themodified MMT was still not as high as that researchersexpected.

TiO2 is not only a popular and low-cost material inmany fields such as photocatalyst, catalyst, solar energycell, gas sensor, and pigment (Yu et al. 2007; Yang andGao 2005; Pietron et al. 2007) but also an importantER material due to high dielectric constant. However,the ER effect of TiO2 is very weak under dc electricfield, which has been attributed to its relatively lowconductivity. According to dielectric design, rare-earth-doped TiO2 and mesoporous rare-earth-doped TiO2

ER materials have been prepared and demonstrated tohave high ER effect (Zhao and Yin 2002; Yin and Zhao2002, 2004), but rare earth elements is of high cost.To attempt to combine the advantages of MMT andTiO2, we previously prepared a composite ER materialcomposed of MMT coated by TiO2 nanocrystallites bya sol-gel technique (Xiang and Zhao 2006).

On the other hand, besides the composition andstructure, the morphology of ER particles is also acritical factor influencing ER performances (Zhao andYin 2006; Zhao et al. 2006). The ER particles withrough surface possibly have stronger ER effect com-pared to smooth particles according to the recent the-ory analysis and experiment result (López-López et al.2009; Yin et al. 2009). Our group has prepared a kindof micro/nano-structured kaolinite/titanate compositewith rough surface state and did a preliminary inves-tigation about the ER property (Wang and Zhao 2005).However, it was also found that the kaolinite clay waseasily destroyed by the strong base medium in the

reaction, which resulted in the difficulty of morphologycontrol and reproduction.

In the present study, we choose MMT and TiO2 asthe ER bases for their low cost and special proper-ties but employ a one-pot solvothermal approach todevelop novel micro/nano-structured particles made ofMMT enclosed by TiO2 nanorods for ER fluid applica-tion. The approach is simple and highly reproducible.Especially, the micro/nano-structured MMT/TiO2 par-ticles possess unique rough surface due to the assemblyof TiO2 nanorods. It is found that the ER fluid basedon micro/nano-structured MMT/TiO2 particles possessnot only significantly decreased leaking current densitybut also stronger ER properties compared to the ERfluids of pure MMT, pure TiO2 rods, and the simplemixture of MMT and TiO2 nanorods. We present thepreparation, characterization, and ER properties andgive a further discussion about ER properties in termsof dielectric analysis.

Experimental section

Material preparation

Na+-MMT, from Qinghe Chemical Cooperation ofZhangjiakou, China, was used without further pu-rification. Besides SiO2 and Al2O3, the MMT also con-tains some other cations, such as Na+, Mg2+, and Ca2+.The molar ratio of ions is about 74.35% (Si), 14.46%(Al), 3.83% (Mg), 1.66% (Ca), and 5.70% (Na). All ofthe other chemical reagents used in experiments wereanalytical grade without further purification.

A typical preparation process was shown as follows:Firstly, titanium tetrachloride (TiCl4) was dissolvedinto distilled water in an ice-water bath to obtain TiCl4aqueous solution, in which the mole concentration ofTiCl4 was 1.5 M. Secondly, 1 g of MMT was swelled in30 ml of toluene and the resulted mixture was furtherstirred for 2 h at room temperature to obtain a uniformsuspension. Thirdly, 6 ml of tetrabutyl titanate (TBT)was slowly added dropwise into the MMT/toluene sus-pension above and further stirred for 2 h in an ice-waterbath. Fourthly, 2 ml of TiCl4 aqueous solution (1.5 M)above was added slowly into the suspension consistingof TBT, MMT, and toluene. After stirring for 1 h,the suspension was transferred into a 50-ml stainlesssteel autoclave lined with Teflon and held at 120◦C for24 h to form precipitates. Finally, the precipitates werefiltered and washed with ethanol for several times, andthen dried at 70◦C to obtain the product. The pure TiO2

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nanorods were synthesized by the same process whileno MMT was added.

Characterization

The crystallographic structure of the samples was char-acterized by the powder X-ray diffraction (XRD) pat-terns at a Philips X’Pert Pro X-ray diffractometer withCuKα irradiation (40 kV/35 mA) and step size of 0.033◦in the 2θ range of 5–70◦. The weight fraction of anatasephase in titania nanorods was estimated by the equationfa = (1 + 1.26Ir/Ia)

−1, where fa was weight fractionof anatase phase and Ir and Ia were the intensitiesobtained from the integral areas of peaks of the (110)of rutile and (101) of anatase reflections, respectively.The content of titania nanorods in the micro/nano-structured particles was estimated by the Ti/Al or Ti/Siratio in the energy-dispersive spectra (JSM-6700F). Theparticle morphology was observed by the scanning elec-tron microscopy (SEM, JSM-6700F) and the Fouriertransform infrared (FT–IR) spectra in the mid-IR range(4,000–400 cm−1) were obtained on a JASCO FT/IR-470 plus Fourier infrared spectrometer at a resolutionof 4 cm−1 using KBr pellets of samples.

Preparation of ER fluids

The as-made particles were further dehydrated in vac-uum for 10 h at 150◦C for preparation of ER fluidsand then were mixed quickly with dried silicone oil(η = 50 mPa s at 25◦C, Shin-Etsu Silicone Co, Japan).The suspensions were milled by the carnelian mortarfor 1 h in order to well disperse particles in siliconeoil. According to the optical photo (not shown here),the obtained ER fluids are uniform and most of theparticles are independently dispersed in silicone oilafter milling for 1 h. It should be pointed out that theincrease of surface roughness of MMT after coatingnano-structured TiO2 is found to have little influenceon the particle dispersion in silicone oil. This maybe ascribed to the low surface tension of silicone oil,which makes it possess strong wetting ability for variousmaterials (Svitova et al. 2002). In order to compareER properties, the ER fluids based on pure MMT,pure TiO2 nanorods, and the simple mixture of MMTand TiO2 nanorods (the ratio of MMT to TiO2 is thesame as that of the micro/nano-structured particles)were also prepared with the same volume faction. Thevolume fraction of particles in fluids was defined by theratio of particle volume to total fluid volume. The den-sity of particles was measured by a pycnometer filledwith pure water at ambient temperature. To decrease

the effect of porosity on density, the pycnometer wasplaced in an ultrasonic cleaning bath and connectedto a vacuum pump. After sonication under reducedpressure for 5 min, the density was measured.

ER measurements

The ER properties were measured by a THERMALHAAKE RS600 stress-controlled electro-rheometerwith a parallel plate system (the diameter of plateis 35 mm and the gap is 1.0 mm), a WYZ-010 dchigh-voltage generator (the maximum voltage of dchigh-voltage generator was 10.0 kV and the currentlimitation was 2.0 mA), a PC-controlled Phoenix cir-cular oil bath system (Silicone oil, −25.0–125.0◦C) fortemperature control, and an ampere meter for detect-ing current through ER fluids. The rheological curvewith increasing shear rate was measured by the con-trolled shear rate (CR) mode. The static yield stresswas obtained by the controlled shear stress (CS) mode.Before reading stresses, we initially applied the electricfield on suspensions for 10.0 s and then sheared. Allmeasurements were finished at 23.0 ± 0.1◦C.

Dielectric analysis

The dielectric spectra of ER fluids were measured by animpedance analyzer (HP 4284A) within the frequencyrange from 20 Hz to 1 MHz using a measuring fixture(HP 16452A) for liquids to investigate their interfacialpolarization. The 1 V of bias electrical potential wasapplied to ER fluids during measurement. It was smallenough that no chain formation within ER fluids wasinduced, thus we could obtain the true behavior of theinterfacial polarization between the particles and themedium and well compare the dielectric behaviors.

Results and discussion

Figure 1 shows the SEM images of MMT, micro/nano-structured MMT/TiO2 particles, and pure TiO2

nanorods. The MMT particles mainly possess a flake-like structure and the particle size is about 1–3 μm.The micro/nano-structured MMT/TiO2 particlesapproximately maintain the shape of MMT, but theirsurface is very rough due to enclosed nanorod-likeTiO2 (Fig. 1b). From the high-resolution SEM imagein Fig. 1c, it can be found that the diameter of enclosedTiO2 nanorods is about 30 nm and the length is about300 nm. If MMT is not added into reaction system,pure TiO2 nanorods are formed but the diameter is

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Fig. 1 SEM images ofpure MMT (a),micro/nano-structuredMMT/TiO2 particles andlocal amplificatory SEM of(b, c), and pure TiO2nanorods (d). Scale bar =1 μm for (a) and (b);scale bar = 500 nm for(c) and (d)

about 40 nm (Fig. 1d), which is larger than that of theTiO2 nanorods on MMT/TiO2 particles.

Figure 2 is the XRD patterns of the MMT,micro/nano-structured MMT/TiO2 particles, and pureTiO2 nanorods. It is found that anatase and rutile

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phases coexist in the pure TiO2 nanorods, and theweight fraction of anatase is less than that of rutile.Compared with XRD peaks of pure MMT and pureTiO2 nanorods, there are two differences in the XRDpeaks of micro/nano-structured MMT/TiO2 particles.One is that anatase and rutile phases also coexist, butthe weight fraction of anatase is significantly higherthan that of rutile. This indicates that the growth ofTiO2 nanorods on the surface of MMT can affect thecrystal phase and content. The other difference is thatthe low angle diffraction peak of MMT at ∼6.81◦,ascribing to the interlayer spacing, shifts towards thelower diffraction angle (∼5.80◦, noted by arrow) in themicro/nano-structured MMT/TiO2 particles, indicatingthat some TiO2 might have inserted into interlayer ofMMT besides being deposited on the surface.

Figure 3 shows the FT–IR spectra of MMT andmicro/nano-structured MMT/TiO2 particles. The ab-sorption bands of MMT at 3,616, 3,445, and 1,646 cm−1

are attributed to O–H stretching vibrations of chem-ical and physical adsorbed water, those at 1,090 and1,036 cm−1 to Si–O stretching vibrations, and 796 cm−1

bands to Al–O stretching vibrations. The bands at 467,520, and 625 cm−1 are attributed to bending vibrations

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Fig. 3 FT–IR spectra of MMT (a) and micro/nano-structuredMMT/TiO2 particles (b)

of Si–O, Al–O, and Si–O–Al, respectively. It is seenthat no new absorption bands appears in the spectra ofMMT/TiO2-nanorod hybrid composite, because the Ti–O bending absorption band is at about 500 cm−1, whichis superposed with bending absorption bands of Si–O,Al–O, and Si–O–Al. Furthermore, the absorption bandat 3,445 cm−1 shifts to 3,395 cm−1 and becomes broad,which can be attributed to the O–H on the surface ofTiO2.

Figure 4 shows the typical flow curves measured bythe CR mode for micro/nano-structured MMT/TiO2

ER fluid under various electric fields. In the absenceof electric field, the flow behavior shows a departurefrom the Newtonian fluid. The shearing-thin phenom-

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ena can be seen in not only the micro/nano-structuredMMT/TiO2 ER fluids but also in the pure MMT ERfluid (see Fig. 5). The shear-thin behavior of MMTfluid is more significant than that of MMT/TiO2 fluid.This may be because plate-like clay particles tend toflow parallel to shearing direction with the increase ofshear rate (Schmidt et al. 2002). However, the roughMMT/TiO2 particles, in particular at high shear rate,probably easily subject to large viscous drag force andinterparticle friction, which cause the rapid climb ofshear stress with shear rate and higher zero field viscos-ity at high shear rate region (>50 s−1). In the presenceof electric field, we can observe a typical ER behavior,i.e., the increase of shear stress or shear viscosity withelectric fields. But the flow curve slightly departs fromthe Bingham model. The shear stress as a function ofshear rate initially decreases to a minimum value at acritical shear rate (this critical shear rate is differentunder various electric fields and it shifts towards highershear rate region with increasing field strength, seearrows in Fig. 4), and then increases as the shear rateincreases. This phenomenon is also observed in otherER fluids because, for the region of higher shear rates,broken fibrillated structures in ER fluids do not haveenough time to reform themselves by an electric fieldand thus the shear stress declines and hydrodynamicforces start to dominate the flow of ER fluids (Janget al. 2001; See et al. 2002; Cho et al. 2005; Yin and Zhao2006; Kim et al. 2007).

To well evaluate the yield stress, we measure theshear stress–shear rate relationship using the controlledshear stress mode because it is more reliable comparedwith an extrapolation of the curly course of the shear

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stress to zero shear rate measured by the controlledshear rate mode (Choi et al. 2000; Pavlinek et al. 2006).Figure 6 shows the shear stress as a function of shearrate measured by the CS mode for the micro/nano-structured MMT/TiO2 ER fluid at different electricfields. The static yield stress as marked by arrows isclearly observed. Figure 7 generalizes the dependenceof static yield stress on electric field strengths. It isfound that the yield stress of micro/nano-structuredMMT/TiO2 ER fluid is about 750 Pa at 3 kV/mm, whichis about 2.8 times that (270 Pa) of pure TiO2 nanorodER fluid and 1.3 times that (590 Pa) of pure MMT ERfluid. It can also be observed that the yield stress of ER

Fig. 7 Static yield stress as a function of electric field strengthfor ER fluids of micro/nano-structured MMT/TiO2, the mixtureof MMT and TiO2 nanorods, MMT, and pure TiO2 nanorods(10 vol.%, T = 23◦C)

fluid containing the simple mixture of MMT and TiO2

nanorods is lower than that of ER fluid of micro/nano-structured MMT/TiO2 when the same electric field isapplied. The yield stress of the mixture-based ER fluidis about 370 Pa at 3 kV/mm, which is only half as muchas that of micro/nano-structured MMT/TiO2-basedER fluid. This indicates that the formed micro/nano-structure has played an important role in the enhance-ment of ER effect. The increased surface roughnessof the micro/nano-structured MMT/TiO2 particles maybe one factor for this enhancement because it hasbeen testified that the enhanced friction between roughparticles can improve the rheological property (López-López et al. 2009; Yin et al. 2009). The correlation ofstatic yield stress and electric field strength is fittedby the power-law relation τ y ∝ Eα . The exponent α

is 1.74 ± 0.07, 1.23 ± 0.06, 1.53 ± 0.07, and 1.43 ±0.15 for pure MMT, pure TiO2 nanorods, micro/nano-structured MMT/TiO2, and MMT/TiO2 nanorod mix-ture ER fluids, respectively. These exponents departfrom 2 which was predicted by the classic polariza-tion model (Klingenberg et al. 1991). This can be at-tributed to the fact that the polarization model treatsthe dielectric sphere as ideal point–dipole and thus theelectrostatic interaction between dielectric spheres istreated as a dipole–dipole interaction only. However,in the factual ER fluids, the several factors, such asparticle concentration, shape, multi-disperse, nonlinearconductivity of oil, etc., often make α differ from 2 (Choet al. 1998; Otsubo 1999; Choi et al. 2001).

Figure 8 shows the current density of MMT,the mixture of MMT and TiO2, TiO2 nanorods,

Fig. 8 Current density as a function of electric field strengthfor ER fluids of micro/nano-structured MMT/TiO2, the mixtureof MMT and TiO2 nanorods, MMT, and pure TiO2 nanorods(10 vol.%, T = 23◦C)

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and micro/nano-structured MMT/TiO2 ER fluids. Thecurrent densities of pure MMT ER fluid and the simplemixture of MMT and TiO2 ER fluid are very large,and continuously increase with time. When the elec-tric field is higher than 3 kV/mm, the current evenexceeds the limitation of the measured system and thedielectric breakdown happens. However, the currentdensity of micro/nano-structured MMT/TiO2 ER fluidis significantly decreased, which is less than 6 μA/cm2

at 3 kV/mm and less than 15 μA/cm2 at 5 kV/mm.And the current density can be remained stable withtime under electric fields. This indicates that assem-bling TiO2 nanorods on the surface of MMT mighthave effectively confined the long-range movement ofactive cations in the interlayer of MMT (Xiang andZhao 2006). Therefore, the formation of micro/nano-structured MMT/TiO2 not only enhances ER effectof MMT but also largely decreases the current den-sity. Furthermore, compared with other clay-based ERfluids, such as polymer conductor intercalated MMT(Kim et al. 1999; Lu and Zhao 2002) and polar mole-cular modified kaolinite (Wang and Zhao 2002), thepresent ER fluid of micro/nano-structured MMT/TiO2

possesses higher yield stress and lower current densityat a comparable volume fraction.

Figure 9 shows the flow curves of shear stress vs.shear rate of the micro/nano-structured MMT/TiO2 ERfluids with different volume fraction under zero and2 kV/mm electric field. It can be found that the electricfield-induced shear stress increases with particle con-centration, while the zero field-viscosity of fluids alsoincreases with particle concentration. In particular, as

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Fig. 9 Flow curves of shear stress vs. shear rate for ER fluidof micro/nano-structured MMT/TiO2 with different volume frac-tions under 0 kV/mm (open symbol) and 2 kV/mm (solid symbol)electric field (T = 23◦C)

the particle concentration increases, the fluids departfrom Newtonian behavior more significantly at zeroelectric field and even show a weak yield stress (∼50 Pafor 22 vol.% ER fluid). This indicates that the fluid hasformed a flocculated suspension even in the absenceof electric fields (López-López et al. 2008). For manyapplications, however, the high no-field viscosity isnot desired because of increased power consumption.Moreover, the current density of fluid is also in-creased with particle concentration. For example, thecurrent density of MMT/TiO2 ER fluid is increasedto ∼16 μA/cm2 (2 kV/mm) at 22% particle volumefraction, which is four times as high as ∼3 μA/cm2 at10%. This can be attributed to the fact that the leakingcurrent mainly comes from ER particles (Foulc et al.1996).

Besides the increased surface roughness, the forma-tion of the micro/nano-structured MMT/TiO2 has beenfound to influence the dielectric properties. Figure 10shows the frequency ( f ), dependence of real part (ε′),and image part (ε′′) of complex permittivity of ERfluids. There is not a loss peak in pure MMT ERfluid within 100 Hz–100 kHz but its loss tends to startto increase after 1 MHz, which indicates that a losspeak will appear at higher frequency above 1 MHz.This can be attributed to its high conductivity (σ )because ε′′ is related to σ /(2π f) (Huang 2004). Thus,this high polarization rate is not helpful to optimal EReffect. There is no loss peak in pure TiO2 nanorodsER fluid and its ε′ is very small. However, a clearloss peak at 0.8 kHz and a larger ε′ within 100 Hz–100 kHz appears in micro/nano-structured MMT/TiO2

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ER fluid. Meanwhile, the Cole–Cole plot can fit the di-electric data of micro/nano-structured MMT/TiO2 ERfluid and pure TiO2 nanorod ER fluid (Choi et al.1998; Cho et al. 2003), but it is not suitable to thepure MMT ER fluid due to its large leaking current.It has been accepted that the particle polarization,in particular interfacial polarization, is important toER effect and the dielectric property plays a domi-nant role (Hao 1997; Hao et al. 1998; Ikazaki et al.1998). Therefore, in terms of dielectric analysis, theimprovement of dielectric properties after formationof micro/nano-structured MMT/TiO2 is another factorto its better ER effect. Having considered the uniquestructure of micro/nano-structured MMT/TiO2 parti-cles by the previous structural characterization, wefurther suppose that assembling TiO2 nanorods ontothe surface of MMT particles might have effectivelyconfined the long-range movement of weakly boundedcations in interlayer of MMT. As a consequence, theleaking current density is decreased but the interfacialpolarization is improved. Furthermore, the rheologicalcurve of ER fluids is a competition result between in-terparticle interaction forces and hydrodynamic forces.Therefore, the dielectric loss peak is also related to thestability of flow curves as shown in Fig. 4 because theadequate polarization rate of ER particles, which canbe approximately evaluated by the relaxation time τ

(τ = 12π fmax

, where fmax is the frequency of dielectric losspeak), reflects the stability of interparticle electrostaticinteraction during shearing deformation (Ikazaki et al.1998). The analogical results can be also found in otherER fluids (Cho et al. 2004; Yin et al. 2010).

Conclusion

By a one-pot solvothermal method, we preparednovel low-cost ER material of micro/nano-structuredMMT/TiO2 particles. This material composed of lay-ered montmorillonite enclosed by TiO2 nanorods. Therheological experiments showed that the micro/nano-structured MMT/TiO2 ER fluid exhibited more ex-cellent ER effect compared to the ER fluids of pureMMT, pure TiO2 nanorods, and the simple mixtureof MMT and TiO2 nanorods. The yield stress ofmicro/nano-structured MMT/TiO2 ER fluid was about750 Pa at 3 kV/mm, which was about 2.8 times that(270 Pa) of pure TiO2 nanorods ER fluid, 1.3 timesthat (590 Pa) of pure MMT ER fluid, and 2.0 timesthat (370 Pa) of ER fluid of the mixture of MMTand TiO2 nanorods. In particular, the leaking currentdensity of the micro/nano-structured MMT/TiO2 ER

fluid was less than 6 μA/cm2 at 3 kV/mm and 15 μA/cm2

at 5 kV/mm, which was largely decreased compared tothe corresponding ER fluids of MMT and the mixtureof MMT and TiO2. Dielectric analysis indicated thatassembling TiO2 nanorods onto the surface of MMTnot only decreased the leaking electrical conductionbut also induced the interfacial polarization within theadequate frequency range of 100 Hz–100 kHz. Theimproved polarization and surface roughness due tothe formation of micro/nano-structure on MMT maybe responsible for the stronger ER effect of the micro/nano-structured MMT/TiO2 ER fluid.

Acknowledgements The authors would like to acknowledgethe support from the Natural Science Foundation of China (nos.60778042 and 50602036)

References

Bartl MH, Boettcher SW, Frindell KL et al (2005) 3-D molecu-lar assembly of function in titania-based composite materialsystems. Acc Chem Res 38:263–271

Block H, Kelly JP (1988) Electro-rheology. J Phys D: Appl Phys21:1661–1677

Cho MS, Choi HJ, To K (1998) Effect of ionic pendent groupson a polyaniline-based electrorheological fluid. MacromolRapid Commun 19:271–273

Cho MS, Choi HJ, Ahn WS (2004) Enhanced electrorheology ofconducting polyaniline confined in MCM-41 channels. Lang-muir 20(1):202–207

Cho MS, Cho YH, Choi HJ, Jhon MS (2003) Synthesisand electrorheological characteristics of polyaniline-coatedpoly(methyl methacrylate) microsphere: size effect. Lang-muir 19(14):5875–5881

Cho MS, Kim JW, Choi HJ, Jhon MS (2005) Polyaniline andits modification for electroresponsive material under appliedelectric fields. Polym Adv Technol 6:352–356

Choi HJ, Cho MS, Jhon MS (1998) Electrorheological proper-ties of poly(acene quinone) radical suspensions. Polym AdvTechnol 8:697–700

Choi HJ, Lee YH, Kim CA, John MS (2000) Microencapsu-lated polyaniline particles for electrorheological materials. JMater Sci Lett 19:533–535

Choi HJ, Cho MS, Kim JW, Kim CA, Jhon MS (2001) A yieldstress scaling function for electrorheological fluids. ApplPhys Lett 78:3806–3808

Couter SP, Weiss KD, Carlson JD (1993) Engineering appli-cations of electrorheological materials. J Intell Mater SystStruct 4:248–259

Dong YC, Feng SS (2005) Nanoparticles of montmorillonite(MMT)/poly (d,l-lactide-co-glycolide) (PLGA) for oral de-livery of anticancer drugs. Biomaterials 26:6068–6076

Fang FF, Choi HJ, Joo J (2008) Conducting polymer/claynanocomposites and their applications. J Nanosci Nanotech-nol 8(4):1559–1581

Foulc JN, Atten P, Boissy C (1996) Correlation between electri-cal and rheological properties of electrorheological fluids. JIntell Mater Syst Struct 17:579–582

Gast P, Zukoski CF (1989) Electrorheological fluids as colloidalsuspensions. Adv Colloid Interfacial Sci 30:153–202

Rheol Acta (2011) 50:87–95 95

Halsey TC (1992) Electrorheological fluids. Science 258:761–766

Han ZH, Zhu HY, Ratinac KR, Ringerc SP, Shi J, Liu JW (2008)Nanocomposites of layered clays and cadmium sulfide:similarities and differences in formation, structure and prop-erties. Microporous Mesoporous Mater 108:168–182

Hao T (1997) The role of the dielectric loss of dispersed materialin the electrorheological effect. Appl Phys Lett 70:1956–1958

Hao T, Kawai A, Ikzaki F (1998) Mechanism of the electrorhe-ological effect: evidence from the conductive, dielectric,and surface characteristics of water-free electrorheologicalfluids. Langmuir 14:1256–1262

Huang JP (2004) Force acting on the microparticles in electrorhe-ological solids under the application of a nonuniform ac elec-tric field. Chem Phys Lett 390:380–383

Ikazaki F, Kawai A, Uchida K, Kawakami T, Edmura K, SakuraiK, Anzai H, Asako Y (1998) Mechanisms of electrorheology:the effect of the dielectric property. J Phys D: Appl Phys31:336–347

Jang WH, Kim JW, Choi HJ, Jhon MS (2001) Synthesis andelectrorheology of camphorsulfonic acid doped polyanilinesuspensions. Colloid Polym Sci 279:823–827

Kim JW, Kim SG, Choi HJ, Jhon MS (1999) Synthe-sis and electrorheological properties of polyaniline-Na+-montmorillonite suspensions. Macromol Rapid Commun20:450–452

Kim SG, Lim JY, Sung JH, Choi HJ, Seo Y (2007) Emulsionpolymerized polyaniline synthesized with dodecylbenzene-sulfonic acid and its electrorheological characteristics: tem-perature effect. Polymer 48:6622–6631

Klingenberg DJ, Swol F, Zukoski CF (1991) The small shear rateresponse of electrorheological suspensions: I. Simulation inthe point–dipole limit. J Chem Phys 94:6160–6163

Lim YT, Park JH, Park OO (2002) Improved electrorheologicaleffect in polyaniline nanocomposite suspensions. J ColloidInterface Sci 245:198–203

López-López MT, Kuzhir P, Bossis G, Mingalyov P (2008) Prepa-ration of well-dispersed magnetorheological fluids and effectof dispersion on their magnetorheological properties. RheolActa 47:787–796

López-López MT, Kuzhir P, Bossis G (2009) Magnetorheologyof fiber suspensions: I. Experimental. J Rheol 53:115–126

Lu J, Zhao XP (2002) Electrorheological properties of suspen-sions based on polyaniline-montmorillonite clay nanocom-posite. J Mater Res 15:2258–2265

Lu J, Zhao XP (2004) A new approach of enhancing theshear stress of electrorheological fluids of montmoril-lonite nanocomposite by emulsion intercalation of poly-N-methaniline. J Colloid Interface Sci 273:651–657

Manikandana D, Divakara D, Valentine RA, Revathia S, EstherLeena Preethia M, Sivakumar T (2007) Synthesis of plat-inum nanoparticles in montmorillonite and their catalyticbehaviour. Appl Clay Sci 37:193–200

O’Dwyer C, Navas D, Lavayen V, Benavente E, Santa AnaMA, González G, Newcomb SB, Sotomayor Torres CM(2006) Nano-urchin: the formation and structure of high-density spherical clusters of vanadium oxide nanotubes.Chem Mater 18:3016–3022

Otsubo Y (1999) Electrorheology of whisker suspensions. Col-loids Surf A 153:459–466

Parthasarathy M, Klingenberg DJ (1996) Electrorheology: mech-anisms and models. Mater Sci Eng R 17:57–103

Pavlinek V, Saha P, Perez-Gonzalez J, Vargas L, Stejskal J,Quadrat O (2006) Analysis of the yielding behavior of elec-trorheological suspensions by controlled shear stress experi-ments. Appl Rheol 16:14–18

Pietron JJ, Stux AM, Compton RS, Rolison DR (2007) Dye-sensitized titania aerogels as photovoltaic electrodes forelectrochemical solar cells. Sol Energy Mater Sol Cells91:1066–1074

Schmidt G, Nakatani AI, Han CC (2002) Rheology andflow-birefringence from viscoelastic polymer–clay solutions.Rheol Acta 41(1–2):45–54

See H, Kawai A, Ikazaki F (2002) Differences in the electrorhe-ological response of a particle suspension under direct cur-rent and alternating current electric fields. Colloid Polym Sci280:24–29

Sudha JD, Sasikala TS (2007) Studies on the formation of self-assembled nano/microstructured polyaniline–clay nanocom-posite (PANICN) using 3-pentadecyl phenyl phosphoricacid (PDPPA) as a novel intercalating agent cum dopant.Polymer 48:338–347

Svitova T, Theodoly O, Christiano S, Hill RM, Radke CJ(2002) Wetting behavior of silicone oils on solid sub-strates immersed in aqueous electrolyte solutions. Langmuir18(18):6821–6829

Wang BX, Zhao XP (2002) Electrorheological behavior ofkaolinite-polar liquid intercalation composites. J MaterChem 12:1865–1869

Wang BX, Zhao XP (2005) Wettability of bionic nanopapilla par-ticles and their high electrorheological activity. Adv FunctMater 15:1815–1820

Whitesides GM, Grzybowski B (2002) Self-assembly at all scales.Science 295:2418–2421

Xiang LQ, Zhao XP (2006) Preparation of montmoril-lonite/titania nanocomposite and enhanced electrorheolog-ical activity. J Colloid Interface Sci 296:131–140

Yang SW, Gao L (2005) Fabrication and characterizationof nanostructurally flowerlike aggregates of TiO2 via asurfactant-free solution route: effect of various reaction me-dia. Chem Lett 34:1044–1045

Yin JB, Zhao XP (2002) Preparation and electrorheological ac-tivity of mesoporous rare-earth-doped TiO2. Chem Mater14:4633–4640

Yin JB, Zhao XP (2004) Giant electrorheological activity of highsurface area mesoporous cerium-doped TiO2 templated byblock copolymer. Chem Phys Lett 398:393–399

Yin JB, Zhao XP (2006) Enhanced electrorheological activity ofmesoporous Cr-doped TiO2 from activated pore wall andhigh surface area. J Phys Chem B 110:12,916–12,925

Yin JB, Zhao XP, Xiang LQ, Xia X, Zhang ZS (2009) Enhancedelectrorheology of suspensions containing sea-urchin-like hi-erarchical Cr-doped titania particles. Soft Matter 5:4687–4696

Yin JB, Xia X, Xiang LQ, Zhao XP (2010) Conductivity and po-larization of carbonaceous nanotubes derived from polyani-line nanotubes and their electrorheology when dispersed insilicone oil. Carbon 48:2958–2967

Yu HG, Yu JG, Cheng B, Lin J (2007) Synthesis, charac-terization and photocatalytic activity of mesoporous tita-nia nanorod/titanate nanotube composites. J Hazard Mater147:581–587

Zhao XP, Yin JB (2002) Preparation and electrorheologicalcharacteristics of rare-earth-doped TiO2 suspensions. ChemMater 14:2258–2263

Zhao XP, Yin JB (2006) Advances in electrorheological fluidsbased on inorganic dielectric materials. J Ind Eng Chem12:184–198

Zhao XP, Yin JB, Tang H (2006) New advances in electrorheo-logical fluids and devices. In: Reece PL (ed) Smart materialsand structures: new research. Nova Science Publishing, NewYork, pp 1–66