Transparent thiourea treated silica suspension through refractive index matching method and its electrorheology

Colloids and Surfaces A: Physicochem. Eng. Aspects 397 (2012) 80–84

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Transparent thiourea treated silica suspension through refractive index matching method and its electrorheology Ying Dan Liu a , Bo Mi Lee a , Ji Eun Kim a , Hyoung Jin Choi a,∗ , Tae-Sang Park b , Seong-Woon Booh b a b

Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Samsung Advanced Institute of Technology, Samsung Electronics Co., Yongin 446-712, Republic of Korea

a r t i c l e

i n f o

Article history: Received 28 November 2011 Received in revised form 21 January 2012 Accepted 26 January 2012 Available online 4 February 2012 Keywords: Electrorheological fluid Silica Transparent Yield stress

a b s t r a c t Silica nanoparticles treated with thiourea were adopted as the solid dispersed phase of an eletrorheological (ER) fluid that underwent orders of magnitude changes in its rheological properties upon the application of an electric field. To fabricate a transparent ER fluid, medium oils with three different refractive indices (RI) were prepared by mixing halocarbon oil and silicone oil, in which the amount of oils needed was estimated based on the Lorentz–Lorentz equation. Thiourea-modified silica nanoparticles dispersed in the oil mixture with the same RI as these particles exhibited the transparency of an ER fluid as well as typical ER characteristics, based on flow curve and dielectric spectra. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Electrorheological (ER) fluids, which are generally composed of electrically polarizable particles dispersed in insulating oils, exhibit fascinating field-induced rheological properties including a rapid and reversible change in their suspension structure under an applied electric field. A microstructural transition from a liquid-like to solid-like phase can be obtained by controlling the electric field strength to vary the fibrillar structures [1–7] due to the fact that in an applied electric field, these particles form chains that span the gap between two field-generating electrodes. The changes in all the physical and mechanical properties induced by the applied electric field were virtually instantaneous within a millisecond order and were reversible upon the removal of the field. The reversible nature of the ER response, the large change in shear viscosity and yield stress, and the short response times observed in these systems are desirable characteristics for many engineering applications [8]. Similar to ER fluids, magnetorheological fluids under an applied magnetic field strength are also considered as smart materials [9,10]. Many powdery materials of both organic and inorganics display ER behavior, and the ER performance is related closely to the dielectric and conducting properties of the suspension particles [11–13]. Extrinsically polarizable particles, such as silica [14], alumina and cellulose [15], which contains a range of additives, including water

∗ Corresponding author. Tel.: +82 32 860 8777; fax: +82 32 865 5178. E-mail address: [email protected] (H.J. Choi). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2012.01.037

molecules, to induce polarizability have attracted enormous attention from researchers. On the other hand, these hydrous ER particles have some drawbacks in their applications including device corrosion, water evaporation, narrow operation temperature range and dispersion instability [16]. Materials possessing polarizable species, such as electrons and ions, are introduced to overcome these shortcomings [17]. The shear viscosity increment is caused by the reorientation of polarized molecules along the direction of the applied electric field and ion aggregation near the electrode. In addition, the particle shape and particularly the size have significant effects on the ER efficiency [18,19], among which nanoparticles based ER fluids are frequently reported in recent research [20–22]. Currently, special attention has been paid to suspensions of nanoparticles with a core/shell structure, which showed extremely high ER performance. Among them, a remarkable increase in the static yield stress of barium titanyl oxalate suspension particles coated with urea under the influence of an electric field was demonstrated [23]. In this study, we applied an analogue of urea thiourea which has higher molecular polarizability, to modify silica nanoparticles for preparing a transparent ER fluid with enhanced ER performance. Transparency of the ER fluid is realized by a refractive index matching method [24,25]. In the general suspension system, the suspension becomes transparent when dispersed particles and medium oil have a similar refractive index. Therefore, to produce a transparent ER fluid, oil with a range of refractive indices determined from the Lorentz–Lorentz equation was produced and the transparent fluid was fabricated. It is possible that transparent ER fluids are able to be applied to display panels [26]. A large change in the viscosity of transparent

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ER fluid with or without an external electric field support tactilefeedback when applied to touch screens in various electronic products. In the touch screen, electric fluid exists all the time, while the electric fluid disappears if people touch the screen. Therefore, the change in viscosity of the ER fluid could be felt and sensed as being similar to the touch of a keypad. 2. Experimental 2.1. Materials Nano-silica particles with a mean particle size of approximately 20 nm (RhodoxaneTM 34, Rhodia) were used. Thiourea (Sigma–Aldrich, USA) and oils of halocarbon (0.8 cSt, Halocarbon, USA), silicone HIVAC F-4 (37 cSt, Shinetsu, Japan) with the refractive indices of 1.383, 1.555 and densities of 1.71, 1.065 g/cm3 respectively, were used directly without any purification. Distilled water and methanol were used as washing reagent. 2.2. Preparation of thiourea modified silica To modify the silica nanoparticles with thiourea, 20 g of nanoparticles were immersed in 900 ml of distilled water at room temperature with magnetic stirring for 1 h. In another beaker, 4 g of thiourea was dissolved in 100 ml of distilled water with magnetic stirring for 30 min. Subsequently, an aqueous solution of thiourea was added slowly to the silica suspension and the mixture was stirred for another 16 h. The products were then washed with distilled water and methanol using a centrifuge and dried at 70 ◦ C in a vacuum oven for 24 h [27]. 2.3. Preparation of transparent ER fluid To fabricate a transparent ER fluid, both the medium oil and dispersing silica particle must have a similar refractive index (RI). Medium oil with a range of RIs (1.45, 1.46 and 1.47) was prepared by mixing halocarbon 0.8 oil and HIVAC F-4 silicone oil at different volume ratios. The following well-known Lorentz-Lorentz equation was used for a quantitative determination of the RI of a binary mixture in this study [24]. n2 − 1 = n2 + 1



n21 − 1 n21 − 2





1 +

n22 − 1 n22 − 2



2

(1)

where n1 , n2 and 1 , 2 represent the refractive index and volume fraction of the pure component 1 and 2, respectively; and n is the refractive index of the final mixture. Table 1 shows the RI of each amount of two oils using the Lorentz–Lorentz equation to produce mixed oil with a wide range of RIs (1.45, 1.46 and 1.47). A vortex shaker was used to mix the two oils for 1 h. The mixture was then left in a vacuum oven at room temperature for another 1 h to remove any bubbles remaining in the mixture. The density of the thiourea-modified silica was 1.58 g/cm3 . Before dispersing in the mixed oil, the thiourea modified silica particles were dried completely in a vacuum oven at 80 ◦ C for 24 h to remove any residual moisture. The nanoparticles were then dispersed homogeneously in the mixed oil by shaking using a vortex for 3 h. 2.4. Characterization Field emission-scanning electron microscopy (FE-SEM, Hitachi S-4300, Japan) was performed to confirm the morphology of the pure silica particles compared to thiourea modified silica particles. Thermal gravimetric analysis (TGA, Perkin Elmer) was carried out

Fig. 1. SEM images of (a) pure silica (inset: TEM image of pure silica nanoparticles), and (b) thiourea modified silica.

at a heating rate of 10 ◦ C per minute from 25 to 800 ◦ C under a nitrogen atmosphere to detect the mass composition and thermal stability of both pure silica and thiourea modified silica. A spectrometer (LAMBDA-900, PerkinElmer) was used to estimate the transparency of the ER fluid based on thiourea modified silica particles. The density of fabricated particles was also examined using a pyconometer. The ER properties of the thiourea-modified silica suspension were characterized by steady shear experiments using a rotational rheometer (MCR 300, Physica, Germany) with a Couette geometry system (CC17 ERD, the gap between a cup and bob was 0.71 mm) and a DC high voltage generator. The flow curves of shear stress vs. shear rate were measured in a controlled shear rate test mode within a shear rate range of 0.01–1000 s−1 . All the ER measurement was conducted at room temperature while the temperature effect is not discussed in this study. Note that recently temperature effect of ER fluid was reported for polyaniline derived carbonaceous nanotubes [28]. The measurement was repeated three times for every sample to obtain accurate data. The dielectric relaxation spectra of the prepared ER fluid was examined using an LCR meter (HP 4284A) with HP 16452A Liquid Test Fixture. The frequency of the AC electric fields was varied from 20 Hz to 1 MHz at room temperature. 3. Results and discussion Fig. 1 shows images of pure silica particles and thiourea modified silica particles. Both samples exhibit serious aggregation especially in the thiourea modified silica due to the further treatment with thiourea and the drying process. The inset in Fig. 1(a) is the TEM image of pure silica particles exhibiting the average size of the particles is about 20 nm. However in Fig. 1(b), the change in particle size is hard to be identified due to the aggregations of the nanoparticles. The same result is obtained by TEM observation as the thin layer of thiourea on the surface of silica particles is not so obvious to be recognized. On the other hand, after being treated with thiourea,

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Table 1 Calculated values using Lorentz–Lorentz equation to prepare the mixed oil. No. of sample

RI of mixture

1 2 3

1.45 1.46 1.47

Volume fraction ()

Weight (g)

Density of mixture (g/cm3 )

HIVAC F-4

Halocarbon 0.8

HIVAC F-4

Halocarbon 0.8

0.405 0.463 0.522

0.595 0.537 0.478

4.31 4.93 5.55

10.18 9.18 8.18

the blocky silica has the size in micron range from 5 to 30 ␮m. For further confirmation, FT-IR was also carried out. However, the characteristic stretching vibration peak of thiocarbonyl (C S) at 1230 and 1030 cm−1 overlaps the peak of silicate at about 1100 cm−1 . By coincidence, the C–N stretching vibration also appears at the same region. Therefore, other characteristics are needed for proofing the existence of thiourea after modification. The weight composition and thermal stability were investigated using TGA, as shown in Fig. 2 for both pure silica and thiourea modified silica. Below 100 ◦ C, the weight loss (∼2 wt%) of both pure silica and thiourea modified silica was considered to be caused by moisture. Pure silica shows the only single peak in the derivative weight loss curve. In the case of thiourea modified silica, a second weight loss peak was observed in the range of 170 and 240 ◦ C, which was attributed to the degradation of thiourea on the surface of the silica particles [29]. Note that the degradation temperature of thiourea is 210 ◦ C, therefore, a weight loss of 1.5 wt% at this temperatures stage is believed to be thiourea. The transparency of the ER fluids, which was made by dispersing the thiourea modified silica particles of 5 vol% in each mixed oil with different RIs (1.45, 1.46 and 1.47), was confirmed by the naked eye. The measurement was performed as shown in Fig. 3 by injecting the ER fluid between a sliding glass and a poly(ethylene terephthalate) (PET) film with a 1.4 mm gap. In the case of the 1.46 mixed oil, the ER fluid was the most transparent among the three samples tested. The ER fluid prepared using the mixed oil with a RI of 1.46 was called 5 vol% TER fluid. To examine the transparency of the TER fluid more accurately, two other experiments were conducted using a spectrometer. In the first experiment, the sample was placed in front of a 1 in. diameter opening and the quantity of light penetrating the opening was measured. The other experiment was to measure the quantity of light from the opening without a sample. The transparency of the TER fluid was determined from the two estimated quantities of light, as listed in Table 2. The change in transmission of the ER fluid along with the wavelength of the light is considered as the influence of the glass and PET film which all exhibit transmission or absorption properties to light [30]. Fig. 4 shows the flow behavior of the 5 vol% TER fluid under an external electric field as a function of the shear rate for different

Fig. 2. TGA thermograms for pure silica and thiourea modified silica particles.

1.45 1.41 1.37

applied electric fields from 0 up to 2.5 kV/mm. Different to common ER fluid [31], the TER fluid behaves like a non-Newtonian fluid without an external electric field, in which the shear thinning is attributed to the dispersion state of the nanoparticles in the suspension. While under an applied electric field, typical Bingham fluid behavior is observed because of the fibrillation of the thiourea modified silica particles perpendicular to the electrodes [32]. The Bingham fluid equation given below is used widely to describe the steady shear response of many ER fluids:  = y + ˙

(2)

where  is the shear stress;  y is the dynamic yield stress, which is a function of the electric field; ␥˙ is the shear rate and  is shear viscosity. In addition, the shear stress increased dramatically with increasing electric field strength. The particle possesses net charges, arising from uniform polarization or electrostatic force under an applied electric field. This indicates that the chain structure or clustering of ER particles are sustained in such a high shear field [33]. The dependence of  y on various electric field strengths obtained from the controlled shear rate (CSR) mode measurements is shown in Fig. 5, in which  y is the extrapolated value obtained from flow curve in Fig. 4. The ER fluid is stressed by an external mechanical torque until the particle chain structure is completely broken so that the shear takes place. Therefore, the shear rate is observed when the flow of the ER fluid starts. As a typical ER fluid, the transparent ER fluid also presented the ER property that the yield stress increases along with increasing of the electric field strength. In general, the scaling function for the normalized yield stress via scaling of the applied electric field strength is represented by the

Fig. 3. Photo of transparency test using the slide glasses which were adhered with double-sided adhesive tape.

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Table 2 Transparency (%) of the ER fluid at each wavelength (nm). Wavelength (nm) Transmission (%)

400 81.84

500 89.28

600 90.72

700 90.64

800 90.33

Fig. 6. Dielectric spectra (a) and Cole–Cole fitting curves (b) for 5 vol% transparent ER fluid. Fig. 4. Shear stress curve (a) and shear viscosity curve (b) as function of shear rate for 5 vol% transparent ER fluid under different electric fields applied.

power law:  y ∝ E˛ [34–37]. The value of ˛ in this study approaches 2 for entire electric field strengths. This result indicates that the polarization of thiourea particles plays the important role in the surface polarization under applied electric fields. In other words,

the thiourea particles lead to enhance the ER properties of this ER system through strong polarization. The dielectric properties of the ER fluid were measured to further investigate the ER performance of the ER fluid. The dielectric constant (ε ) and loss factor (ε ) are typical results for the interfacial polarization of suspensions including an ER fluid, which consists of a polarizable phase dispersed in insulating media [38]. The lines in Fig. 6 are fitted from the Cole–Cole formula [22,39] ε∗ = ε + iε = ε∞ +

Fig. 5. Dynamic yield stress vs. electric field strength for electrorheological fluid.

ε 1 + (iω )

1−˛

(3)

where is the relaxation time, the exponent (1 − ˛) characterizes the broadness of the relaxation time distribution. When ˛ = 0, Eq. (3) reduces to Debye’s well-known single relaxation time model. The parameters in Eq. (3) for the ER fluids in oils of different RIs are summarized in Table 3. ε = ε0 − ε∞ is the achievable polarizability in ER fluids, which also shows positive effect on the ER performance. In addition, the relaxation time ( ) of the interfacial polarization of ER fluids is related to the yield stress and stress enhancement under an applied electric field [40]. It is discovered in Table 3 that ε values of the ER fluids are much different each other as the RI of the oil is changed. Even though the sample 2 (RI = 1.46) showed the best transparency, ε of which is the lowest among the three samples, indicating that the RI match may have an effect on the dielectric properties of the ER fluids. Further investigation is needed for this point of view.

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Table 3 Parameters in Eq. (3) for 5 vol% transparent ER fluid. Parameter samplea

ε0

ε∞

ε

(s)

˛

1 2 3

5.6 5.1 5.9

2.5 2.8 2.2

3.1 2.3 3.7

0.07 0.09 0.08

0.40 0.62 0.40

a

The number of each sample is consisted with that in Table 1.

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