Development of Novel Colorful Electrorheological Fluids

Herein, the electrorheological (ER) performances of ER fluids were correlated with their colors to allow for the visual selection of the appropriate fluid for a specific application using naked eyes. A series of TiO2-coated synthetic mica materials colored white, yellow, red, violet, blue, and green (referred to as color mica/TiO2 materials) were fabricated via a facile sol–gel method. The colors were controlled by varying the thickness of the TiO2 coating layer, as the coatings with different thicknesses exhibited different light interference effects. The synthesized color mica/TiO2 materials were mixed with silicone oil to prepare colored ER fluids. The ER performances of the fluids decreased with increasing thickness of the TiO2 layer in the order of white, yellow, red, violet, blue, and green materials. The ER performance of differently colored ER fluids was also affected by the electrical conductivity, dispersion stability, and concentrations of Na+ and Ca2+ ions. This pioneering study may provide a practical strategy for developing new ER fluid systems in future.


Introduction
Electrorheological (ER) fluids, which are well-dispersed suspensions of polarizable materials in insulating media, such as silicone oils, have received widespread attention owing to their various advantages, such as rapid response, low power consumption, and reversible phase transition [1][2][3][4][5]. When an external electric field (E-field) is applied, ER fluids exhibit a sudden and rapid change from liquids to solid-like states within a few milliseconds [6][7][8][9][10]. ER fluids can therefore be used in various industrial applications which require fast response times, including dampers, clutch systems, engine mounts, and haptic devices [11][12][13]. ER activities can be improved by controlling the mechanical and chemical properties of dispersing materials [14][15][16]. Additionally, the polarizability of the materials plays a pivotal role in ER performance [17,18]: in general, materials with high polarizability exhibit high ER efficiencies [19]. Various polarizable materials, including metallic, polymeric, organic, and inorganic materials, have been used as ER materials [20][21][22].
Among various inorganic and metal oxides, TiO 2 is widely used as a dispersing material for ER fluids owing to its advantages, such as availability for mass production and simple fabrication process via the sol-gel method [23][24][25]. TiO 2 and its derived materials also exhibit numerous chemical advantages, such as stability, resistance to corrosion, and photocatalytic ability [26][27][28][29][30]. Previous ER studies have developed numerous TiO 2 -derived materials, such as polymer-coated TiO 2 , shape-controlled TiO 2 , and alkaline earth metaldoped TiO 2 [31,32]. Furthermore, TiO 2 materials can be incorporated or coated onto a SiO 2 template to fabricate multilayered materials to achieve core/shell structures, porous characteristics, and precursors for hollow-type materials [33][34][35]. Silicon-containing materials,

Synthesis of Color-Mica/TiO 2 Materials
Various color mica/TiO 2 materials were synthesized using a typical sol-gel method. Mica (10.5 g) was dispersed in deionized water (100 mL) under vigorous magnetic stirring for 1 h. A constant heat of 90 • C was then applied to the solution, and TiCl 4 (2.0 mL) was continuously added dropwise with constant stirring-the amount of TiCl 4 added determined the color of the material. The sol-gel reaction proceeded for 4 h, during which NaOH (20.0%) was slowly added to the suspension to maintain the pH value at 1.8; pH preservation is vital for the successful coating of the TiO 2 layer on mica. The resulting color mica/TiO 2 materials were collected via vacuum filtration, washed several times with water to remove residues, and dried in an oven overnight at 90 • C. The final dry materials obtained were: white mica/TiO 2 , yellow mica/TiO 2 , red mica/TiO 2 , violet mica/TiO 2 , blue mica/TiO 2 , and green mica/TiO 2 . Figure 1 shows a schematic of the synthesis process. Aldrich (Burlington, MA, USA). Sodium hydroxide (40.0%) and ethanol (EtOH, 99.9%) were purchased from Samchun Chemical Company (Seoul, Korea). All chemicals were used as received without any additional purification.

Synthesis of Color-Mica/TiO2 Materials
Various color mica/TiO2 materials were synthesized using a typical sol-gel method. Mica (10.5 g) was dispersed in deionized water (100 mL) under vigorous magnetic stirring for 1 h. A constant heat of 90 °C was then applied to the solution, and TiCl4 (2.0 mL) was continuously added dropwise with constant stirring-the amount of TiCl4 added determined the color of the material. The sol-gel reaction proceeded for 4 h, during which NaOH (20.0%) was slowly added to the suspension to maintain the pH value at 1.8; pH preservation is vital for the successful coating of the TiO2 layer on mica. The resulting color mica/TiO2 materials were collected via vacuum filtration, washed several times with water to remove residues, and dried in an oven overnight at 90 °C. The final dry materials obtained were: white mica/TiO2, yellow mica/TiO2, red mica/TiO2, violet mica/TiO2, blue mica/TiO2, and green mica/TiO2. Figure 1 shows a schematic of the synthesis process.

Characterization
The morphological structures of the mica and color mica/TiO2 materials were investigated via field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Tokyo, Japan); cross-sectional samples of the color mica/TiO2 materials were prepared for the analysis using a focused ion beam (FIB, LYRA3 GMH, TESCAN, Brno, The Czech Republic) system. The elemental compositions (Si and Ti) of the color mica/TiO2 materials were obtained using an energy-dispersive X-ray spectroscopy (EDS) system (EX-250, HORIBA, Ltd., Kyoto, Japan) integrated with the FE-SEM instrument. The molecular structures of the materials were analyzed using a Fourier-transform infrared (FT-IR) instrument (Nicolet iS10, Thermo Fisher Scientific, Waltham, MA, USA). The concentrations of Na + and Ca 2+ ions in the materials were determined using an ion chromatography system (930 Compact IC Flex, Metrohm, Herisau, Switzerland). The electrical conductivities of the materials in the pellet form were obtained using a two-point probe system (MCP-HT450, Mitsubishi, Tokyo, Japan).

Investigation of ER Properties
To prepare the color mica/TiO2-based ER fluids (3.0 wt%), the dried ER materials (0.3 g) were ultrasonically dispersed in silicone oil (11 mL, viscosity = 100 cST), followed by vigorous magnetic stirring overnight. No additional additives were added to the prepared ER fluids. The ER investigation of the samples was performed using a rheometer (MCR 302, Anton Parr, Graz, Austria) with a cup, concentric cylinder conical geometry, and high-voltage generator (HCN 7E-12500, Fug Elektronik, Schechen, Germany). The gap between the cup and the geometry was set to 1.00 mm on each side without any interference.

Characterization
The morphological structures of the mica and color mica/TiO 2 materials were investigated via field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Tokyo, Japan); cross-sectional samples of the color mica/TiO 2 materials were prepared for the analysis using a focused ion beam (FIB, LYRA3 GMH, TESCAN, Brno, The Czech Republic) system. The elemental compositions (Si and Ti) of the color mica/TiO 2 materials were obtained using an energy-dispersive X-ray spectroscopy (EDS) system (EX-250, HORIBA, Ltd., Kyoto, Japan) integrated with the FE-SEM instrument. The molecular structures of the materials were analyzed using a Fourier-transform infrared (FT-IR) instrument (Nicolet iS10, Thermo Fisher Scientific, Waltham, MA, USA). The concentrations of Na + and Ca 2+ ions in the materials were determined using an ion chromatography system (930 Compact IC Flex, Metrohm, Herisau, Switzerland). The electrical conductivities of the materials in the pellet form were obtained using a two-point probe system (MCP-HT450, Mitsubishi, Tokyo, Japan).

Investigation of ER Properties
To prepare the color mica/TiO 2 -based ER fluids (3.0 wt%), the dried ER materials (0.3 g) were ultrasonically dispersed in silicone oil (11 mL, viscosity = 100 cST), followed by vigorous magnetic stirring overnight. No additional additives were added to the prepared ER fluids. The ER investigation of the samples was performed using a rheometer (MCR 302, Anton Parr, Graz, Austria) with a cup, concentric cylinder conical geometry, and highvoltage generator (HCN 7E-12500, Fug Elektronik, Schechen, Germany). The gap between the cup and the geometry was set to 1.00 mm on each side without any interference. The ER measurements were initiated by adding well-dispersed ER fluids into the cup along with a rotor. A mechanical shear rate of 10.0 s −1 was applied for 5 min to achieve stability and homogeneous placement of the fluid in the cup. Finally, the E-field was applied to analyze the ER performances of the different ER fluids under various conditions.

Structure and Morphology of Color-Mica/TiO 2 Materials
To visualize the color appearance, the color mica/TiO 2 materials were dissolved in EtOH and observed by the naked eye. Clearly, each color mica/TiO 2 material expressed a distinct color (Figure 2a). The color change of a color mica/TiO 2 material can be explained as follows: When incident light penetrates the color mica/TiO 2 material, a certain amount of light is reflected from the nica-TiO 2 interface. This reflected light encounters new incident light, facilitating the light interference effect. Varying the thickness of the TiO 2 layer changes the traveling distances of the incident light, causing different interference effects involving various wavelengths, thereby producing different colors. This correlation between the ER material color and TiO 2 layer thickness is shown in Figure 2b. The refractive indices of mica and TiO 2 are 1.6 and 2.5, respectively [49,50]. According to Fresnel's law of reflection, the larger the difference in the refractive index between two materials, the higher the light reflectance at the interface of the materials [51,52].
along with a rotor. A mechanical shear rate of 10.0 s −1 was applied for 5 min to achieve stability and homogeneous placement of the fluid in the cup. Finally, the E-field was applied to analyze the ER performances of the different ER fluids under various conditions.

Structure and Morphology of Color-Mica/TiO2 Materials
To visualize the color appearance, the color mica/TiO2 materials were dissolved in EtOH and observed by the naked eye. Clearly, each color mica/TiO2 material expressed a distinct color (Figure 2a). The color change of a color mica/TiO2 material can be explained as follows: When incident light penetrates the color mica/TiO2 material, a certain amount of light is reflected from the nica-TiO2 interface. This reflected light encounters new incident light, facilitating the light interference effect. Varying the thickness of the TiO2 layer changes the traveling distances of the incident light, causing different interference effects involving various wavelengths, thereby producing different colors. This correlation between the ER material color and TiO2 layer thickness is shown in Figure 2b. The refractive indices of mica and TiO2 are 1.6 and 2.5, respectively [49,50]. According to Fresnel's law of reflection, the larger the difference in the refractive index between two materials, the higher the light reflectance at the interface of the materials [51,52].  The morphologies of the color mica/TiO 2 materials were investigated using FE-SEM ( Figure 3). The particle size of the color mica/TiO 2 materials was determined to be approximately 25 µm. Significantly, a clear separation between the particles was observed for all the color mica/TiO 2 materials, which is in contrast with the aggregated mica particles; this particle separation occurred owing to the presence of the TiO 2 layer. Notably, the surfaces of mica and color mica/TiO 2 materials were completely different: compared to the smooth surface of mica, the color mica/TiO 2 materials displayed rough surfaces owing to the TiO 2 coating. In addition, the size of the TiO 2 particles increased with increasing TiO 2 thickness. The morphologies of the color mica/TiO2 materials were investigated using FE-SEM ( Figure 3). The particle size of the color mica/TiO2 materials was determined to be approximately 25 μm. Significantly, a clear separation between the particles was observed for all the color mica/TiO2 materials, which is in contrast with the aggregated mica particles; this particle separation occurred owing to the presence of the TiO2 layer. Notably, the surfaces of mica and color mica/TiO2 materials were completely different: compared to the smooth surface of mica, the color mica/TiO2 materials displayed rough surfaces owing to the TiO2 coating. In addition, the size of the TiO2 particles increased with increasing TiO2 thickness. To further investigate the layer of each color mica/TiO2 material, FIB analysis was conducted on cross-sectional samples of mica and the color mica/TiO2 materials, and the thickness of the TiO2 layers was determined ( Figure 4). For the mica sample, the measured thickness was approximately 728.6 nm. For the color mica/TiO2 materials, the thicknesses of the TiO2 layers of the white mica/TiO2, yellow mica/TiO2, red mica/TiO2, violet mica/TiO2, blue mica/TiO2, and green mica/TiO2 materials were approximately 52.3, 84.4, 115.7, 138.8, 164.9, and 187.5 nm, respectively. Clearly, TiO2 layers were successfully introduced onto the mica in the color mica/TiO2 materials, and the color of each material changed according to the different light interference effects of the respective TiO2 layers.
EDS analysis was conducted to examine the elemental compositions (Si and Ti) of mica and various color mica/TiO2 materials. For mica, only Si was detected, and no Ti was detected. In contrast, Ti was detected in all the color mica/TiO2 materials. The Ti proportions for white mica/TiO2, yellow mica/TiO2, red mica/TiO2, violet mica/TiO2, blue mica/TiO2, and green mica/TiO2 were 21.0%, 31.5%, 41.0%, 49.2%, 55.7%, and 61.9%, respectively. The total Ti composition of each color mica/TiO2 material increased with increasing TiO2 thickness, which is in accordance with the cross-sectional FE-SEM observation. The detailed elemental compositions of mica and the color mica/TiO2 materials are listed in Table 1.
FT-IR analysis was conducted to investigate the molecular structures of mica and the color mica/TiO2 materials, as shown in Figure 5. In the case of mica, characteristic peaks were observed at 970, 800, and 695 cm −1 , which correspond to silicon-containing oxides with asymmetric Si-O-Si stretching vibrations, Si-O symmetric stretching vibrations, and Si-O symmetric bending, respectively [53][54][55]. Significantly, the peaks characteristic to mica diminished in the absorbance curves of all the color mica/TiO2 materials, indicating the successful coating of TiO2. Moreover, the absorbance increased after 830 cm −1 for the color mica/TiO2 materials owing to the presence of the TiO2 layers, which absorbs in the near-infrared (NIR) region. To further investigate the layer of each color mica/TiO2 material, FIB analysis was conducted on cross-sectional samples of mica and the color mica/TiO 2 materials, and the thickness of the TiO 2 layers was determined ( Figure 4). For the mica sample, the measured thickness was approximately 728.6 nm. For the color mica/TiO 2 materials, the thicknesses of the TiO 2 layers of the white mica/TiO 2 , yellow mica/TiO 2 , red mica/TiO 2 , violet mica/TiO 2 , blue mica/TiO 2 , and green mica/TiO 2 materials were approximately 52.3, 84.4, 115.7, 138.8, 164.9, and 187.5 nm, respectively. Clearly, TiO 2 layers were successfully introduced onto the mica in the color mica/TiO 2 materials, and the color of each material changed according to the different light interference effects of the respective TiO 2 layers.
EDS analysis was conducted to examine the elemental compositions (Si and Ti) of mica and various color mica/TiO 2 materials. For mica, only Si was detected, and no Ti was detected. In contrast, Ti was detected in all the color mica/TiO 2 materials. The Ti proportions for white mica/TiO 2 , yellow mica/TiO 2 , red mica/TiO 2 , violet mica/TiO 2 , blue mica/TiO 2 , and green mica/TiO 2 were 21.0%, 31.5%, 41.0%, 49.2%, 55.7%, and 61.9%, respectively. The total Ti composition of each color mica/TiO 2 material increased with increasing TiO 2 thickness, which is in accordance with the cross-sectional FE-SEM observation. The detailed elemental compositions of mica and the color mica/TiO 2 materials are listed in Table 1.
FT-IR analysis was conducted to investigate the molecular structures of mica and the color mica/TiO 2 materials, as shown in Figure 5. In the case of mica, characteristic peaks were observed at 970, 800, and 695 cm −1 , which correspond to silicon-containing oxides with asymmetric Si-O-Si stretching vibrations, Si-O symmetric stretching vibrations, and Si-O symmetric bending, respectively [53][54][55]. Significantly, the peaks characteristic to mica diminished in the absorbance curves of all the color mica/TiO 2 materials, indicating the successful coating of TiO 2 . Moreover, the absorbance increased after 830 cm −1 for the color mica/TiO 2 materials owing to the presence of the TiO 2 layers, which absorbs in the near-infrared (NIR) region.

Suitability Tests of Color-Mica/TiO2 Materials for ER Application
The suitability of various color mica/TiO2 materials for ER applications was investigated by determining their ion concentration, electrical conductivity, and dispersion stability. First, ion chromatography was used to identify the ion concentrations of the color mica/TiO2 materials. Ion chromatography revealed the presence of Na + and Ca 2+ in all the color mica/TiO2 materials. Notably, the concentrations of both Na + and Ca 2+ ions decreased as the thickness of the TiO2 layer increased, indicating that the ion species originated from the mica material, which contains various minerals. Detailed concentrations of Na + and Ca 2+ ions in various color mica/TiO2 materials are listed in Table 2. Previous studies have demonstrated that ion species enhance the ER activity via ion polarization effects [17]. Therefore, the ER performance of color mica/TiO2 may be affected by the ion concentration, and a high concentration may have a benign effect on the ER performance. The electrical conductivities of various color mica/TiO2 materials were investigated to examine their suitability for ER applications, as electrical conductivity is a key factor that affects the ER performance of a material. Previous studies have demonstrated that the electrical conductivity should be near ~10 −9 S m −1 for achieving the ideal ER performance of a material [56]. Moreover, the electrical conductivity mismatch between the materials and dispersing medium can facilitate the ER effect [25,57]. The electrical conductivities for white mica/TiO2, yellow mica/TiO2, red mica/TiO2, violet mica/TiO2, blue mica/TiO2, and green mica/TiO2 were determined to be 2.4 × 10 -9 , 2.1 × 10 -9 , 1.7 × 10 -9 , 1.4 × 10 -9 , 1.1 × 10 -9 , and 0.9 × 10 -9 S m −1 , respectively; the conductivity of silicone oil is ~1 × 10 -13 S m −1 . Therefore, the electrical conductivities of various color mica/TiO2 materials were

Suitability Tests of Color-Mica/TiO 2 Materials for ER Application
The suitability of various color mica/TiO 2 materials for ER applications was investigated by determining their ion concentration, electrical conductivity, and dispersion stability. First, ion chromatography was used to identify the ion concentrations of the color mica/TiO 2 materials. Ion chromatography revealed the presence of Na + and Ca 2+ in all the color mica/TiO 2 materials. Notably, the concentrations of both Na + and Ca 2+ ions decreased as the thickness of the TiO 2 layer increased, indicating that the ion species originated from the mica material, which contains various minerals. Detailed concentrations of Na + and Ca 2+ ions in various color mica/TiO 2 materials are listed in Table 2. Previous studies have demonstrated that ion species enhance the ER activity via ion polarization effects [17]. Therefore, the ER performance of color mica/TiO 2 may be affected by the ion concentration, and a high concentration may have a benign effect on the ER performance. The electrical conductivities of various color mica/TiO 2 materials were investigated to examine their suitability for ER applications, as electrical conductivity is a key factor that affects the ER performance of a material. Previous studies have demonstrated that the electrical conductivity should be near~10 −9 S m −1 for achieving the ideal ER performance of a material [56]. Moreover, the electrical conductivity mismatch between the materials and dispersing medium can facilitate the ER effect [25,57]. The electrical conductivities for white mica/TiO 2 , yellow mica/TiO 2 , red mica/TiO 2 , violet mica/TiO 2 , blue mica/TiO 2 , and green mica/TiO 2 were determined to be 2.4 × 10 −9 , 2.1 × 10 −9 , 1.7 × 10 −9 , 1.4 × 10 −9 , 1.1 × 10 −9 , and 0.9 × 10 −9 S m −1 , respectively; the conductivity of silicone oil is~1 × 10 −13 S m −1 . Therefore, the electrical conductivities of various color mica/TiO 2 materials were in a suitable range for ER applications, and the electrical conductivity of each material was at least two orders of magnitude higher than that of silicone oil, thereby consequently promoting the ER effect of the materials [25].
Finally, the dispersion stabilities of the color mica/TiO 2 materials were examined ( Figure 6). For ER applications, dispersion stability is another key factor affecting ER performance, as the sedimentation of materials from the media may disrupt the formation of rigid fibril-like structures. To examine the dispersion stability, each color mica/TiO 2 material was thoroughly mixed with silicone oil at a concentration of 3.0 wt%. Each fluid was well-dispersed in the original state, but the dispersed materials gradually sedimented over time, reaching the equilibrium state at 20 h. After 20 h, each color mica/TiO 2 -based ER fluid exhibited favorable dispersion stability for ER application. The dispersion stability of the materials decreased in the order of white mica/TiO 2 > yellow mica/TiO 2 > red mica/TiO 2 > violet mica/TiO 2 > blue mica/TiO 2 > green mica/TiO 2 , indicating that the dispersion stability of the material decreased with increasing TiO 2 content. in a suitable range for ER applications, and the electrical conductivity of each material was at least two orders of magnitude higher than that of silicone oil, thereby consequently promoting the ER effect of the materials [25]. Finally, the dispersion stabilities of the color mica/TiO2 materials were examined ( Figure 6). For ER applications, dispersion stability is another key factor affecting ER performance, as the sedimentation of materials from the media may disrupt the formation of rigid fibril-like structures. To examine the dispersion stability, each color mica/TiO2 material was thoroughly mixed with silicone oil at a concentration of 3.0 wt%. Each fluid was well-dispersed in the original state, but the dispersed materials gradually sedimented over time, reaching the equilibrium state at 20 h. After 20 h, each color mica/TiO2-based ER fluid exhibited favorable dispersion stability for ER application. The dispersion stability of the materials decreased in the order of white mica/TiO2 > yellow mica/TiO2 > red mica/TiO2 > violet mica/TiO2 > blue mica/TiO2 > green mica/TiO2, indicating that the dispersion stability of the material decreased with increasing TiO2 content.

ER Performances of Color-Mica/TiO2-Based ER Fluids
The ER characteristics of the six differently colored ER fluids were examined by determining the shear stress under various conditions: as a function of shear rate, in E field onoff conditions, and at various E field strengths (Figure 7). First, ER activities were measured as a function of shear rate when an E-field strength of 3.0 kV mm −1 was applied (Figure 7a). All the ER fluids experienced immediate shear stress upon the application of the E field, owing to the electrostatic forces between the dispersed materials in the dispersing media [58]; the shear stress resulted in the formation of fibril-like structures. In the low shear rate region, all ER fluids exhibited Bingham plastic behavior, which was ascribed to the electrostatic forces between materials that were stronger than the hydrodynamic forces generated by mechanical shear [59]. Past the critical shear rate (τc), Newtonian behavior was exhibited as the shear stress increased proportionally with shear rate, indicating the dominance of hydrodynamic forces over electrostatic forces [60]. The determined ER activities for the

ER Performances of Color-Mica/TiO 2 -Based ER Fluids
The ER characteristics of the six differently colored ER fluids were examined by determining the shear stress under various conditions: as a function of shear rate, in E field on-off conditions, and at various E field strengths (Figure 7). First, ER activities were measured as a function of shear rate when an E-field strength of 3.0 kV mm −1 was applied (Figure 7a). All the ER fluids experienced immediate shear stress upon the application of the E field, owing to the electrostatic forces between the dispersed materials in the dispersing media [58]; the shear stress resulted in the formation of fibril-like structures. In the low shear rate region, all ER fluids exhibited Bingham plastic behavior, which was ascribed to the electrostatic forces between materials that were stronger than the hydrodynamic forces generated by mechanical shear [59]. Past the critical shear rate (τ c ), Newtonian behavior was exhibited as the shear stress increased proportionally with shear rate, indicating the dominance of hydrodynamic forces over electrostatic forces [60]. The determined ER activities for the white, yellow, red, violet, blue, and green mica/TiO 2 -based ER fluids were 95.1, 81.9, 71.2, 63.3, 57.5, and 51.1 Pa, respectively; the ER performance of the white mica/TiO 2 -based ER fluid was the highest among the various color mica/TiO 2based ER fluids. This trend mirrors that of the ER performance, which was in the order of white > yellow > red > violet > blue > green materials. These trends in relation to ER activity are in accordance with the experimental results of various suitability tests. As discussed earlier, the white mica/TiO 2 material displayed the highest ion concentrations, the largest electrical conductivity difference with the dispersing media, and excellent dispersion stability compared to the other color mica/TiO 2 materials. In contrast, the green mica/TiO 2 material demonstrated low ion concentrations, electrical conductivity, and dispersion stability, thereby exhibiting the poorest ER performance. The tentative mechanism for the different ER performances of white and green mica/TiO 2 -based ER fluids is shown in Figure 7b. centrations, electrical conductivity, and dispersion stability, thereby exhibiting the poorest ER performance. The tentative mechanism for the different ER performances of white and green mica/TiO2-based ER fluids is shown in Figure 7b.
E-field on-off tests were performed to examine the reversibility of color mica/TiO2based ER fluids (Figure 7c). Without an applied E field, all ER fluids were in the rest position, experiencing a shear stress of less than 1.0 Pa. However, when an E field of 3.0 kV mm −1 was applied, shear stresses were observed immediately for all the ER fluids. Similarly to previous ER measurements, the magnitude of shear stresses experienced by each fluid followed the order of white > yellow > red > violet > blue > green. When the applied E field was turned off, the shear stresses immediately returned to their initial states. Therefore, all the color mica/TiO2-based ER fluids clearly exhibited reversible ER features. Finally, dynamic yield stresses of various color mica/TiO2-based ER fluids were measured as a function of E field strength at a fixed shear rate of 0.1 s −1 (Figure 7d). The applied E field was then elevated from 1.0 to 4.0 kV mm −1 in increments of 1.0 kV mm −1 . The dynamic yield stress of all the ER fluids increased by approximately 1.5 times the power of the E field strength in the region. All the ER fluids exhibited stable dynamic yield stresses without any electrical shorts.  E-field on-off tests were performed to examine the reversibility of color mica/TiO 2based ER fluids (Figure 7c). Without an applied E field, all ER fluids were in the rest position, experiencing a shear stress of less than 1.0 Pa. However, when an E field of 3.0 kV mm −1 was applied, shear stresses were observed immediately for all the ER fluids. Similarly to previous ER measurements, the magnitude of shear stresses experienced by each fluid followed the order of white > yellow > red > violet > blue > green. When the applied E field was turned off, the shear stresses immediately returned to their initial states. Therefore, all the color mica/TiO 2 -based ER fluids clearly exhibited reversible ER features. Finally, dynamic yield stresses of various color mica/TiO 2 -based ER fluids were measured as a function of E field strength at a fixed shear rate of 0.1 s −1 (Figure 7d). The applied E field was then elevated from 1.0 to 4.0 kV mm −1 in increments of 1.0 kV mm −1 . The dynamic yield stress of all the ER fluids increased by approximately 1.5 times the power of the E field strength in the region. All the ER fluids exhibited stable dynamic yield stresses without any electrical shorts.
To examine the real-time response of each ER fluid, optical microscopy (OM) was performed ( Figure 8). The well-dispersed color mica/TiO 2 -based ER fluids were placed between two electrodes. When an E field of 1.0 kV mm −1 was applied, the dispersed materials formed fibril-like structures within a few tens of milliseconds. Notably, each structure exhibited the complementary color of the corresponding color mica/TiO 2 material; specifically, the structures in the white, yellow, red, violet, blue, and green mica/TiO 2 -based ER fluids displayed complementary colors of black, blue, green, yellowish-green, yellow, and red, respectively. Therefore, the OM analysis proved the rapid response time of the ER fluids, while also clearly evidencing the successful synthesis of the color mica/TiO 2 -based ER fluids with their distinct colors. To examine the real-time response of each ER fluid, optical microscopy (OM) was performed ( Figure 8). The well-dispersed color mica/TiO2-based ER fluids were placed between two electrodes. When an E field of 1.0 kV mm −1 was applied, the dispersed materials formed fibril-like structures within a few tens of milliseconds. Notably, each structure exhibited the complementary color of the corresponding color mica/TiO2 material; specifically, the structures in the white, yellow, red, violet, blue, and green mica/TiO2based ER fluids displayed complementary colors of black, blue, green, yellowish-green, yellow, and red, respectively. Therefore, the OM analysis proved the rapid response time of the ER fluids, while also clearly evidencing the successful synthesis of the color mica/TiO2-based ER fluids with their distinct colors.

Conclusions
In conclusion, Mica/TiO2 materials with various colors-white, yellow, red, violet, blue, and green-were successfully prepared by the facile coating of a TiO2 layer on mica material using a sol-gel method, utilizing TiCl4 as a precursor. To control the color of each material, the amount of TiCl4 added was adjusted during the reaction to vary the thickness of the TiO2 layer, and consequently vary the light interference effects. ER suitability tests,

Conclusions
In conclusion, Mica/TiO 2 materials with various colors-white, yellow, red, violet, blue, and green-were successfully prepared by the facile coating of a TiO 2 layer on mica material using a sol-gel method, utilizing TiCl 4 as a precursor. To control the color of each material, the amount of TiCl 4 added was adjusted during the reaction to vary the thickness of the TiO 2 layer, and consequently vary the light interference effects. ER suitability tests, including ion concentration, electrical conductivity, and dispersion stability measurements, were conducted for each color mica/TiO 2 material. The results obtained for all the color mica/TiO 2 materials were in suitable ranges for use in ER applications. The ER activities of the color mica/TiO 2 -based ER fluids were measured using various ER testing methods. Notably, with increasing thickness of the TiO 2 layer, the ER performance decreased: the white mica/TiO 2 -based ER fluid exhibited the highest ER performance (95.1 Pa) owing to its high ion concentration and dispersion stability, and the green mica/TiO 2 -based ER fluid demonstrated the lowest performance (51.1 Pa). Hence, our novel colored ER fluids showed a correlation of color with ER performance, which may establish a new standard for selecting ER fluids by visually evaluating the colors.