An Electrophotorheological (EPR)

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Smart Fluid System Dually Responsive to Light and Electric Fields: An Electrophotorheological Fluid Chang-Min Yoon,‡ Yoonsun Jang,‡ Jungchul Noh, Jungwon Kim, and Jyongsik Jang* School of Chemical and Biological Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Korea S Supporting Information *

ABSTRACT: Electrophotorheological (EPR) fluids, whose rheological activity is dually responsive to light and electric fields (E fields), is formulated by mixing photosensitive spiropyrandecorated silica (SP-sSiO2) nanoparticles with zwitterionic lecithin and mineral oil. A reversible photorheological (PR) activity of the EPR fluid is developed via the binding and releasing mechanism of lecithin and merocyanine (MC, a photoisomerized form of SP) under ultraviolet (UV) and visible (VIS) light applications. Moreover, the EPR fluid exhibits an 8fold higher electrorheological (ER) performance compared to the SP-sSiO2 nanoparticle-based ER fluid (without lecithin) under an E field, which is attributed to the enhanced dielectric properties facilitated by the binding of the lecithin and SP molecules. Upon dual application of UV light and an E field, the EPR fluid exhibits high EPR performance (ca. 115.3 Pa) that far exceeds its separate PR (ca. 0.8 Pa) and ER (ca. 57.5 Pa) activities, because of the synergistic contributions of the PR and ER effects through rigid and fully connected fibril-like structures. Consequently, this study offers a strategy on formulation of dual-stimuli responsive smart fluid systems. KEYWORDS: electrophotorheology, photorheology, electrorheology, smart fluid, spiropyran, dual-responsive mart fluids (i.e., rheological systems) that are responsive to various environmental stimuli are of widespread interest.1 The three major classes of rheological systems most frequently reported are electrorheological (ER), magnetorheological (MR), and photorheological (PR), which are categorized by the response to specific stimuli: electric fields (E fields), magnetic fields, and light, respectively.2,3 Smart fluids are composed of solid particles or molecules, additives, and media that are sensitive to these specific stimuli.4 Accordingly, external stimuli can be used effectively to modulate and tune a rheological property, such as shear stress and viscosity.5 In this manner, various smart fluids have been designed for a wide range of applications, such as dampers, haptic devices, and microfluidic valves.6 An ER fluid consists of a mixture of electrically polarizable particles suspended in insulating oils.7 These oils can be mineral, vegetable, or silicone oils. The application of an E field causes well-dispersed particles to form fibril-like structures via dipole−dipole interactions, resulting in a liquid- to solid-like phase transition.8 ER applications have several advantages, including low operating powers, simple mechanisms, reversibility, simplicity of preparation, and fast responses.9,10 Numerous metal, inorganic, organic, polymer, and composite materials have been adopted as ER materials.11,12 Colloidal stability and the dielectric properties of the materials are the

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© 2017 American Chemical Society

most important factors affecting the performance of an ER fluid. According to previous studies, ER activity can be greatly enhanced using low density, small, and highly polarizable materials.13−15 Although ER fluids possess many positive virtues, they are limited in terms of the choice of dispersing medium. ER applications require high operating voltages (up to 5.0 kV mm−1); hence, the dispersing medium must be able to withstand high voltages to avoid electrical short-circuiting and operating difficulties. Therefore, ER media are limited to oils; other solutions, such as organic solvents, are excluded. The operating mechanism and selection of dispersing medium for MR fluids are similar to those for ER fluids, but magnetic materials and magnetic fields are used instead.16 Conventional ER and MR smart fluids have been widely studied for more than 50 years, and innumerable reports and applications have been produced. PR fluids are the latest smart fluid development. These fluids are composed of photosensitive organic materials, surfactants, gelators, and polymers, and rheological change is induced by light.17 Compared to ER and MR fluids, PR fluids have distinct Received: April 26, 2017 Accepted: September 29, 2017 Published: September 29, 2017 9789

DOI: 10.1021/acsnano.7b02894 ACS Nano 2017, 11, 9789−9801

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Cite This: ACS Nano 2017, 11, 9789-9801

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Figure 1. Schematic illustration of (a) the fabrication method of the spiropyran-decorated silica (SP-sSiO2) nanoparticles and (b) the reversible photoisomerization of spiropyran (SP) into the merocyanine (MC) form on SP-sSiO2 nanoparticles under ultraviolet (UV) (λ = 365 nm) and visible (VIS) light illumination.

advantages of spatiality, precision, and fine control: The intensity of light from various sources can be used and simply needs to be pointed at the desired spot.18 PR fluids have great potential in nano- or microscale applications, including microrobotics, microvalves, flow sensors, and other microfluidic devices.19 However, most PR materials are very complex organic materials, and it is difficult to design a well-operating PR system without understanding the underlying complex chemistry. Thus, still relatively little attention has been paid to the study of PR fluids compared to conventional ER and MR fluids. In addition, PR fluids show relatively low shear stress and viscosity compared to ER or MR fluids, which directly creates the fibril-like structure under applied field, since PR activity is solely based on the perplexed chemical interactions including micelle formation, molecular entanglements, and photoisome-

rization of photosensitive molecules and other additive chemicals.20−22 In this regard, a PR system using simple molecules and commercially available materials is highlighted herein. Various photosensitive molecules, such as azobenzene, diarylethene, stilbene, phenoxynaphthacene, quinones, and spiropyran (SP) are used in a wide variety of photoapplications.23,24 Among these molecules, SP is of particular interest because of its commercial availability and the lightinduced reversibility of its two isomers. The SP molecule photoisomerizes into the merocyanine (MC) form under ultraviolet (UV) irradiation and reverts back to the original form under visible (VIS) light. The reversible SP → MC photoisomerization can be easily observed and is distinguished by a color change. Previous studies reported that SP and SP9790


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advantages of PR and ER fluids and demonstrated a synergistic effect of PR and ER activity in a single fluid.

derived molecules display a yellowish or transparent color, while MC molecules exhibit darker colors such as purple or red (or a reddish color).25,26 The most fascinating characteristic of the SP molecule is that its two isomers possess completely different properties. The original SP isomer is nonpolar, but cleavage of the C−O bond of the chromene group converts the MC molecule into a zwitterionic polar molecule, with a negative charge at the oxygen atom and a positive charge at the nitrogen atom of the indole group.27 Consequently, the two isomers interact quite differently with solvents, matrices, and chemicals. This versatility, reversibility, and commercial availability make SP a good candidate for PR applications. Lee et al. reported the reversible viscosity change of a PR system consisting of SP, sodium deoxycholate, lecithin, and cyclohexane under the alternating application of UV and VIS irradiation.28 However, unlike its use in numerous photoapplications, using SP in the PR field is not widespread. The above-mentioned smart fluid systems have distinct advantages and limitations, and some researchers have attempted to exploit certain synergistic effects to create fluid systems that are responsive to multiple stimuli. A fluid system responsive to both electric and magnetic fields is called an electromagnetorheological (EMR) fluid. EMR materials are usually fabricated as core/shell materials containing magnetic/ polarizable particles. For instance, Minagawa et al. synthesized TiO2-coated iron particles to investigate the drastic increase in viscosity observed with EMR fluids.29 Choi et al. prepared polyaniline-coated iron oxide (PANI/Fe 3 O 4 ) to study enhanced EMR activity under electric and magnetic fields.30 Specifically, the colloidal stability of heavy magnetic particles is improved by coating with lighter polarizable materials, and a more rigid fibril-like structure is formed by superimposing (in parallel) electric and magnetic fields.31 Although synergistic EMR effects have been reported, there is no study concerning the combination of the effects of using PR fluid in conjunction with other smart fluids. This is because of the markedly different operating mechanisms and dispersing medium required for PR fluids, as noted above. Accordingly, this research focused on the creation of smart fluids having both PR and ER activities. In this paper, we report a smart fluid, that is, electrophotorheological (EPR) fluid, which is dually responsive to light (UV−VIS) and E fields. The EPR system was carefully designed by dispersing SP-doped silica (SP-sSiO2) nanoparticles in a specially formulated medium. The photosensitive SP molecule photoisomerizes under UV light irradiation to form the ring-opened zwitterionic MC form and reverts back to its original form under VIS light illumination. This EPR system thus has high reversibility. Also, the EPR dispersing medium, composed of commercially available mineral oil and zwitterionic lecithin at the optimal concentration, greatly enhanced the dispersion stability of the SP-sSiO2 nanoparticles and chemically enabled EPR activity. Under our experimental conditions, the UV light-induced EPR system manifested its PR activity as increased yield stress through interactions between MC and lecithin molecules under UV illumination and decreased yield stress under VIS light illumination. Moreover, the EPR fluid showed positive ER activity under an applied E field without any electric short-circuiting. Finally, the yield stress was further increased by dual application of UV light and an E field according to the combined contributions of PR and ER activity. Consequently, this EPR system successfully leveraged the

RESULTS AND DISCUSSION Synthesis of the Photosensitive EPR Material. The formulation strategy for the EPR system was divided into two parts: (1) synthesis of an EPR material that was responsive to both light and an E field and (2) development of a dispersing medium that would support PR and ER activities. Figure 1a shows the overall synthesis approach. First, core SiO 2 nanoparticles were fabricated using the well-known Stöber method.32 The surface of the SiO2 nanoparticles was then coated with a silane shell (silylated) by dropwise addition of (3glycidyloxypropyl)trimethoxysilane (GPTS) to obtain sSiO2 nanoparticles. Finally, the epoxide group of the silane shell of sSiO2 and the hydroxyl group of SP were connected via a ringopening reaction to obtain photosensitive SP-decorated nanoparticles (SP-sSiO2). These SP-sSiO2 nanoparticles are hereafter referred to as the EPR material. The connection between SP and GPTS molecules was verified by 1H nuclear magnetic resonance (1H NMR) spectroscopy. The reaction mechanism and 1H NMR peaks are described in Figure S1. In this study, SiO2 nanoparticles served as both an ER material and the core template material to accept the PR-active SP molecules and form the SP-sSiO2 nanoparticles. SiO2 is one of the most widely used materials in ER applications because of its numerous advantages, including suitable conductivity, capability for mass production, particle size controllability, and possibility of surface treatment.33−35 From the perspective of PR applications, the final SP-sSiO2 nanoparticles acted as one large photosensitive material. The mechanism for reversible photoisomerization between SP and MC molecules on the SP-sSiO2 nanoparticle surface under UV (λ = 365 nm) and VIS light illumination is schematically shown in Figure 1b. The resulting photoisomerized EPR materials displayed different characteristics, such as polarity and interaction capability toward other chemicals, due to the presence of the zwitterions in MC form.36 Two parameters were controlled during the fabrication of the EPR materials to ensure successful operation of the EPR system, that is, the size of the core SiO2 nanoparticles and the thickness of the silane shell. Generally, PR activity is generated by chemical interactions within the dispersing medium; thus, dispersion stability must be enhanced to prevent degradation of PR performance by sedimentation. To increase the dispersion stability, the core SiO2 nanoparticles needed to be very small. The silane shell thickness also had to be optimized to maximize the SP loading amount to achieve high PR activity. The SiO2 nanoparticles were prepared having diameters of ca. 50, 100, and 250 nm, and their dispersion stabilities were compared by measuring the sedimentation ratio (R), as shown in Figure S2. After 60 h, the 50 nm SiO2 nanoparticles showed a high dispersion stability of 0.95, which was extremely high compared to that for the 100 nm (R = 0.82) and 250 nm (R = 0.59) SiO2 nanoparticles. Therefore, 50 nm SiO2 nanoparticles were adopted as the core template for the EPR material. The silane shell thickness was controlled by changing the silylation times (Figure S3). Notably, the silane shell thickness with the 50 nm-sized sSiO2 nanoparticles was ca. 1, 4, 6, 10, and 19 nm for reaction times of 1, 3, 6, 12, and 24 h, respectively. The sSiO2 nanoparticles (100 mg) reacted with SP molecules, and the resulting SP decoration level of the final SP-sSiO2 nanoparticles was calculated by determining the amount of 9791


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nanoparticle were further revealed by Brunauer−Emmett− Teller (BET) and Barrett−Joyner−Halenda (BJH) N2-sorption analyses (Table 1). The surface area of the silane-coated

unreacted SP left in the supernatant solution using UV−VIS spectroscopy (Figure S4) and the equations described in the Experimental Section. The mass of SP molecules attached to the 100 mg of SPsSiO2 nanoparticles increased from 58.0 to 66.4 mg when the reaction time was increased from 1 to 24 h. However, there was no significant difference between the SP loading amount at 12 h (66.6 mg) and 24 h (66.4 mg). Hence, the optimal silane treatment time was 12 h for the SP loading amount of 66.6 mg (1.89 × 10−4 mol). The loading of SP molecules onto the SPsSiO2 nanoparticles as a function of silylation times is given in Table S1. The morphology of the nanoparticle precursors (SiO2 and sSiO2) and the final EPR material (SP-sSiO2) was studied by transmission electron microscopy (TEM; Figure 2). Highly

Table 1. BET Surface Area and Pore Volume of SiO2, sSiO2, and SP-sSiO2

a

Morphology

BET surface area (m2 g−1)a

Pore volume (cm3 g−1)b

SiO2 sSiO2 SP-sSiO2

48.43 35.31 43.68

0.253 0.273 0.378

Calculated by BET method. bTotal pore volume.

nanoparticles (sSiO2) decreased compared to that of the uncoated SiO2 nanoparticles because of the increased particle size resulting from the silylation. The subsequent increased surface area of the SP-sSiO2 nanoparticles was ascribed to the successful deposition of SP moieties. Furthermore, scanning transmission electron microscopy (STEM) was used to determine the elemental composition of the SiO2, sSiO2, and SP-sSiO2 nanoparticles (Figure 3). Only Si and O were detected for the SiO2 and sSiO2 nanoparticles. Elemental N was additionally observed for the SP-sSiO2 nanoparticles and was due to the N atom present in the indole portion of the SP molecule. In addition, elemental ratio of SiO2, sSiO2, and SPsSiO2 nanoparticles were quantified by elemental analyzer (EA) and inductively coupled plasma-atomic emission spectrometer (ICP-AES), as shown in Table S2. Particularly, SiO2 nanoparticles were mainly composed of Si and O elements, and small amounts of C and H elements were detected due to slight leftover of organic residues from the synthesis process. In the case of sSiO2 nanoparticles, detected amounts of C and H elements were greatly increased compared to SiO2 nanoparticles attributed to the silane coating on the outer shell. Interestingly, N element was only acquired from SP-sSiO2 nanoparticles, indicating the successful decoration of SP molecule on the outermost shell. These results confirmed that the EPR material was successfully synthesized with high uniformity and with SP molecules decorating the outer shell. Development of the EPR System. For the second stage of EPR fluid preparation, the dispersing medium needed to be compatible for both PR and ER activities. However, PR and ER dispersing media are completely different from each other. For instance, PR dispersing media are usually organic solvents or an aqueous mixture that can enable micelle formation, photogelation, and photoisomerization of chemicals. However, dispersing media for ER applications are limited to oils such as silicone, mineral, and vegetable oils. These oils have high breakdown strengths and so can withstand high voltages. The amounts of moisture, ions, and other additives in ER fluids are minimized to prevent the electric short-circuiting induced by the large leakage current, which deteriorates the ER effect. Differences between PR and ER applications are summarized in Table 2. Accordingly, our task was to develop a single EPR dispersing medium having the distinguishing features of PR and ER media. The medium was prepared in two experimental steps: (a) selecting an appropriate base oil and (b) adding lecithin, which induced PR activity by micelle formation and prevented aggregation of the EPR nanoparticles. Each step of the EPR fluid preparation process was verified and supported by various analyses.

Figure 2. TEM micrographs of (a, b) SiO2, (c, d) sSiO2 (silylated), and (e, f) SP-sSiO2 (SP-decorated) nanoparticles, respectively (highly magnified images (b, d, and f)).

uniform SiO2, sSiO2, and SP-sSiO2 nanoparticles were synthesized having diameters of ca. 50, 60, and 63 nm, respectively. It was evident that the silane shell coating and SP decoration process gradually increased the size of the nanoparticles. Scanning electron microscopy (SEM) also revealed the well-defined structures of the nanomaterials (Figure S5). Notably, the SP-sSiO2 nanoparticles had a slightly bumpy outer surface that was attributed to the deposition of SP molecules across the shell. Structural changes of each 9792


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Figure 3. STEM elemental mapping images of (a) SiO2, (b) sSiO2, and (c) SP-sSiO2 nanoparticles, respectively (detected elements: Si, O, and N).

capability for reversible photoisomerization of the SP molecules was verified by subjecting to UV (λ = 365 nm) and VIS light irradiation (Figure 4). The silicone oil dispersion of SP-sSiO2 nanoparticles turned a purplish color when illuminated with the UV light, which indicated the SP → MC transformation, but no reversible color change was observed after subsequent exposure to VIS light (Figure 4a). This nonreversible photoisomerization may have originated from the stabilization of MC molecules in the silicone oil. Specifically, the polar MC molecules had improved stability in silicone oil through dipole−dipole interactions of the Si and O atoms of the siloxane bond and did not convert back to the nonpolar SP form via the ringclosing mechanism.

Table 2. Comparison of ER and PR Fluid ER fluid External stimuli Dispersing medium Dispersant Phase Conductivity

Electric field (up to 5.0 kV mm−1) Oils like silicone, mineral, vegetable, etc. Polarizable particle (nano to microsized) Two (colloid + oil) Low (best near 10−7 S cm−1)

PR fluid UV or VIS light Organic solvent (polar or nonpolar) Photosensitive molecule, additives, gelator, etc. One (molecules dissolved in solvent) Depends on the composition

To select an appropriate base oil, SP-sSiO2 nanoparticles were dispersed in widely used silicone or mineral oil, and the

Figure 4. Photographs of the EPR material dispersed in silicone and mineral oils after treatment with UV (λ = 365 nm) and VIS light. (a) Silicone oil. Left: blank oil; middle: containing the EPR material; right: containing the EPR material + lecithin. (b) Mineral oil. Left: blank oil; middle: containing the EPR material; right: containing the EPR material + lecithin. 9793


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as intended (Figure S8). Specifically, the zwitterionic heads of lecithin molecules formed charge−charge interactions with MC molecules, thereby preventing self-aggregation of the EPR material. With the application of VIS light, the MC form reverted back to its non-ionic SP form, and the EPR material was released from the lecithin. These binding and releasing sequences of the EPR material and lecithin not only prevented MC aggregation but also produced the PR activity which will be discussed in the later section. The UV−VIS spectra of the EPR fluid were examined to further investigate the light-induced effect (Figure 6). The EPR

The mineral oil dispersion of the EPR material also turned a reddish color after UV light illumination, which indicated successful photoisomerization. However, the color-changed EPR material suddenly aggregated and rapidly settled (Figure 4b). This phenomenon, referred to as MC aggregation, was caused by the charge−charge interaction of zwitterionic MC molecules.25,37 Lecithin was added to the mineral oil to solve this aggregation issue. Our hypothesis was that lecithin can interact with MC molecules via charge−charge interactions and thereby prevent the aggregation of the EPR material. Lecithin is a type of phospholipid whose molecules have long hydrophobic tails and zwitterionic heads. Because of the hydrophobicity of the tail part, lecithin molecules were well dissolved in the hydrophobic mineral oil, but not in silicone oil having siliphilicity (Figure S6). Therefore, further experiments only used mineral oil, which seemed to be the more appropriate choice as the base oil for preparation of the EPR fluids. The effect of lecithin on the mineral oil was investigated by measuring the viscosity and leakage current as a function of added lecithin. Lecithin is a viscous organic molecule, and the addition of too much could induce electric short-circuiting of the fluid and degrade the ER activity. The molar ratio of the lecithin added to the mineral oil was based on the maximum SP loading amount (1.89 × 10−4 moles) and ranged from 0.5 to 0.75, 1.0, 1.25, and 1.5. The varying amount of lecithin did not drastically change the viscosity, and only a slight increment was observed with higher addition levels (Figure S7). However, the leakage current varied considerably for the lecithin + mineral oil mixtures under an applied E field strength of 2.0 kV mm−1. The leakage currents for the 0.5, 0.75, 1, 1.25, and 1.5 ratio mixtures were 0.001, 0.003, 0.013, 0.025, and 1.222 mA, respectively. The increasing leakage current with increasing added lecithin was attributed to the higher number of zwitterions. The 1.5 ratio mixture displayed a large leakage current with intermittent electric short-circuiting; this rendered it inappropriate for ER purposes. Based on these results, the amount of added lecithin was set at the lecithin:SP ratio of 1.25:1.0. Additionally, the photoreversibility and dispersion stability of the as-prepared EPR fluid were examined. The EPR fluid manifested a reversible color change, which indicated successful photoisomerization between SP and MC (Figure 5). Moreover, the UV-irradiated EPR fluid displayed excellent dispersibility without any aggregation or sedimentation, which implied that the lecithin played a pivotal role in preventing MC aggregation,

Figure 6. (a) UV−VIS spectra of EPR fluids under UV (λ = 365 nm) and VIS light irradiation. (b) Absorbance changes of the UV− VIS spectra.

fluid was exposed to UV light (λ = 365 nm), and the absorption spectra were recorded at 1 min intervals. In the absence of UV irradiation, the EPR fluid showed almost no absorption peak in the range of wavelengths examined. Application of the UV light caused a strong absorption peak to appear at 540 nm, indicating the SP → MC isomerization of the EPR material.38 Notably, there was no further absorbance change in the spectrum after 3 min of UV irradiation, which indicated that the conversion was complete within 3 min. Moreover, the absorption peak at 540 nm decreased greatly in intensity after VIS light was

Figure 5. Reversible color change of the EPR fluid composed of SPsSiO2 nanoparticles, lecithin, and mineral oil under UV and VIS light illumination. No sedimentation occurred. 9794


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→ 0) and fictitious polarizability (ε∞, f → ∞) of the dielectric constant curve. The Δε of the SiO2, sSiO2, and SP-sSiO2 nanoparticle-based ER fluids were 0.37, 0.46, and 0.81, respectively. The relatively large Δε of the SP-sSiO2 nanoparticles suggested that they had a larger polarization tendency compared to the SiO2 and sSiO2 nanoparticles. This was ascribed to the presence of the organic SP molecule, which has a highly polarizable conjugated π bond in its structure. Fortunately, the deposited SP molecules contributed both photoresponsiveness and high dielectric property to the SPsSiO2 nanoparticles. The relaxation time of each material was estimated using the following equation:41

subsequently applied for 3 min, suggesting successful MC → SP reverse isomerization. Considering these results, reversible photoisomerization of the EPR material was complete within 3 min, and PR activity was induced in this time interval. The EPR fluid was successfully prepared by optimizing various parameters, including the template size (50 nm SiO2), silylation time (12 h), medium selection (mineral oil), and additives (lecithin). EPR Activity of the SP-sSiO2-Based EPR Fluid. Prior to measuring the rheological activity, the dielectric properties of the SP-sSiO2 nanoparticles were compared to those of SiO2 and sSiO2 nanoparticles to evaluate the potential of SP-sSiO2 as an ER material. The dielectric property of a material is closely related to its polarizability, which directly affects the ER activity. Figure 7a graphs the dielectric constant (ε′) and dielectric loss

λ=

1 2πfmax

(1)

where f max is the maximum E field frequency of the dielectric loss factor curve. The calculated λ of the SiO2, sSiO2, and SPsSiO2 nanoparticles was 0.048, 0.043, and 0.039 s, respectively. Therefore, the SP-sSiO2 nanoparticles polarized relatively faster than the SiO2 and sSiO2 nanoparticles. The dielectric properties of the lecithin added EPR fluid were investigated after exposing to UV−VIS light to gain deeper insight into the effect of lecithin and its binding mechanism with EPR material on polarization and ER activity (Figure 7b). To clearly observe the lecithin effect, the lecithin−mineral oil mixture free of EPR material was prepared as well as the EPR fluid. The lecithin− mineral oil mixture manifested an extremely large Δε of 16.66 compared to normal ER fluids. This drastic increment of Δε was attributed to the generation of strong electrode polarization. Previous studies have reported that types of polarization were varied in fluids according to their composition and mixture state.42,43 The polarization induced in the heterogeneous ER fluids were known as an interfacial polarization, generated by space charges created within the interface of solid and dispersing medium.6 With the addition of conductive molecule-like lecithin, polarization type of fluids was dominated by the electrode polarization. It is known that electrode polarization is rather slow process, but strong polarization is facilitated by double-layer formation of polarizable molecules near the electrode.44,45 Thus, presence of lecithin caused large Δε and slow λ of the lecithin-mineral oil mixture distinguished from ER fluids bearing the interfacial polarization. Furthermore, UV-irradiated and VIS-illuminated EPR fluids displayed quite a different Δε of 20.43 and 8.99, indicating that polarizability of EPR fluid was greatly affected by the binding of lecithin and EPR material. Since lecithin and MC can bind via charge− charge interaction, an enlarged size of polarizable entities resulted in a very high polarizability of EPR fluid under UV irradiation. The determined λ of UV-irradiated EPR fluid also further verified the UV-induced size increment of polarizable entities. Particularly, double-layer formation was retarded in the UV-irradiated EPR fluids due to slow movement of enlarged assemblies of lecithin-EPR material toward electrodes, resulting in slow λ. On the other hand, VIS-illuminated EPR fluid displayed decreased Δε attributed to the nonbinding of lecithin and EPR material, but faster λ was observed with reduced size of polarizable material. Consequently, EPR fluid can develop high polarizability under UV application if given sufficient time for polarization. These dielectric experiments successfully investigated the polarizability of EPR fluids and effect of binding of lecithin and EPR material on the polarization. The

Figure 7. Dielectric constant (ε′) and loss factor (ε″) of (a) SiO2, sSiO2, and SP-sSiO2 nanoparticle-based ER fluids and (b) EPR fluids under UV and VIS light applications as a function of E field frequency ( f).

factor (ε″) of SiO2, sSiO2, and SP-sSiO2 nanoparticles dispersed in the mineral oil (the normal ER fluid) as a function of E field frequency (f). Generally, good ER activity is characterized by a large achievable polarizability (Δε) and small relaxation time (λ).39,40 The former is related to the polarization tendency and the latter to the polarization rate of the material. The achievable polarizability is determined from the difference, Δε, between the static polarizability (ε0, where f 9795


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Figure 8. Shear stress curves. (a) Three ER fluids and the EPR fluid. (b) The EPR fluid exposed to UV and VIS light (3 min) as a function of shear rate (s−1) under 2.0 kV mm−1 of E field strength. (c) Yield stress of the EPR fluid exposed to UV and VIS light (3 min) as a function of E field strength at a fixed shear rate of 0.1 s−1. (d) Cyclic on−off testing of the EPR fluid by alternating the E field at a frequency of 0.1 s−1.

fluid, respectively. This result suggested that interactions of zwitterionic MC and lecithin molecules (UV-induced effect) affected the ER activity of the EPR fluid. Also, the ER results were similar for the EPR fluid subjected to UV−VIS exposure three times, which suggested excellent stability and lack of degradation of the EPR materials by numerous applications of external stimuli. The stability of the EPR fluid was further evaluated as a function of the E field strength and by an on−off test. Specifically, the ER performance of the EPR fluids exposed to VIS and UV light for 3 min was measured as a function of E field strength (Figure 8c). The field strength was increased at increments of 0.5 kV mm−1 to 2.0 kV mm−1. The yield stress of the EPR fluids linearly and stably increased by the 1.5 power of the E field strength up to 2.0 kV mm−1. However, the EPR fluid could not withstand an E field strength above 2.0 kV mm−1 because electric short-circuiting occurred from the numerous lecithin zwitterions. Figure 8d shows the results of an E field cyclic on−off test that was also conducted on the EPR fluids. Under an applied E field, the EPR fluids manifested immediate yield stresses that dissipated when the E field was turned off. This on−off process was repeated five times; the EPR fluids exhibited similar yield stresses during the cycles, which indicated good reversibility of the fluid. Therefore, the designed EPR fluids had sufficient stability and reversibility and did not degrade under high E field strengths and exposure to light sources. The PR and EPR activities were examined by the in situ application of external stimuli during the rheological tests for in-depth study of the effect of UV−VIS light on the EPR system. Figure 9a shows the rheological changes of the EPR

detailed dielectric properties of the EPR fluid, lecithin + mineral oil, and three ER fluids are listed in Table S3. The ER activities of the EPR fluid and three ER fluids (only nanoparticles + mineral oil) were visualized by graphing the shear stress (τ) as a function of shear rate (s−1) under an E field strength of 2.0 kV mm−1 (Figure 8a). The EPR and ER fluids exhibited shear stress curves consistent with the formation of fibril-like chains within the fluids. The Bingham plastic behavior observed for all of the samples in the low shear rate region was originated from the balancing of the electrostatic static force of the arranged particles with the hydrodynamic force from the mechanical shearing.46 After passing the critical shear stress (τcrit), all samples showed Newtonian fluid behavior, denoted by the proportional increment of shear stress with shear rate.47 Among ER fluids without lecithin, the SP-sSiO2-based ER fluids had the best performance due to the enhanced dielectric property derived from the SP molecules. The measured ER performances of the SiO2-, sSiO2-, SP-sSiO2-based ER fluids and EPR fluids were ca. 7.7, 9.4, 14.1, and 57.5 Pa, respectively. It was noticeable that the EPR fluids manifested a 4-fold higher ER activity than the SP-sSiO2-based ER fluids. These results indicated that the EPR fluids were well-designed to provide outstanding ER performance via combined enhancement of the dielectric properties of the SP and lecithin molecules. To investigate the effect of photoisomerization of SP on ER activity, the EPR fluids were exposed to UV and VIS light for 3 min prior to the ER investigation, and the shear stress curves were then measured under an E field strength of 2.0 kV mm−1 (Figure 8b). Notably, the UV-irradiated EPR fluid displayed a shear stress of ca. 115.3 Pa, which was 2- and 8-fold higher than VIS light-illuminated EPR fluid and the SP-sSiO2-based ER 9796


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UV irradiation + E field application from 1000 to 1300 s. Specifically, the PR activities of the EPR fluid were examined over the range of 100 to 700 s when only UV and VIS light were applied, and EPR activities were investigated by the simultaneous application of light and an E field (2.0 kV mm−1) in the range from 700 to 1300 s. Figure 9b shows the PR performance of the EPR fluid over the range from 100 to 700 s. Under UV irradiation, the yield stress gradually increased to ca. 0.8 Pa (400 s). With the VIS light application, the yield stress then steadily decreased to the original state of ca. 0.45 Pa. This increasing and decreasing yield stress behavior under UV and VIS light illumination clearly demonstrated the reversible PR property of the EPR fluid. Real-time observation by optical microscopy (OM) was used to gain deeper insight into the PR activity mechanism (Movie S1). Initially, the EPR fluid was exposed to UV light (MC form) and placed on a slide glass for imaging. As soon as the VIS light was applied, the well-distributed EPR materials gradually lost their arrangement and displayed a somewhat aggregated state. These OM observations can be explained as follows: When the EPR material is in its MC form, the head parts of the lecithin molecules bind to the surface of the EPR material to form a micellar structure, while the hydrophobic tail parts bind to the hydrophobic mineral oil. This results in a high dispersibility of lecithin-EPR material assmblies in mineral oil, which leads to an increased yield stress. Nonpolar EPR materials, such as the EPR material in its SP form, may be attracted by intermolecular forces, which results in a nonhomogeneous and relatively low dispersibility. The EPR material in the MC form exhibited higher PR activity due to the stabilization and well-arranged state that arose from the sequential binding of the EPR material, lecithin, and mineral oil. EPR performances were assessed in the interval between 700 and 1300 s (Figure 9a). Under the dual stimuli of VIS light and an E field, the EPR fluid exhibited a greatly increased yield stress of ca. 57.5 Pa compared to the PR region because of the inherently high ER activity. Notably, the EPR fluid manifested the highest yield stress of ca. 115.3 Pa under simultaneous application of UV light and an E field. Real-time OM observations were carried out for two different conditions to further clarify the drastic enhancement of the EPR performance

Figure 9. (a) Real-time observation of EPR activity under the application of various external stimuli: UV light irradiation (4 W cm−2), VIS light illumination, and E field (2.0 kV mm−1) at the fixed shear rate of 0.1 s−1. (b) Magnified PR activity over the time interval from 0 to 700 s.

fluid under various conditions including simultaneous applications of external stimuli. The application conditions for the various stimuli were divided over 300 s, as follows: (a) no stimuli from 0 to 100 s, (b) VIS light illumination from 100 to 400 s, (c) UV irradiation from 400 to 700 s, (d) VIS light illumination + E field application from 700 to 1000 s, and (e)

Figure 10. Proposed mechanism of the distributed state of EPR materials under (a) VIS light and (b) UV light, and resulting differences of fibril-like structure formations under applied E field strength of 2.0 kV mm−1. 9797


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ACS Nano noted under the UV + E field condition. The EPR materials formed partially disconnected or weak fibril-like structures under the VIS + E field condition (Movie S2). However, they displayed extensive formation of fully connected fibril-like structures under the UV + E field condition (Movie S3). Figure 10 illustrates our tentative mechanisms for enhanced EPR activity of the EPR fluid under the UV + E field condition and the formation of the fibril-like structures. Notably, the EPR materials were well-distributed throughout the dispersing medium, and a more rigid and fully connected fibril-like structure can be created with the E field application compared to the case of the inhomogeneously distributed VIS-induced EPR fluid. These results indicate that the binding mechanism of the SP-sSiO2 nanoparticles, lecithin, and mineral oil provided the PR activity and successfully contributed to the high EPR activity under the dual application of UV light and the E field. The ER, PR, and EPR activities of the EPR fluid under various conditions of external stimuli are summarized in Table 3.

decorated on the surface of SiO2 nanoparticles via silylation and a ring-opening reaction of the GPTS epoxide. The resultant SPsSiO2 nanoparticles demonstrated reversible SP and MC photoisomerization under UV and VIS light. Furthermore, addition of lecithin prevented MC aggregation while providing PR activity by zwitterionic interactions with MC molecules. Additionally, the EPR fluid manifested much higher ER activity than normal ER fluids (without lecithin) due to enhanced dielectric properties originating from the lecithin and SP molecules. Furthermore, EPR performance was exploited by simultaneous application of light (UV and VIS) and an E field. The EPR fluid displayed the highest yield stress of ca. 115.3 Pa under the combination of UV irradiation with an E field to illustrate the synergistic effect of PR and ER activity. In detail, EPR materials were well-distributed in the medium through the binding process of MC and lecithin, which helped to create more rigid and fully connected fibril-like structures under the application of an E field. Conclusively, EPR fluid successfully demonstrated the EPR activity under multiple stimuli applications of light and E field.

Table 3. Summarization of ER, PR, and EPR Activities of ER and EPR Fluids Samplea SiO2-based ER fluid sSiO2 -based ER fluid SP-sSiO2-based ER fluid EPR fluid (PR activity) EPR fluid (ER activity) EPR fluid (EPR activity)

E field strength (kV mm−1)

Light sourceb

EXPERIMENTAL SECTION

Highest yield stressc (Pa)

2.0 2.0

− −

ca. 7.7 ca. 9.4

2.0

−

ca. 14.1

−

UV light

ca. 0.8

2.0

VIS light

ca. 57.5

2.0

UV light

ca. 115.3

Materials. 1-(2-Hydroxyethyl)-3,3-dimethylindolino-6′-nitrobenzopyrylospiran, (SP-OH molecule 93.0%) was purchased from TCI Co. Tetraethyl orthosilicate (TEOS), (3-glycidyloxypropyl)trimethoxysilane (GPTS, 98.0%), and silicone oil [poly(methylphenylsiloxane), viscosity = 100 cSt] were obtained from Aldrich Chemical Co. Absolute ethyl alcohol (EtOH, 99.5%), ammonium hydroxide (NH4OH, 28.0−30.0%), and mineral oil (paraffin) were purchased from Samchun Chemical Co. (Korea). Lecithin phospholipid (from soybean, MW: 779.76 g mol−1) was obtained from Avanti Polar Lipids, Inc. All chemicals were used as received without any further purification. Fabrication of SP-sSiO2 Nanoparticles. Synthesis of spiropyrandecorated silica (SP-sSiO2) nanoparticles started with preparation of core template. A 50 nm-sized core silica (SiO2) nanoparticle was fabricated according to the typical Stöber method.32 In a typical synthesis of a SiO2 nanoparticle, EtOH (79 mL), DI water (1.4 mL), and NH4OH (2.0 mL) were mixed by vigorous stirring. Then, TEOS (2.0 mL) was injected into the mixtures, and the reaction proceeded for 12 h at 50 °C. Resulting SiO2 nanoparticles were collected by centrifugation. For the silylation, as-synthesized SiO2 nanoparticles were redispersed into DI water (7 mL) by sonication. Sequentially, EtOH (100 mL) was added to a SiO2 dispersed solution, and GPTS (2 mL) was added dropwise into the colloidal solution with magnetic stirring. The silane shell thickness was controlled by the reaction time from 1 to 24 h according to the necessity. Resulting silylated silica (sSiO2) nanoparticles were collected by centrifugation and redispersed into EtOH (30 mL) solution with sonication and pipetting. Lastly, SPOH (100 mg) was added to the sSiO2 colloidal solution with vigorous stirring for 24 h. Final SP-sSiO2 nanoparticles were centrifuged with EtOH (30 mL)m and the supernatant solution was saved for calculation of the unreacted SP molecule. In addition, 100 and 250 nm-sized SiO2 nanoparticles were fabricated by changing the reaction temperature and amount of reactants. For synthesis of 100 nm SiO2, all other steps are the same to 50 nm SiO2 except that the reaction proceeded at room temperature. In case of 250 nm SiO2, EtOH (200 mL), DI water (32 mL), and NH4OH (42.3 mL) was mixed. Then, TEOS (21 mL) was added to the mixture, and the reaction was carried out at room temperature for 2 h. Preparation of EPR Fluid. EPR fluid (3.0 wt %) was formulated by following procedure: First, as-prepared SP-sSiO2 nanoparticles (0.3 g) were finely ground by using a mortar and pestle. Then, ground SPsSiO2 nanoparticle was dispersed into mineral oil (11 mL) containing lecithin molecules (0.185 g, 2.36 × 10−4 mole) using sonication (1 h) and magnetic stirring (12 h). The concentration of EPR fluid was set to 3.0 wt %, and no other additives were added. The ER fluid (without

a Concentrations of EPR and ER fluids were set to 3.0 wt % in mineral oil. bIntensity of UV light was 4 W cm−2. cHighest yield stress was determined from the origin program.

Lastly, yield stress of EPR fluid was measured as a function particle weight fraction (wt %) to clarify the dependence of rheological performance on concentration of particles (Figure S9). Specifically, EPR fluids were prepared in five concentrations of 1.0, 3.0, 5.0, 7.0, and 9.0 wt %, respectively. The rheological tests were assessed under the aforementioned UV + E field and VIS + E field conditions to examine the effect of UV−VIS light on highly concentrated EPR fluids. With the increasing particle concentration, yield stresses of both UVirradiated and VIS-illuminated EPR fluids gradually increased owing to the formation of more fibril-like structures. Notably, yield stresses of UV-irradiated EPR fluids far exceeded the performance of equivalently concentrated VIS-illuminated EPR fluids, indicating that EPR activities were successfully facilitated in higher concentrations. However, yield stress of 9.0 wt % EPR fluid under the UV + E field condition was unable to measure due to electrical short-circuiting caused by an increased number of conductive lecithin and MC molecules. Our EPR fluid successfully demonstrated ER, PR, and EPR activity depending on the sources and combination of external stimuli.

CONCLUSION The EPR smart fluid was successfully prepared by mixing SPsSiO2 nanoparticles, zwitterionic lecithin, and mineral oil in the appropriate concentrations. Photosensitive SP molecules were 9798


Article

ACS Nano lecithin, 3.0 wt %) was prepared by a similar method of adding finely ground SiO2, sSiO2, and SP-sSiO2 (0.3 g) into mineral oil (10 mL) with sonication (1 h) and magnetic stirring (12 h). Characterization. Morphologies of SiO2, sSiO2, and SP-sSiO2 nanoparticles were analyzed by TEM observation (JEM-200CX) and FE-SEM characterization (JEOL-6700). BET surface area and pore volume of nanoparticles were determined by BJH measurements (ASAP-2010, Micrometrics). Elemental mappings (Si, O and N atoms) of nanoparticles were carried out by STEM (Tecnai F20, FEI) with installation of a Gatan image filter (Gatan, Inc.). Quantified elemental compositions of nanoparticles were acquired by elemental analyzer (EA1110, CE Instruments) and inductively coupled plasma atomic emission spectrometer (ICPS-7500, Shimadzu). NMR peaks of GPTS connected SP-OH molecule were analyzed by 300 MHz NMR (Avance-300, Bruker) [GPTS connected SP-OH molecule: 1H NMR (300 MHz, MeOD) δ = 0.94 (m, 2H), 1.20 (s, 6H) 1.37 (m, 2H), 3.26−3.41 (m, 15H), 3.64−3.75 (m, 4H) 4.23−4.27 (m, 1H), 6.02− 6.05 (d, 1H), 6.65−6.68 (d, 1H), 6.79−6.86 (m, 2H), 7.06−7.17 (m, 3H), 7.65−7.72 (d, 1H), 8.11−8.12 (d, 1H)]. Viscosities of lecithin dispersed mineral oils were determined using a rheometer (AR 2000, TA Instruments) with parallel plate geometry (diameter: 20 mm). Leakage current of lecithin dispersed mineral oils was obtained by a high-voltage generator (Trek 677B). UV−VIS absorption spectra of EPR fluid were determined using an UV−VIS spectrophotometer (Lambda 35, PerkinElmer). Dielectric properties of EPR fluid and ER fluids were assessed by dielectric interface (Solatron 1296) and frequency response analyzer (Solartron 1260). Real time PR effect of EPR fluid and fibril-like structure formation was observed by OM instrument (Nikon Lv100 microscope, Nikon). Calculation of the SP Doping Level. The SP-sSiO2 nanoparticle dispersion was centrifuged, and the supernatant solution (ca. 30 mL) retained; any unreacted SP was left in the supernatant. This solution (0.5 mL) was diluted with EtOH (2.5 mL). A reference sample was prepared by dispersing SP (100 mg) in EtOH (30 mL), which was the same SP concentration used for synthesis of the SP-sSiO2 nanoparticles. This reference sample (0.5 mL) was also diluted with EtOH (2.5 mL). The UV−VIS spectra of the diluted supernatant and reference solutions were measured (Lambda 35, PerkinElmer). The absorbance of the supernatant solution was compared to that of the reference to calculate the excess SP according to the following relation:

A supernatant /msupernatant SP = A reference /mreference SP

10 mL) were placed into a cup, and geometry was inserted. Sequentially, shear rate (10.0 s−1) was applied to obtain equilibrium state of EPR fluid. Finally, external stimuli of light and E field were simultaneously applied to determine the EPR activities of EPR fluids. For the PR activity of EPR fluid, single irradiation of UV−VIS light was applied during the in situ rheological measurement.

ASSOCIATED CONTENT

S Supporting Information *

The contents of Supporting Information may include the following: This material is available free of charge via Internet at The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02894. (1) Reaction mechanism and 1H NMR peaks of GPTS and SP; (2) dispersion stability of nanoparticles; (3) TEM micrographs of various silane treated silica (sSiO2) nanoparticles; (4) UV−VIS absorption spectra of leftover SP in supernatant solution; (5) attached amount of spiropyran on SP-sSiO2 nanoparticles; (6) SEM micrographs of SiO2, sSiO2, and SP-sSiO2 nanoparticles; (7) elemental compositions of SiO2, sSiO2, and SP-sSiO2 nanoparticles; (8) digital photograph of lecithin mixed silicone and mineral oil; (9) viscosities of lecithin-mineral oil mixture in various concentrations; (10) digital photograph of EPR fluid under UV and VIS light irradiation; (11) dielectric properties of EPR and ER fluids; and (12) yield stress of various concentrated EPR fluids (PDF) Movie S1: Real-time observation by OM for insight into the PR activity mechanism (AVI) Movie S2: EPR materials formed partially disconnected or weak fibril-like structures under the VIS + E field condition (AVI) Movie S3: Extensive formation of fully connected fibrillike structures under the UV + E field condition (AVI)

AUTHOR INFORMATION

(2)

Corresponding Author

*E-mail: [email protected].

where Asupernatant is the absorbance of the supernatant solution, msupernatant SP is the amount of excess SP in the supernatant solution, Areference is the absorbance of the reference, and mreference SP is the amount of SP in the reference solution (100 mg). Solving this relation provided the amount of excess SP. Finally, the mass of SP decorated on the SP-sSiO2 nanoparticles, mSP, was calculated with the following relation: mSP = mtotal SP − msupernatant SP (3)

ORCID

Jyongsik Jang: 0000-0002-0415-802X Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

where mtotal SP is the mass of SP added in the reaction (100 mg). Investigation of Various Rheological (ER, PR, and EPR) Activities. All of ER, PR, and EPR activities were measured using sets of rheometer (AR 2000 Advanced rheometer, TA Instruments), cup (diameter: 30.0 mm and height: 30.0 mm), and concentric cylinder geometry (diameter: 28.0 mm and height: 30.0 mm). External stimuli were generated by high-voltage generator (E field strength, Trek 677B), UV lamp (ENF-240C, 4 W cm−2), and xenon lamp (Siriu 300P, Zolix). In the case of ER activity, rheological measurements were started by placing the ER or EPR fluid (3.0 wt %, 10 mL) into a cup and submerging the concentric cylinder geometry. For the EPR fluid, intended UV−VIS light was applied for 3 min prior to placing the samples into the rheological instrument. Mechanical shear (10.0 s−1) was applied continuously to samples for 10 min, ensuring the homogeneous distribution of dispersants. Finally, E field strength was applied, and ER activities of ER and EPR fluids were determined. On the other hand, EPR activities were investigated by in situ stimulation of light and E field at the same time. Particularly, EPR fluids (3.0 wt %,

ACKNOWLEDGMENTS This work was supported by the Development Fund of Seoul National University funded by Dongjin Semichem Co., South Korea (0668-20150016). REFERENCES (1) Stanway, R. Smart fluids: Current and Future Developments. Mater. Sci. Technol. 2004, 20, 931−939. (2) Lee, S.; Noh, J.; Hong, S.; Kim, Y. K.; Jang, J. Dual StimuliResponsive Smart Fluid of Graphene Oxide-Coated Iron Oxide/Silica Core/Shell Nanoparticles. Chem. Mater. 2016, 28, 2624−2633. (3) Avó, J.; Cidade, M. T.; Rodriguez, V.; Lima, J. C.; Parola, A. J. Photorheological Ionic Liquids. J. Phys. Chem. B 2015, 119, 6680− 6685. (4) Kim, S. D.; Zhang, W. L.; Choi, H. J. Pickering EmulsionFabricated Polystyrene−Graphene Oxide Microspheres and Their Electrorheology. J. Mater. Chem. C 2014, 2, 7541−7546. 9799


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