Enhanced Electrorheological Performance of Mixed Silica

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Enhanced Electrorheological Performance of Mixed Silica Nanomaterial Geometry Chang-Min Yoon, Yoonsun Jang, Jungchul Noh, Jungwon Kim, Kisu Lee, and Jyongsik Jang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08298 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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ACS Applied Materials & Interfaces

Enhanced Electrorheological Performance of Mixed Silica Nanomaterial Geometry Chang-Min Yoon, Yoonsun Jang, Jungchul Noh, Jungwon Kim, Kisu Lee and Jyongsik Jang* *

School of Chemical and Biological Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-

gu, Seoul 151-742, Korea RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) *

Tel.: 82-2-880-7069; Fax: 82-2-888-7295; e-mail: [email protected]

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ABSTRACT The mixed geometrical effect on electrorheological (ER) activity of bimodal ER fluids was investigated by mixing SiO2 spheres and rods of different dimensions. To gain an in-depth understanding of the mixed geometrical effect, 12 bimodal ER fluids were prepared from 4 sizes of SiO2 spheres (50, 100, 150, and 350 nm) and 3 types of SiO2 rods with different aspect ratios (L/D = 2, 3, and 5). Five concentrations of SiO2 spheres and rods were created for each bimodal ER fluid, resulting in a total of 60 sets of comprehensive ER measurements. Some bimodal ER fluids exhibited enhanced ER performance, as high as 23.0%, compared with single SiO2 rod-based ER fluids to reveal the mixed geometrical effect of bimodal ER fluids. This interesting experimental result is based on the structural reinforcement provided by spheres to fibrillated rod materials, demonstrating the mixed geometrical effect on ER activity.

KEYWORDS Electrorheology, Bimodal ER fluid, Mixed geometrical effect, Silica, Smart fluid.


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INTRODUCTION Electrorheological (ER) fluids are non-aqueous suspensions composed of polarizable particles dispersed in insulating mediums such as silicone and mineral and vegetable oils.1–3 ER fluids can change their rheological properties from liquid-like to solid-like by application of an external electric field (E-field), and are regarded as a smart fluid system due to their unique properties such as rheological reversibility, fast response, and low power consumption.4–6 ER fluids have received much attention in academic and industrial fields and have been practically applied in haptic devices, clutch systems, and dampers.7–9 The general working mechanism of an ER fluid is that an applied E-field induces polarization in the material. The resulting electrostatic interactions cause the formation of fibrillike structures along the E-field direction. This fibril-like structure formation increases the rheological and mechanical properties (i.e., the viscosity and shear stress) of ER fluids.10 Based on the pioneering discovery of the ER effect by W. Winslow, various methods have been developed to enhance ER performance.11 In the early stages of ER studies, a variety of materials were used in ER applications, including polymers and inorganic and organic materials.12,13 It was observed that each material exhibited different ER activity arising from differences in intrinsic and physical properties such as dielectric permittivity, electrical conductivity, structure, density, and size.14–16 Further ER studies reported the manipulation of these properties using techniques including controlling the size from the micro- to the nanoscale, changing the structure and material hybridization.17,18 As a result, standards and trends have been developed for obtaining positive ER effects. For example, Lee et al. reported that reducing the size of silica (SiO2) nanospheres and increasing the aspect ratio (L/D) of SiO2 rod-like materials results in high ER performances.19,20 Hao et al. and Zhang et al. showed that coating and introduction of high dielectric materials on template materials can significantly increase ER activity.21,22 Yoon et al. described the enhanced ER effect of low-density materials relative to highdensity materials.23 However, these standards for ER activities are limited to single particles or


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homogeneous mixtures of those particles. There have been few reports on ER fluids composed of two particulate (bimodal) systems. Some researchers including Wu et al. and See et al. have investigated the effect of mixed particle sizes on ER activity by mixing two differently sized ER materials;24,25 however, in these studies, the particle sizes were limited to the micrometer (µm) scale and only sphere-shaped materials were investigated. More recently, ER materials with nanometer (nm)-size scales and non-spherical onedimensional (1D)-structures (fiber, rod, and tube) with high L/D values have received considerable attention.26,27 Nano-sized ER materials can induce higher ER activity by creating a more rigid and dense fibril-like structure compared with larger materials. In addition, 1D ER materials can exhibit high ER performances through advantages such increased flow resistance, mechanical stability, and slippage reduction relative to spherical materials.28–36 However, no previous reports have investigated a bimodal ER fluid composed of mixed spherical and 1D rod-like materials of nanometer-scale dimensions. SiO2 is one of the most widely utilized materials for ER applications due to a range of advantages, including the capacity for mass production, a facile preparation method, and size controllability. SiO2 materials are particularly useful for studying structural or geometrical effects on ER activity, as their size and structure can be modified easily by changing the molar ratio of reagents, the reaction temperature, or the reaction time.37–40 Moreover, 1D rod-like SiO2 materials can be synthesized by the addition of surfactants such as cetyltrimethylammonium bromide (CTAB).41 Therefore, many ER studies investigating the effects of size, dimensions, or structure have been conducted using SiO 2 and derivative materials.42,43 In this study, we investigated the ER activity of bimodal ER fluids composed of various sizes of SiO 2 spheres and rods, to establish mixed geometrical effects. SiO2 spheres of four sizes (50, 100, 150, and 350 nm), and SiO2 rods with three length/diameter ratios (L/D = 2, 3, and 5) were synthesized. These ER materials were mixed in 12 bimodal ER fluids with 5 mixture compositions (concentrations of SiO2 spheres and rods), to make a total of 60 bimodal ER fluid sets. The ER performance of each ER fluid ACS Paragon Plus Environment

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was examined under an applied E-field and the mixed geometrical effect observed. Interestingly, some bimodal ER fluids exhibited increased ER activity compared with single SiO2 sphere- or rod-based ER fluids. A plausible mechanism for this phenomenon is provided through examination of dielectric properties and practical structural observations of the fibril-like structures of each bimodal ER fluid.


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RESULTS AND DISCUSSION Preparation of SiO2 sphere, SiO2 rod, and bimodal ER fluid. Figure 1 shows a schematic illustration of the bimodal ER fluids composed of four sizes of SiO2 spheres and three types of SiO2 rods with different L/D values. The SiO2 spheres and rods were synthesized using the Stöber method and a modification with added surfactant (cetyltrimethylammonium bromide, CTAB). To prepare each bimodal ER fluid, one type of SiO2 sphere (50, 100, 150, or 350 nm) and one type of SiO2 rod (L/D = 2, 3, or 5) were mixed to form a bimodal ER fluid labelled as “S#LD#”. The bimodal ER fluid is also known as a bimodal ER fluid depending on the context. Five mixture compositions of each bimodal ER fluid, denoted as M-1 through M-5, were created by changing the concentrations of SiO2 spheres and rods. Rods were expected to exhibit a higher ER performance than spheres. M-1 contained 97.0 wt% SiO2 rods and 3.0 wt% SiO2 spheres; the concentration of spheres increased in increments of 3.0%, with a corresponding decrease in rods, for subsequent mixtures. A full description of the bimodal ER fluid compositions is provided in Table 1.

Figure 1. Schematic illustration of SiO2 spheres (50, 100, 150, and 350 nm), SiO2 rod (L/D = 2, 3, and 5), and bimodal ER fluid composed of SiO2 spheres and rods.


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Table 1. Descriptions of bimodal ER fluids (12 “S#LD#” ER fluids) composed of various concentrations of SiO2 rod and SiO2 sphere materialsa Rods (L/D = 2, 3, or 5): concentration in the Spheres (size = 50, 100, 150, or 350 nm): Labelb mixture (wt%) concentration in the mixture (wt%) M-1 97.0 3.0 M-2 94.0 6.0 M-3 91.0 9.0 M-4 88.0 12.0 M-5 85.0 15.0 a The total combined weight of the SiO2 sphere and rod mixtures was 0.3 g, and the resulting mixture was dispersed in silicone oil (11 mL) to attain bimodal ER fluids (3.0 wt%). b Each bimodal ER fluid (12 types) was sorted into five mixture concentrations.

TEM analysis was conducted to investigate the morphologies of the SiO2 materials (Figure 2). To clearly visualize the size differences in each bimodal ER fluid, TEM samples were prepared using a mixture of SiO2 spheres and rods. TEM images revealed that all SiO2 spheres and rods were welldefined, highly uniform structures; the diameters of the four types of SiO2 spheres were ca. 50, 100, 150, and 350 nm and the diameters of the three types of SiO2 rods were ca. 50 ± 3, 70 ± 5, and 160 ± 15 nm, with corresponding lengths of ca. 110 ± 5, 200 ± 10, and 800 ± 20 nm, respectively. Hence, the SiO2 rods were successfully synthesized with L/D values of ~2, ~3, and 5. Enlarged TEM micrographs of each type of SiO2 sphere and rod are shown in Figure S1 and S2.


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Figure 2. TEM micrographs of SiO2 spheres (50, 100, 150, and 350 nm), SiO2 rods (L/D: 2, 3, and 5), and the 12 bimodal mixtures of SiO2 spheres and rods.

FT-IR spectra were obtained to examine the compositions of various SiO2 materials (Figure S3). Firstly, four types of SiO2 spheres and three kinds of rods a showed typical characteristic peaks of SiO2 material including Si–O–Si stretching at ~1040 cm–1, SiO–H bending near ~950 cm–1, and Si–O–Si bending at ~800 cm–1. However, peaks related to water and organic molecules were minimized or not detected from samples, indicating that the several washing and drying process effectively removed the residues. Considering these results, it was confirmed that all spheres and rods were successfully fabricated as similarly composed SiO2 materials. Thus, it was expected that the ER activities of all SiO2


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materials and their bimodal mixtures were based on the geometrical differences of various SiO2 materials.

ER activity of normal ER fluid and bimodal ER fluid. Our strategy for investigating the mixed geometrical effect of a bimodal ER fluid included the following steps: (1) The ER activities of the ER fluids composed of each type of SiO2 sphere (50, 100, 150, or 350 nm) were determined. (2) The ER activities of the ER fluids composed of each type of SiO2 rod (L/D = 2, 3, or 5) were determined. (3) The ER performance of each of the 12 combinations of bimodal ER fluid composed of SiO 2 spheres and rods was investigated for the 5 composition mixtures, to provide ER activities for a total of 60 sets. (4) The ER performances of the SiO2 spheres, SiO2 rods, and each bimodal ER fluid were compared and a plausible mechanism for the mixed geometrical effect on ER activity suggested.


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Figure 3. Shear stresses of a) SiO2 sphere- and b) SiO2 rod-based ER fluids (3.0 wt%) under an electric field (E-field) strength of 3.0 kV mm−1 with a fixed shear rate of 0.1 s−1.

Prior to measurement of ER activities, all ER fluids (including the bimodal mixtures) were prepared at the same concentration of 3.0 wt% for direct comparison between each result to minimize experimental errors. In addition, all ER measurements were conducted using an E-field on-off test method to examine the stability of the fluids and ensure the reliability of the ER data. First, the ER activities of the four SiO2 sphere-based ER fluids were investigated under an E-field strength of 3.0 kV mm−1 at a steady shear rate of 0.1 s−1 (Figure 3a). The SiO2 sphere-based ER fluids exhibited immediate shear stresses under the applied E-field, and the ER performances dropped off suddenly when the E-field was removed. All SiO2 sphere-based ER fluids showed similar and stable shear stresses without fluctuation during the ACS Paragon Plus Environment

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E-field on-off test. The ER activities of the SiO2 spheres increased with decreasing particle size, in agreement with previous reports on trends in ER activity observed in nano-sized sphere materials.6,19 Second, the ER activities of the three SiO2 rod-based ER fluids were investigated using the same method (Figure 3b). All SiO2 rod-based ER fluids exhibited sudden ER activities under application of the E-field, and the shear stresses disappeared when the applied voltage was turned off. However, the SiO2 rod-based ER fluids exhibited much higher (up to 20-fold greater) ER activities compared with the SiO2 spheres. These large ER enhancements in rod-like materials originate from structural advantages; previous studies have reported that rod-like materials can form highly linked fibril-like structures resulting in high mechanical strength and increased flow resistance against shear forces generated from ER instruments.44 Hence, the SiO2 rod-based ER fluids exhibited increased ER performances compared with the SiO2 spheres. In addition, the ER performance of the SiO2 rods increased with increasing L/D.


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Figure 4. ER performances of bimodal ER fluids (3.0 wt%) composed of 50-nm SiO2 spheres mixed with a) rods of L/D = 2, (S50LD2), b) rods of L/D = 3, (S50LD3), and c) rods of L/D = 5, (S50LD5) in various concentrations measured under an alternating E-field strength of 3.0 kV mm−1.

The practical investigations of bimodal ER fluids were started with bimodal ER fluids composed of 50-nm SiO2 spheres and each of the three types of SiO2 rods, labelled S50LD2, S50LD3, and S50LD5 ACS Paragon Plus Environment

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(Figure 4). To clearly visualize the mixed geometrical effect, the ER activities of the SiO 2 spheres and rods and the bimodal ER fluids are included in each case. For S50LD2, the SiO2 rod-based (L/D = 2) ER fluids exhibited the highest shear stress, and the ER performances of the bimodal ER fluids decreased from M-1 to M-5 as the concentration of spheres increased from 3.0 wt% to 15.0 wt%. Under our experimental conditions, it was noticeable that some concentrations of bimodal ER fluids showed increment in the shear stresses, which surpassing the ER activity of corresponding single SiO2-rods based ER fluids. In particular, the M-1 of S50LD3 and M-1 and M-2 of S50LD5 bimodal ER fluids exhibited higher shear stresses compared with that of SiO2 rods-based ER fluids. Apart from these compositions, the other bimodal ER fluid combinations of 50 nm-sized SiO2 spheres and rods displayed decreased ER performance with increased sphere concentrations.


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Figure 5. ER performances of bimodal ER fluids (3.0 wt%) composed of 100-nm-sized SiO2 spheres mixed with a) rods of L/D = 2, (S100LD2), b) rods of L/D = 3, (S100LD3), and c) rods of L/D = 5, (S100LD5) in various concentrations, measured using an alternating E-field of 3.0 kV mm−1.

For the S100LD2, S100LD3, and S100LD5 bimodal ER fluids, consisting of 100-nm SiO2 spheres and various rods, only the M-1 formulations of S100LD3 and S100LD5 showed an increase in ER ACS Paragon Plus Environment

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performance compared with rods, and other concentrations showed decreasing ER performance with increasing sphere concentrations (Figure 5). In contrast, bimodal ER fluids with larger 150 and 350 nmsized SiO2 spheres (S150LD# and S350LD#) contained no mixtures where the ER performance exceeded that of rods alone through the mixed geometrical effect; however, decreasing shear stresses with increasing sphere concentrations were observed (Figure S4 and S5). Overall, the ER performances of the S150LD# and S350LD# bimodal ER fluids were lower than the ER activities of S50LD# and S100LD#, most likely due to the deteriorating ER performance associated with the larger size of the sphere. It was confirmed that five bimodal ER fluids of M-1 / S50LD3, M-1 / S50LD5, M-2 / S50LD5, M-1 / S100LD3, and M-1 / S100LD5 manifested the mixed geometrical effects of surpassing the ER performance of corresponding single SiO2-rods based ER fluids. Furthermore, flow curve investigations of bimodal ER fluids, SiO2 spheres- and rods-based ER fluids (3.0 wt%) were assessed to gain deeper insight into the mixed geometrical effect. Particularly, flow curves were measured as a function of shear rate under two conditions of zero-field (without E-field) and on-field (E-field of 3.0 kV mm–1). Under zero-field conditions, all ER fluids manifested very low shear stresses (~0.1 Pa), since particles cannot form fibril-like structures without E-field applications (Figure S6). It was noticeable that no mixed geometrical effect was detected in all bimodal ER fluids (Figure S7, S8, S9, and S10). On the other hand, notable shear stresses were immediately appeared under on-field conditions, and similar behavior change of flow curves for all ER fluids were observed under varying shear rate (Figure S11, S12, S13, S14, and S15). In the low shear rate regions, all flow curves manifested Bingham plastic-like behavior of plateau curves. After passing the critical shear rate, flow curves showed Newtonian fluid-like behavior, depicted as linear increase of shear stresses. As similar to the result of E-field on-off test, mixed geometrical effect was also observed in the flow curves of aforementioned five bimodal ER bimodals. Considering these results, mixed geometrical effect of bimodal ER fluids were only observable under an applied E-field. Additionally, ER activity of all bimodal ER fluids composed of higher concentrations of spheres and smaller concentrations of rods, which was reversely formulated as high spheres-concentrated ER fluid, ACS Paragon Plus Environment

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was investigated under an applied E-field of 3.0 kV mm–1 (Figure S16, S17, S18, and S19). Specifically, high spheres-concentrated bimodal ER fluids were prepared in three different concentrations by mixing 97.0, 94.0, and 91.0 wt% of SiO2 spheres to 3.0, 6.0, and 9.0 wt% of SiO2 rods, respectively. With the applied E-field, all high-spheres-concentrated bimodal ER fluids manifested immediate shear stresses. However, none of ER performance have surpassed the shear stress of corresponding single SiO2 rodbased ER fluid, indicating that no mixed geometrical effect was observed from bimodal ER fluids containing high concentration of SiO2 spheres and low concentrations of SiO2 rods. To clearly visualize the mixed geometrical effect on ER activity, the highest ER performances of all bimodal ER fluids were plotted as a function of SiO2 sphere and rod concentrations (Figure 6). As discussed previously, five bimodal ER fluids exhibited ER performances exceeding that of rods alone. Specifically, the ER efficiencies of M-1 / S50LD3, M-1 / S50LD5, M-2 / S50LD5, M-1 / S100LD3, and M-1 / S100LD5 increased by 23.0, 17.0, 4.3, 3.9, and 6.5% compared with the corresponding SiO 2 rodbased ER fluids, respectively. Also, M-1 / S50LD3, M-1 / S50LD5, M-2 / S50LD5, M-1 / S100LD3, and M-1 / S100LD5 bimodal ER fluids showed 439.4, 755.7, 660.5, 540.5, and 992.0% increment in ER efficiencies compared to the corresponding single SiO2 sphere-based ER fluids, respectively. Detailed ER activities of various SiO2 sphere-based, SiO2 rod-based, and bimodal ER fluids are listed in Table S1. Three important similarities were observed between the five performance-enhanced bimodal ER fluids: they were all composed of small SiO2 spheres (50 and 100 nm) and elongated rods (L/D = 3 and 5), and had a lower concentration of spheres compared with rods (3.0 and 6.0 wt%, respectively). Despite the observation of the mixed geometrical effect from these five bimodal ER fluid samples, further examination is required to verify that this result is solely due to geometrical effects rather than other properties affecting ER performance.


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Figure 6. Shear stresses of various bimodal ER fluids (3.0 wt%) as a function of mixture state under an E-field strength of 3.0 kV mm−1 at a fixed shear rate of 0.1 s−1 (dashed line: shear stresses of SiO2 rodbased ER fluids).

Plausible mechanism of mixed geometrical effect of bimodal ER fluid. According to previous studies, the dielectric properties of materials have a significant effect on ER activity. Generally, ER materials with high permittivity (ε') show high polarizability under an E-field, resulting in the formation of stronger fibril-like structures and enhanced ER performance.45 Hence, we investigated the permittivities of the five bimodal ER fluids that showed increased ER activities compared with rods, to gain deeper insight into the correlation between the mixed geometrical effect and dielectric properties. First, the permittivities of various SiO2 sphere- and SiO2 rod-based ER fluids were examined (Figure 7). The relationship between permittivity and ER activity is explained by the achievable polarizability (△ε, ACS Paragon Plus Environment

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polarization tendency), which is determined from the difference between the fictitious (ε0, f → 0) and static (ε∞, f → ∞) permittivities.46 The △ε values of the 50, 100, 150, and 350 nm SiO2 sphere-based ER fluids were determined as 0.73, 0.62, 0.62, and 0.61, respectively, and the △ε values of the SiO2 rods of L/D 2, 3, and 5 were determined as 1.1, 1.6, and 2.19, respectively. Notably, the SiO 2 spheres all showed similar △ε values; however, the SiO2 rods exhibited an increasing △ε trend with larger L/D, which can be ascribed to an increasing dipole moment.47 Hence, rods (or elongated materials) exhibit higher ER activities than spheres and low L/D materials, due to increased polarizability as well as mechanical stability and flow resistance originating from structural advantages.

Figure 7. Permittivities of (ε') of various a) SiO2 spheres-based ER fluids (3.0 wt%) and b) SiO2 rods-based ER fluids (3.0 wt%) as a function of E-field frequency.


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Figure 8. Permittivities of (ε') of 5 performance-enhanced bimodal ER fluids (3.0 wt%), manifesting the mixed geomtrical effect, as a function of E-field frequency (dashed line: permittivities of rod-based ER fluids).

The permittivities of the five bimodal ER fluids exhibiting the mixed geometrical effect were determined (Figure 8). Our hypothesis was that if the ER performance enhancements observed in these fluids arose only from geometry, without any dielectric influence, then the permittivities of these five fluids must be lower than the corresponding SiO2 rod-based ER fluids due to the presence of spheres. All five bimodal ER fluids had lower △ε values than the SiO2 rod-based ER fluids, because the presence of low dielectric spheres in the bimodal ER fluids will decrease the dielectric properties. Judging from this result, we can confirm that the increased ER performance of the five bimodal ER fluids arose only from geometrical effects and not from the dielectric properties of the material. The dielectric properties of various SiO2 spheres, rods, and bimodal ER fluids are listed in Table S2.


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Figure 9. Dispersion stabilities of SiO2 spheres (50, 100, 150, and 350 nm)- and SiO2 rods (L/D = 2, 3, and 5)-based ER fluids dispersed in silicone oil (3.0 wt%) [inset: definition of sedimentation ratio].

In addition to dielectric property, dispersion stability of materials is known as another key factor affecting the ER performance. In general, it is known that ER fluids with high dispersion stability can exhibit enhanced ER performance due to the readiness of creating more and rigide fibril-like structures with well-dispersed materials.9,27 Therefore, dispersion stability of bimodal ER fluids were explored to achieve an in-depth insight into the relationship between the dispersion stability and mixed geometrical effect. Figure 9 shows the dispersion stabilities of various SiO2 spheres- and SiO2 rods-based ER fluids dispersed in silicone oil (3.0 wt%). The dispersion stabilities of SiO2 materials enhanced in the order of rod (L/D = 5), sphere (350 nm), rod (L/D = 3), sphere (150 nm), rod (L/D = 2), sphere (100 nm), and sphere (50 nm). It was clearly observed that smaller spheres and shorter rods showed enhanced dispersion stability compared to larger SiO2 materials. Furthermore, dispersion stability of all concentrations (M1 to M5) of bimodal ER fluids of S50LD3, S50LD5, S100LD3, and S100LD5 bearing the mixed geometrical effect was investigated (Figure 10). The dispersion stabilities of bimodal ER fluids increased in the order of M-1, M-2, M-3, M-4, and M-5 concentrations. It has been found that dispersion stability of bimodal ER fluids increased with increasing concentrations of small SiO2 spheres ACS Paragon Plus Environment

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(50 and 100 nm), which was the reverse trend of the ER activity enhancement in order of M-5, M-4, M3, M-2, and M-1 concentrations. Prior the determining the dispersion stability of bimodal ER fluids, we made a prediction that if dispersion stability is closely related to the mixed geometrical effect, then M-1 or M-2 concentrations with the mixed geometrical effect may display high dispersion stability compared to M-3, M-4, and M-5 concentrations with lower ER performance. However, the practical dispersion stabilities of bimodal ER fluids revealed that dispersion stability did not affected the ER performance of bimodal ER fluids. Therefore it can be concluded dispersion stability had no impact on the mixed geometrical effect of bimodal ER fluids as well as the dielectric property, but the mixed geometrical effect is solely based on the physical phenomenon of materials under applied E-field.

Figure 10. Dispersion stabilities of S50LD3, S50LD5, S100LD3, and S100LD5 bimodal ER systems dispersed in silicone oil (solid line: specific concentrations showing the mixed geometrical effect and dashed line: all other concentrations).


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Figure 11. OM observations of fibril-like structure formation of a) a SiO2 sphere-based ER fluid (50 nm), b) a SiO2 rod-based ER fluid (L/D = 5), and c) a bimodal ER fluid (S50LD5) under an applied Efield of 1.0 kV mm−1 (dashed box: enlarged image of fibril-like structures).

To achieve an in-depth understanding of the geometrical effect on ER activity, practical OM analysis was conducted to observe fibril-like structure formation of spheres (50 nm), rods (L/D = 5), and a bimodal ER fluid (M-1 of S50LD5) in real-time (Figure 11). It may be expected from the enhanced ER performance that the fibril-like structures of the five bimodal ER fluids must be larger or more rigid than those of the SiO2 sphere- and rod-based ER fluids. Immediate formation of fibril-like structures ACS Paragon Plus Environment

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along the direction of the E-field was observed in all three fluids within a few milliseconds of application of the E-field. Interestingly, the size and rigidity of the fibril-like structures varied across the ER fluids. For spheres, thin and weak fibril-like structures were constructed; in the case of rods, the fibril-like structures showed increased rigidity and thickness relative to spheres. Finally, the bimodal ER fluid S50LD5 exhibited the largest and thickest fibril-like structures, which may originate from the mixed geometrical effect of spheres and rods. In addition, OM images of the other bimodal ER fluids exceeding the performance of rods also showed thick and rigid fibril-like structures (Figure S20). As previously discussed, rod-like materials can form highly linked fibril-like structures and exhibit increased ER activity owing to the layered array of rods under an E-field. Hence, fibrillated rods can effectively produce mechanical stability, prevent slippage between particles, and induce flow resistance against shear forces. In contrast, spheres form relatively weak fibril-like structures and have low ER performances due to the slipperiness of the materials, resulting in low mechanical stability against shear forces. For the five bimodal ER fluids, the sizes of the fibril-like structures were larger than those of the rods alone, a phenomenon that may have arisen from the reinforcing effect of spheres on the fibrillated rods. Our bimodal ER fluids were composed mainly of rods (in concentrations of up to 97.0 wt%). Thus, the fibril-like structures were composed of mostly rods; spheres would reside near or in fill spaces between fibrillated rods resulting in a larger, more rigid fibril-like structures. Proposed schematic illustrations for the formation of fibril-like structures in spheres, rods, and bimodal ER fluids are shown in Figure 12. OM analysis on bimodal ER fluids displaying lower ER activity was conducted to observe the differences between the fibril-like structures of the five ER performance-enhanced bimodal ER fluids and ER activity decreased bimodal ER fluids. Specifically, the fibril-like structures of M-5 / S100LD2, M-2 / S150LD3, and M-3 / S350LD2 bimodal ER fluids were analyzed (Figure S21). Compared with the performance-enhanced bimodal ER fluids, the fibril-like structures of other bimodal ER fluids were relatively weak and small in size, close to the fibrillated forms of sphere- and rod-based ER fluids. ACS Paragon Plus Environment

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Hence, bimodal ER fluids with large spheres or containing high concentrations of sphere materials were unable to manifest the mixed geometrical effect to increase ER performance. Our tentative mechanism for the decreased ER performance of bimodal ER fluids is that the presence of large spheres hinders the layered and ordered assembly of rods and reduces the ER effect. Likewise, high concentrations or concentrations of spheres deteriorates the formation of rigid fibril-like structures. The mechanism for formation of weak fibril-like structures in bimodal ER fluids with high concentrations of large spheres is illustrated in Figure S22. To summarize, the addition of a small concentration of sphere materials to rod-like materials increases ER performance, due to the mixed geometrical effect originating from the structural reinforcement provided by small spheres.

Figure 12. Tentative mechanism of fibril-like structures formed from a) sphere-based ER fluids, b) rodbased ER fluids, and c) performance-enhanced bimodal ER fluids under an applied E-field.


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CONCLUSION In conclusion, various bimodal ER fluids were successfully prepared by mixing a nanometer-scale SiO2 spheres and rods. SiO2 spheres were synthesized in four sizes (50, 100, 150, and 350 nm) using the simple Stöber method, and SiO2 rods were synthesized with varying L/D of 2, 3, and 5 by the addition of a CTAB surfactant. For a comprehensive analysis, all possible combinations of sizes of SiO 2 spheres and rods were mixed to create 12 bimodal ER fluids. Five concentrations of these fluids were produced to provide 60 ER fluid samples in total. In addition, ER fluids consisting of single SiO2 spheres and rods were prepared for comparison. Five bimodal ER fluids composed of small sized spheres (50 and 100 nm), rods with high L/D (3 and 5), and a small concentration of spheres (3.0 and 6.0 wt%) exhibited increased ER efficiencies as high as 23.0% (M-1 / S50LD3) compared with the corresponding single SiO2 rod-based (L/D = 5) ER fluid, demonstrating the mixed geometrical effect. Practical OM analysis of the fibril-like structures of the ER fluids showed that this ER enhancement was solely based on the mixed geometrical effect and that other properties such as dielectric permittivity and dispersion stability of materials had no influence. In particular, the five performance-enhanced bimodal ER fluids formed rigid and strong fibril-like structures relative to spheres, rods, and performance-decreased bimodal ER fluids. Based on these experimental results, we suggest that the mixed geometrical effect is induced by the reinforcing effect of small spheres on the arrayed fibril-like structures of rods, resulting in increased mechanical strength. Lastly, this interesting mixed geometrical effect of bimodal ER fluids induced by the mixing of nanometer-scale sphere and rod materials were not discovered in any previous bimodal ER studies employing micrometer scale particulates. Thus, this study may open up a new possibility and methodology in preparation of bimodal ER fluids in nanometer-scales.


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EXPERIMENTAL SECTION Materials. Tetraethyl orthosilicate (TEOS), hexadecyltrimethylammonium bromide (CTAB), and silicone oil (100 cSt) were purchased from Sigma Aldrich Co. Ethyl alcohol (99.8%), ammonium hydroxide (NH4OH, 28.0–30.0%), and hydrochloric acid (HCl, 35.0–37.0%) were received from Samchun Chemical Co. (Korea). All chemicals and reagents were used as received without purification. Synthesis of four types of SiO2 spheres. SiO2 spheres with four different sizes were prepared according to the Stöber method.39 The size of SiO2 spheres were controlled by changing the reaction temperature or compositions of reagents. In a typical synthesis of SiO2 spheres (100 nm), DI water (1.4 mL), ethyl alcohol (79 mL), and NH4OH solution (2.0 mL) were mixed by magnetic stirring for 10 min. After the equilibrium, TEOS (2.0 mL) was added to the resulting mixture and reaction was allowed for 6 h. Resulting SiO2 spheres were collected by centrifugation (6000 × g) and dried in oven (90 °C) for overnight. For synthesis of 50 nm sized SiO2 sphere, all steps are same as above except that reaction temperature was maintained at 70 °C after injection of TEOS. For 150 nm sized SiO2 sphere, all steps are same as above except that amount of added TEOS was increased to 3.5 mL. In case of 350 nm sized SiO2 sphere, amount of added reagents were slightly changed to DI water (30 mL), ethyl alcohol (180 mL), NH4OH solution (33.7 mL), and TEOS (11.5 mL). Synthesis of three SiO2 rod with different L/D. SiO2 rods with three different L/D were synthesized according to our previous study.29 The L/D of SiO2 rods were readily controlled by amount of added DI water and fixing the compositions of other reagents during the synthesis process. In a typical synthesis of SiO2 rod (L/D = 2), DI water (130 mL), ethyl alcohol (5 mL) and NH4OH solution (1.74 mL) were vigorously mixed for 10 min. Afterwards, CTAB (0. 47 g) was added to the mixture and stirred for another 10 min. Subsequently, TEOS (1 mL) was injected into the prepared mixture and reaction proceeded for 2 h with vigorous magnetic stirring. Synthesized SiO2 rods were collected by centrifugation (6000 × g) and washed several times with ethyl alcohol. To remove the organic residues and leftover surfactants, collected SiO2 rods were dispersed in mixture of DI water (30 mL) and HCl (1M, 3 mL) and heated to 75 °C for 6h. Acid washed SiO2 rods were collected by centrifugation and ACS Paragon Plus Environment

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washed several times with DI water. SiO2 rods with L/D of 3 and 5 were fabricated with exact same experimental method described above except added amount of DI water was changed to 100 mL and 34 mL, respectively. Characterization. The morphologies of four types of SiO2 sphere and three kinds of SiO2 rods with different L/D were assessed by transmission electron microscopy (TEM, JEOL-6700) analysis. Dielectric properties of ER fluids were examined using dielectric interface analyzer (Solartron-1296) equipped with impedance analyzer (Solartron-1260). Practical fibril-like structure formation of ER materials were observed using optical microscope (OM, Nikon Lv100 microscope, Nikon) installed with high voltage generator (Trek 677B). To attain clear OM images, ER fluids were placed in between the small electrode (gap distance: 1.0 mm), and alternative on-off application of E-field was applied until rigidity and size of fibril-like structure is clearly defined. Preparation of single phased ER fluids and bimodal ER fluids. First, ER fluids with a single composition of each SiO2 spheres and SiO2 rods were synthesized for comparison. To prepare the ER fluids (3.0 wt%), dried SiO2 spheres or rods (0.3 g) were dispersed in silicone oil (11 mL) using a pestle and mortar. Well-ground ER fluids were sonicated for 12 h and stirred vigorously for another 12 h to obtain well-dispersed ER fluids. To prepare the bimodal ER fluids, one of the SiO2 sphere types (50, 100, 150, or 350 nm) was mixed with one type of SiO2 rod (L/D = 2, 3, or 5), with a total combined weight of 0.3 g, and dispersed in silicone oil (11 mL) to attain 3.0 wt% bimodal ER fluids. The labelling of each bimodal ER fluid is written as “S#LD#”. For instance, if 50 nm SiO2 spheres were mixed with L/D = 2 SiO2 rods, then the ER fluid was labelled “S50LD2”. Twelve different “S#LD#” bimodal ER fluids were prepared using the four types of SiO2 spheres and three types of SiO2 rods. Five different concentrations of each “S#LD#” bimodal ER fluid were fabricated by changing the concentrations of SiO2 spheres and rods for a more detailed examination of the mixed geometrical effect. Descriptions of the compositions of each bimodal ER fluid are: M-1 (97.0 wt% rod + 3.0 wt% sphere), M-2 (94.0 wt% rod + 6.0 wt% sphere), M-3 (91.0 wt% rod + 9.0 wt% sphere), M-4 (88.0 wt% rod + 12.0 wt% sphere), and M-5 (85.0 wt% rod + 15.0 wt% sphere). In summary, 12 bimodal ER fluids were prepared and ACS Paragon Plus Environment

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compositions of sphere and rod materials were controlled in 5 concentrations to produce a total of 60 bimodal ER fluids. Investigation of ER activity. All ER measurements were carried out using rheometer (AR-2000, TA instruments) equipped with sample loading cup (d = 28.0 mm and h = 30.0 mm), concentric conical geometry (d = 28.0 mm and h = 30.0 mm), and high voltage generator (Trek 677B). To start the ER measurements, well-dispersed ER fluids were loaded to the cup and geometry was slowly inserted into the cup. The gap distance between cup and geometry was fixed to 1.0 mm without any frictions between instruments. Prior to apply E-field strength, pre-shear (10.0 s–1) was applied for 5 min to attain complete dispersion and equilibrium of loaded sample. After the equilibrium process, ER activities were measured in on-off test methods. Specifically, E-field was applied for 10 s (on state) and closed (off state) for 10 s, and this process was repeated for 5 times.


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ACKNOWLEDGEMENT This work was supported by Development Fund of Seoul National University funded by Dongjin Semichem Co., Korea (0458-20130066).

Supporting Information Available: The contents of Supporting Information may include the following: (1) TEM micrographs of four kinds of SiO2 spheres, (2) TEM micrographs of three types of SiO2 wirh different L/D, (3) FT-IR spectra of various SiO2 spheres and rods, (4) ER performances of S150LD2, S150LD3, and S150LD5 bimodal ER fluids, (5) ER performances of S350LD2, S350LD3, and S350LD5 bimodal ER fluids, (6) Zero-field ER activities of bimodal ER fluids and SiO2 spheresand SiO2 rods-based ER fluids, (7) On-field ER activities of bimodal ER fluids and SiO2 spheres- and SiO2 rods-based ER fluids, (8) ER performances of high spheres-concentrated bimodal ER fluids, (9) Detailed descriptions of ER performances of SiO2 spheres-, SiO2 rods-, and various bimodal ER fluids, (10) Dielectric properties of performance-enhanced bimodal ER fluids, (11) OM images of fibril-like structures of various performance-enhanced bimodal ER fluids, (12) OM images of fibril-like structures of various performance-decreased bimodal ER fluids, and (13) Tentative mechanism for decreased ER performance of bimodal ER fluids. This material is available free of charge via Internet at http://pubs.acs.org.


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TABLE OF CONTENTS


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