Chameleon-Inspired Mechanochromic Photonic Films Composed of

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Chameleon-Inspired Mechanochromic Photonic Films Composed of Non-ClosePacked Colloidal Arrays Gun Ho Lee,† Tae Min Choi,† Bomi Kim,† Sang Hoon Han,† Jung Min Lee,‡ and Shin-Hyun Kim*,†

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†

Department of Chemical and Biomolecular Engineering (BK21+ Program), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡ The 4th R&D Institute, Agency for Defense Development, Daejeon 34060, Republic of Korea S Supporting Information *

ABSTRACT: Chameleons use a non-close-packed array of guanine nanocrystals in iridophores to develop and tune skin colors in the full visible range. Inspired by the biological process uncovered in panther chameleons, we designed photonic films containing a non-close-packed face-centered-cubic array of silica particles embedded in an elastomer. The non-closepacked array is formed by interparticle repulsion exerted by solvation layers on the particle surface, which is rapidly captured in the elastomer by photocuring of the dispersion medium. The artificial skin exhibits a structural color that shifts from red to blue under stretching or compression. The separation between inelastic particles enables tuning without experiencing significant rearrangement of particles, providing elastic deformation and reversible color change, as chameleons do. The simple fabrication procedure consists of film casting and UV irradiation, potentially enabling the continuous high-throughput production. The mechanochromic property of the photonic films enables the visualization of deformation or stress with colors, which is potentially beneficial for various applications, including mechanical sensors, sound−vision transformers, and color display. KEYWORDS: chameleons, colloidal crystals, non-close-packed arrays, structural colors, mechanochromism

S

strain-induced color change, or mechanochromism, for various applications. For example, strain or pressure can be measured in a colorimetric manner with mechanochromic films;7−9 fingerprints can be readily recognized by analyzing color pattern.9 In addition, lasing from fluorescent molecules in the photonic crystals can be modulated by strain-induced adjustment of the stop band position.10 Through a local modulation of the elastic modulus, the color pattern can be encrypted; the hidden pattern is disclosed when the films are subjected to extensional stress.11,12

everal species of chameleons have a distinguished talent to rapidly tune their colors between a cryptic (camouflage) state and a highly visible excited state during male−male combats and courtship.1−3 Recent analysis combining microscopy, photometric videography, and photonic band-gap modeling has revealed that panther chameleons change colors primarily through the active tuning of a lattice of guanine nanocrystals within iridophore cells.3 These specialized dermal cells constitute therefore tunable photonic crystals. Photonic crystals are periodic nanostructures in the length scale of half the wavelength, which develops colors through the reflection of selected wavelengths of visible light.4−6 As the period of the photonic crystals dictates the wavelength of reflection, the color can be tuned by adjusting the period with mechanical strain. Colloidal crystals have been used to provide © 2017 American Chemical Society

Received: August 18, 2017 Accepted: November 2, 2017 Published: November 2, 2017 11350

DOI: 10.1021/acsnano.7b05885 ACS Nano 2017, 11, 11350−11357

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Figure 1. Mechanochromic films containing a non-close-packed colloidal array. (a) Relaxed chameleon showing green color (left) and excited, showing yellow color (right). Transmission electron microscope (TEM) images show the change of periodicity in a regular array of guanine nanocrystals (reproduced from Teyssier et al., Nature Communications3 by courtesy of Prof. Michel Milinkovitch). (b) Schematic of mechanochromic photonic films composed of a non-close-packed array of silica particles embedded in an elastomeric matrix. (c, d) Photographs of a photonic film taken at two different conditions: (c) structurally colored when the reflection condition is satisfied and (d) translucent when the condition is unsatisfied. (e) Scanning electron microscope (SEM) image showing the film cross-section containing a non-close-packed array of silica particles. Inset is a fast Fourier transform (FFT) image. (f, g) Series of photographs of a photonic film with three different strains taken in reflection (f) and transmission (g).

electrostatic repulsion and are immobilized by a hydrogel.25−27 Although the non-close-packed arrays embedded in waterswollen gels provide a mechanochromic property, the low mechanical stability of gels against stretching and vaporization of water in the air environment allows only a short-term use in restricted conditions. Thus, a simple and scalable method to create non-close-packed arrays in a liquid-free elastic matrix remains an important yet unmet need for highly stretchable, durable photonic films with reversible full-color tunability. In this work, we design mechanochromic elastomers containing a non-close-packed array of silica particles by mimicking the iridophore structure of chameleons.3 Silica particles are dispersed in a carefully selected rubber precursor to induce interparticle repulsion through the formation of a solvation layer on the surface of particles. The repulsive potential leads to spontaneous crystallization of silica particles above the threshold volume fraction, and the regular array is captured by rapid photopolymerization of the precursor in a liquid-free rubber matrix. The elastic composites exhibit a pronounced reflection color which blue-shifts in full visible range as the film is stretched up to a strain of 70%. During the stretching, silica particles remain untouched by following the positions of the elongated lattice. The negligible rearrangement of particles renders the photonic composites fully reversible in a color shift along the strain, implying that the deformation is elastic and no residual strain remains. The elastic composites also show color change with compression. The compression with stamps causes a blue-shift of color on relief regions and a red-shift on engraved regions, developing multicolor patterns.

To render colloidal crystals mechanochromic, various combinations of materials and processes have been exploited. For example, interstices of close-packed colloidal crystals, or opal structures, are filled with elastomers;13−15 although the composites show strain-induced color shift, strain and color shift are highly limited because the extension entails significant rearrangement of close-packed colloids. Swelling of the elastomeric matrix with organic solvents slightly increases the allowable strain.16,17 Inverse opals composed of close-packed air cavities embedded in elastomers have negligible resistance against deformation of cavities, thereby providing a full-color change.18 However, delicate procedures of fabrication and low mechanical stability of inverse opal structures restrict practical uses; only small extensional strain is allowed. Alternatively, colloids composed of an inelastic core and elastic shell are crystallized and fused by a melt-flow technique to form highly stable, flexible composite structures;19−22 this approach enables the continuous production of the composite films or fibers by roll-to-roll or extrusion process.23,24 Although the composites can display a color shift in the full visible range by stretching, the deformation is not purely elastic and leaves a residual strain as interparticle separation is insufficient.19,21 Furthermore, colloids are crystallized only near the surfaces, achieving a limited photonic performance.19 A non-close-packed array of colloids can provide a large deformation of the elastic matrix and therefore enables a fullcolor change in the absence of colloidal rearrangement. To achieve the large interparticle separation, charged colloids have been spontaneously crystallized in an aqueous medium through 11351

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Figure 2. Threshold volume fraction for crystallization. (a) Cartoons showing the arrangement of particles in a liquid resin with three different volume fractions: Particles are randomly distributed at a volume fraction below the threshold (ϕ < ϕth), whereas they form a non-closepacked crystal at volume fractions above the threshold (ϕ > ϕth). (b, c) Set of optical microscope (OM) images (b) and reflectance spectra (c) of photonic films prepared from dispersions of silica particles with d = 150 nm at different values of ϕ = 0.1, 0.2, 0.26, 0.33, and 0.4. (d) Volume fraction dependence of a reflectance peak position, λmax (black squares), and a full width at half-maximum, Δλ, normalized by λmax (red triangles). The black line indicates Bragg’s equation for stacked (111) planes of a non-close-packed fcc structure.

the dispersion into a gap between two glass slides by capillary force and irradiating with ultraviolet (UV) light (Figure S1 of the Supporting Information). At the same time, the films are translucent (Figure 1d). The films are composed of a nonclose-packed array of silica particles embedded in the matrix of polymerized PEGPEA (pPEGPEA) (Figure 1e). The ordered array develops color, and the low contrast of the refractive index between silica (np = 1.45) and pPEGPEA (nm = 1.502) leads to negligible scattering out of the stop band, rendering the films translucent. These optical properties are further confirmed with reflectance and transmittance spectra, which show a narrow peak and dip, respectively, at the same wavelength of the stop band (Figure S2). The transmittance is as high as 80% out of the stop band, and the sum of reflectance and transmittance is close to 95% even for a short wavelength, indicating negligible scattering in the entire visible range. The photonic film exhibits a blue-shift of reflection color as it is stretched (Movie S1 of the Supporting Information). The red film at normal reflection turns green and blue for strains of 0.20 and 0.47, respectively (Figure 1f). With a backlight, the film shows complementary color as light at the stop band is blocked, while light out of the stop band is transmitted (Figure 1g). Therefore, the red film at reflection appears cyan at transmission, which is the result of color mixing of green and blue. The cyan color turns to magenta and yellow at the transmission as the film is stretched; the magenta is originating from the color mixing of red and blue, and yellow originates from red and green, respectively. Threshold Volume Fraction for Crystallization. The interparticle repulsion is effective only when solvation layers of neighboring particles are overlapped. Therefore, there exists a threshold volume fraction of particles, ϕth, for the formation of

This approach based on a photocurable colloidal dispersion is potentially applicable for roll-to-roll or jetting processes to continuously produce mechanochromic films or fibers for versatile uses.

RESULTS AND DISCUSSION Non-Close-Packed Array Embedded in Elastomeric Matrix. In dermal iridophores of chameleons, particles of rigid guanine nanocrystals are embedded in a matrix of elastic cytoplasm to form non-close-packed crystals.3 As the cytoplasm undergoes a rapid volume change, the lattice parameter is altered in the range of 240−480 nm, while maintaining the lattice structure, thereby leading to a drastic color shift in the full visible range (Figure 1a). We mimic the iridophore structure by embedding a non-close-packed array of rigid silica particles in an elastomeric matrix (Figure 1b). To compose the structures, silica particles are dispersed in a rubber precursor of poly(ethylene glycol) phenyl ether acrylate (PEGPEA). The dispersion is highly stable against agglomeration and sedimentation even at volume fractions higher than 40% at least for several months. The high stability is attributed to repulsive interparticle potential exerted by solvation layers formed on the particle surface. The acrylate groups of PEGPEA are strongly bound on the surface of silica particles through the hydrogen bonds with silanol groups, forming a dense solvation layer.28,29 Therefore, silica particles repel each other due to the disjoining pressure when particles are brought close together. The interparticle repulsion facilitates the formation of a regular array of silica particles. The composite films, produced by photopolymerization of the dispersion, show pronounced reflection colors (Figure 1c); the film is prepared by infiltrating 11352

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Figure 3. Reversible color change in full visible range by extension. (a) Series of OM images and a CIE plot showing a blue-shift of structural color along with extensional strain, ε. The photonic film is prepared with silica particles with d = 195 nm at ϕ = 0.33. (b) Reflectance spectra of the film for various strains. (c) Reversible change of λmax during cycles of extension to ε = 40% and relaxation to ε = 0%. (d) λmax (black squares) and Poisson ratio, ν (red circles), as functions of extensional strain. Insets correspond to model structures with ε = 0, 21.0, 41.2, and 60.8%. (e) Extensional-strain dependence of reflectivity at λmax (black squares) and refractive index contrast, Δn, between two neighboring slices with equal thickness (red circles) estimated from model structures: one slice contains the (111) plane in the middle and the other contains an intermediate plane. Insets show the pairs of two slices with ε = 0, 21.0, 41.2, and 60.8%.

the ordered array (Figure 2a). At volume fractions lower than the threshold (ϕ < ϕth), no overlap among the solvation layers and a disordered array of particles are expected. At the threshold (ϕ = ϕth), the average interparticle separation is comparable to double the thickness of the solvation layer, and particles spontaneously form a regular array with a long-range order. The interparticle separation will be reduced, while maintaining the long-range order, above the threshold (ϕ > ϕth). To estimate the thickness of the solvation layer and the threshold volume fraction responsible for the long-range ordering, composite films are prepared from dispersions of silica particles with various volume fractions in the range of ϕ = 0.1−0.4; silica particles with a diameter of d = 150 nm are used for all volume fractions. The film at ϕ = 0.1 shows no pronounced color. The film at ϕ = 0.2 shows green colors, which blue-shifts along with ϕ for ϕ ≥ 0.20 (Figure 2b). The volume-fraction dependence of the color change is further confirmed with reflectance spectra (Figure 2c). The spectrum peak blue-shifts along with ϕ for ϕ ≥ 0.20, whereas no peak occurs at ϕ = 0.1. The peak positions for ϕ ≥ 0.26 are in a good agreement with the stop band position, λmax, expected from Bragg diffraction from (111) planes of a non-close-packed facecentered-cubic (fcc) lattice (Figure 2d): λmax = 2d111neff

⎛ π ⎞1/3⎛ 8 ⎞1/2 =⎜ ⎟ ⎜ ⎟ dneff ⎝3 2ϕ⎠ ⎝3⎠

The coincidence between the peak positions of spectra and λmax from eq 1 indicates that silica particles form a non-closepacked fcc structure whose (111) planes are aligned along with the film surfaces. The peaks for ϕ ≥ 0.26 have a self-consistent value of a full width at half-maximum normalized by the stop band, Δλ/λmax, as approximately 0.04 (Figure 2d), indicating an insignificant difference in ordering. By contrast, the peak for ϕ = 0.20 has a value of Δλ/λmax as large as 0.081, implying a less ordered colloidal array. In addition, the peak is located at 543 nm, which is smaller than a λmax of 565 nm from eq 1. This indicates that the colloidal array does not fully occupy the whole volume of the film; the value of d111 estimated from the peak position by assuming a perfect fcc crystal is 182 nm, whereas the value of d111 is 189 nm according to eq 1, which assumes non-close-packed crystals that occupy the whole volume. When we assume that all the silica particles in the film constitute the crystal, the film is composed of 89% crystal region and 11% particle-free region. The interparticle separation in the crystal region is 73 nm, from which the thickness of the solvation layer, t, is estimated as 36.5 nm and ϕth is estimated as 0.225 for silica particles with d = 150 nm. The value of ϕth varies with the diameter of the silica particles. The thickness of the solvation layer, t, is expected to be independent of particle diameter, d. Therefore, the surfaceto-surface separation at the threshold volume fraction for large particles is the same as that of small particles. Therefore, larger particles occupy more volume and result in a larger threshold volume fraction than smaller ones. The values of ϕth are estimated as 0.222 for d = 148 nm, 0.248 for d = 166 nm, 0.260 for d = 175 nm, 0.285 for d = 195 nm, and 0.309 for d = 216 nm (see Figure S3 and related paragraph in the Supporting Information for the detailed calculation). We prepared photonic films with various stop band positions with silica

(1)

where d111 is the distance between two neighboring (111) planes and neff is an effective refractive index.30 To determine the value of neff, we use the Maxwell−Garnett average of refractive indices of silica particles and a pPEGPEA matrix: neff 2 = n p2ϕ + nm 2(1 − ϕ)

(2) 11353

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Moreover, the photonic films maintain a resonant wavelength and reflectivity during long-term storage at room temperature, as shown in Figure S8. This indicates that the photocurable dispersions and photonic films contain a negligibly small amount of volatile solvent. The elastic deformation and corresponding color shift across the entire visible range are the results of sufficient separation between two neighboring silica particles; crystals with small separation undergo plastic deformation for large εx.19,21 The average separation between two nearest particles is 60 nm for the film composed of d = 195 nm at ϕ = 0.33. We model the structural change of the colloidal array from the blue-shift of the stop band. The stop band wavelength, λmax(εx), blue-shifts as much as εzλmax (εx = 0) as the film is stretched:

particles with different diameters while maintaining ϕ = 0.33, which is larger than ϕth for all particles (Figure S4). Therefore, a colloidal array with a long-range order fully occupies the films, and reflectance peak positions are in a good agreement with eq 1. Mechanical and Surface Properties of Photonic Films. The particle-free matrix polymer of pPEGPEA is highly elastic and shows a constant Young’s modulus of 0.23 MPa for the entire range of strain below the fracture at 0.91. The composite photonic film has a Young’s modulus of 0.46 MPa for a strain smaller than 0.47, which increases to 1.03 MPa for a strain below the fracture at 0.75 (Figure S5). The modulus at low strain is only 2 times larger than that of particle-free pPEGPEA because rigid silica particles are embedded without contact. As strain increases, the silica particles are brought closer, leading to the increase of the modulus. Strain at the fracture slightly decreases, as the film contains inelastic fillers. The low elastic modulus, as well as finite surface energy, renders the photonic film adhesive. Therefore, the film sticks to itself or other materials to reduce the surface energy, making it difficult to handle. To prevent the self-adhesion, the surfaces of the photonic film are modified by reactive ion etching (RIE) with SF6. The RIE treatment fluorinates the surfaces and therefore reduces the surface energy. We confirm the reduction of the surface energy from the contact angles of water; the angle is 67° for the pristine films and 138° for the fluorinated ones (Figure S6). The fluorinated films are almost free from self-adhesion (Movie S2). The RIE treatment alters only the surface property of the films while retaining the colloidal crystals in the thickness, providing the same color and reflectance spectrum as the pristine sample (Figure S6). The nonsticky property is maintained for several days. High Reversibility of Mechanochromism. The elastic photonic films instantly change color as they are stretched. The stretching reduces the thickness of the film, thereby causing the reduction of d111. The color is therefore dictated by strain along the lateral direction, εx. The photonic film composed of d = 195 nm at ϕ = 0.33 shows a red reflection color and stop band at 618 nm in the absence of strain. As εx increases to 69%, the color and stop band continuously shift from red to blue, covering almost the entire visible range (Figure 3a and b). Compared with swollen opals, inverse opals, or mechanochromic gels,18−22 the photonic films allow significantly high extensional strain (∼70%), as the composite structure is composed of non-close-packed silica particles embedded in a solvent-free elastomeric matrix with high stability against extension. The maximum strains of swollen opals and mechanochromic organic gels are limited to approximately 20% and 22%;16,31 inverse opals and mechanochromic hydrogels allow only compressive strain.18 The color shift induced by the extensional strain is also highly reversible (Figure 3c) as a result of elastic deformation. Because particles do not rearrange during the deformation, no residual strain is observed when the film is free from stress even after applying a maximum strain of 70%, thereby fully recovering the original color and stop band wavelength (Movie S3). Although photonic films prepared by a melt-flow technique using colloids with an inelastic core and elastic shell allow large extensional strain, insufficient particle separation leads to plastic deformation and, therefore, results in a residual strain for color tuning in the full visible range.19,21 The high reversibility is conserved for at least 3 days of stretching with εx = 0.4 in the temperature range between −20 and 70 °C (Figure S7).

λmax (εx) = 2d111(1 − εz)neff = 2d111(1 − νεx)neff

(3)

where ν is Poisson’s ratio. The responses of εz and ν for εx are estimated from reflectance peak positions using eq 3. The lattice structure of the deformed composite film can be constructed from values of εx and εz, where εy is set to εz because the pPEGPEA matrix is isotropic (insets of Figure 3d). The centers of particles follow the deformation of the matrix, which does not cause contact between two particles for given ranges of εx, εy, and εz. The interparticle separation at the strain-free state is sufficient to avoid rearrangement of particles during the stretching, thereby enabling the elastic extension without residual strain. We confirm no rearrangement of silica particles embedded in the stretched photonic film from a crosssectional SEM image (Figure S9). As expected from the lattice model, the silica particles follow the position of the elongated lattice without additional rearrangement. The stop band steeply shifts at small strain, and the slope is lowered as the strain increases (left y-axis of Figure 3d). The value of ν calculated from the slope for εx ≤ 0.077 is 0.45, which is slightly smaller than 0.5 (value for rubber); that is, the deformation of the composite film is governed by the elastic pPEGPEA matrix for small strain as inelastic silica particles are separated. As the strain increases, the value of ν for the small interval of εx decreases (y-axis of Figure 3d); the value of ν for 0.61 ≤ εx ≤ 0.69 is 0.23. For large stain, inelastic particles are brought close, partially losing the property of the elastic matrix. That is, the volume is not conserved as silica particles resist the matrix deformation during the stretching. During the blue-shift of the stop band, reflectivity at the stop band is reduced. Because silica particles are embedded in the elastic matrix, particles maintain the regular array during the stretching. Nevertheless, silica particles in two neighboring (111) planes are partially interpenetrated to each other, reducing the refractive index contrast between a slice containing the (111) plane at the middle and an intermediate slice. The effective refractive indices of the two slices are estimated using eq 2 based on model structures (Figure S10a). The volume fraction of the matrix increases as the matrix is expanded by the strain for the slice containing the (111) plane, whereas the volume fraction of silica particles increases as silica particles invade from the (111) planes for the intermediate slice (Figure S10b). Therefore, the index contrast decreases as strain increases. As the strength of the Bragg reflection linearly varies with the index contrast for a fixed number of alternating layers, the reflectivity is also expected to vary in the same manner.32 It is clearly shown that the tendency of the contrast reduction is comparable to that of reflectance reduction (Figure 3e), 11354

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Figure 4. Compression of photonic films with stamps. (a) Schematic illustration showing compression of a photonic film with a patterned stamp. The staked (111) planes are contracted on relief regions and expanded on engraved regions. (b) Photographs and OM image of a photonic film before (left panels) and after (right panels) compression with a stamp that contains small ‘K’s to form a large ‘K’. (c) Reflectance spectra of uncompressed (red line) and compressed photonic film (green line). Reflectance spectra locally measured from small ‘K’ (dark green line) and background (magenta line) are included. (d, e) Sets of OM images of a stamp and a compressed photonic film, where the stamps have parallel lines (d) and a square array of dots (e).

by reflectance spectra taken at local areas; the spectrum at small ‘K’ has a sharp peak at 591 nm, whereas that at the background has a sharp peak at 619 nm. The photonic film can develop dual-colored patterns at a high resolution. For example, a stamp with a line pattern composed of 100-μm-width reliefs and 100μm-width engraved regions produces alternate green and red lines with comparable dimensions under compression (Figure 4d). A stamp containing a square array of 50-μm-diameter circular pillars develops a green dot array on a red background (Figure 4e). The development of the dual-colored patterns is reversible (Movie S4).

implying that the reflectance reduction is caused by the contrast reduction. Color Change by Compression. The elastic photonic films show a color shift in response to compressive stress. When the film is compressed by stamps, the film under the relief region is contracted, thereby causing a blue-shift of color (Figure 4a). At the same time, the film under the engraved regions is expanded as the volume of the composite is roughly conserved, thereby causing a red-shift.33 To confirm the color shift, the elastic photonic film that exhibits a red reflection color is compressed with a stamp composed of reliefs of small ‘K’s that form a large ‘K’. The single-colored film forms a dualcolored pattern by the compression (Figure 4b). The reflectance spectrum is also changed from a sharp peak at 610 nm to a broad peak (Figure 4c). The broadening is caused by simultaneous blue- and red-shifts of color on the relief and engraved regions, respectively. The shifts are further confirmed

CONCLUSION In summary, we mimic the photonic structure of dermal iridophores of panther chameleons by embedding a non-closepacked array of silica particles in an elastic polymer matrix.3 The sufficient separation between particles enables the 11355

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numerical aperture of 0.30 is used for spectrum measurement, and a field stop is used to measure spectra from local areas in some experiments. The transmission spectra are measured using a spectrometer (SP-2300i/PIXIS: 400B_eXcelon, Princeton Instruments) equipped in an inverted microscope (Eclipse Ti, Nikon). Cross-sections of the films are observed using scanning electron microscopy (S-4800, Hitachi) after coating with osmium tetraoxide. Stress−strain curves are measured using a microforce testing system (8848, Instron Corporation). Contact angles of water drops on the films are measured using a goniometer (DSA 10-Mk2, KRUSS).

reversible color shift in the full visible range in the absence of significant particle rearrangement under extensional stress as the iridophores do. The only remaining problem is the reduction of reflectivity during the stretching process. As we proved with a lattice model, the optical loss is caused by the reduction of the refractive-index contrast. The problem is potentially overcome by swelling the photonic structure with a nonvolatile organic solvent. As the swelling increases the interparticle separation, a larger blue-shift of structural color can be achieved with a small reduction in the index contrast. The mechanochromic materials can visualize strain with colors. This deformation−vision transformation is useful for various colorimetric measurements. For example, surface topologies of materials, such as fingerprints, can be mapped with color by compressing the films with them (Figure 4). In addition, the real-time deformation of skin can be visualized by attaching the photonic film on the surface (Figure S11 and Movie S5); the elastic modulus of the films, approximately 1 MPa, is comparable to that of skin, thereby minimizing mechanical mismatch between the two layers during the deformation.34 The mechanochromic films have great potential as active color reflectors for large-scale wallpaper or signboard displays in combination with site-addressable pneumatic compressors.35,36 In addition, the photonic films with low elastic modulus are potentially able to convert acoustic waves into colors, transforming sound into visual senses.37 The mechanochromic films are simply prepared by casting of a colloidal dispersion and subsequent UV exposure without any tedious processes. This process is compatible with continuous roll-to-roll or slot coating production with high throughput. Moreover, the format of materials can be further modified into granules, fibers, and any 3D architectures by employing microfluidic or 3D printing techniques, extending the applicable area of mechanochromic materials.38

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05885. Reflectance and transmittance spectra of photonic films; color tunability with particle diameter; stress−strain curve and elastic modulus of photonic film; influence of surface modification; reversibility, thermal, long-term stability tests; a lattice model; cross-sectional SEM image of stretched film; film on the skin (PDF) Color change by stretching (AVI) Nonsticky, fluorinated film (AVI) Reversible change of color and spectrum (AVI) Dual-color patterns formed by compression of photonic film with stamps (AVI) Color change of photonic film on the skin by folding and unfolding elbow (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Shin-Hyun Kim: 0000-0003-4095-5779 Author Contributions

EXPERIMENTAL METHODS

G.H.L. carried out all experiment; G.H.L. and T.M.C. analyzed the data; B.K., S.H.H., and J.M.L. helped in the characterization; S.-H.K. designed and supervised the research.

Preparation of Photocurable Colloidal Dispersions. Monodisperse silica particles are synthesized by two steps: Seed particles are synthesized by a two-phase method39 and are then grown into silica particles with a target diameter by the Stöber method.40 The suspension is fully dried in a convection oven at 70 °C for 12 h after washing with ethanol, and the weight of the silica powder is measured. The dried powders are redispersed in ethanol, which is then mixed with PEGPEA (MW 324, Sigma-Aldrich) containing 1 w/w % photoinitiator of 2-hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173, Ciba Chemical), where the amount of PEGPEA is determined from the weight of silica powder to have a target volume fraction of silica particles in ethanol-free basis; the densities of silica particles and PEGPEA are approximated as 2.0 and 1.127 g mL−1, respectively. Ethanol is vaporized in a convection oven at 70 °C for 12 h to prepare a silica-in-PEGPEA dispersion; the protocol to prepare concentrated dispersions has been reported in the literature.41 Fabrication of Mechanchromic Photonic Films. The photocurable dispersion is spontaneously infiltrated by capillary force into a gap between two glass slides separated by 50-μm-thick spacers of polyimide tape (Kapton). The dispersion is irradiated by UV light (Inno-cure 100N, Lichtzen) with an intensity of 2 W cm−2 for 20 s. The polymerized composite film is released from the glass slides. To give the film a low surface energy, both surfaces of the film are treated by reactive ion etching (VSRIE-400A, Vacuum Science Inc.) with sulfur hexafluoride gas (SF6) for 30 s with a power of 150 W and a flow rate of 100 sccm. Characterization. Images of photonic films are taken by an optical microscope in reflection mode (Eclipse L150, Nikon), and reflectance spectra are measured using a fiber-coupled spectrometer (USB 4000, Ocean Optics Inc.) equipped in the microscope; a 10× lens with a

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Agency for Defense Development of Korea (17-113-706-011). REFERENCES (1) Stuart-Fox, D.; Whiting, M. J.; Moussalli, A. Camouflage and Colour Change: Antipredator Responses to Bird and Snake Predators Across Multiple Populations in a Dwarf Chameleon. Biol. J. Linn. Soc. 2006, 88, 437−446. (2) Weiss, S. L. Reproductive Signals of Female Lizards: Pattern of Trait Expression and Male Response. Ethology 2002, 108, 793−813. (3) Teyssier, J.; Saenko, S. V.; Van Der Marel, D.; Milinkovitch, M. C. Photonic Crystals Cause Active Colour Change in Chameleons. Nat. Commun. 2015, 6, 6368. (4) Kim, S.-H.; Lee, S. Y.; Yang, S.-M.; Yi, G.-R. Self-Assembled Colloidal Structures for Photonics. NPG Asia Mater. 2011, 3, 25−33. (5) Lee, H. S.; Shim, T. S.; Hwang, H.; Yang, S.-M.; Kim, S.-H. Colloidal Photonic Crystals toward Structural Color Palettes for Security Materials. Chem. Mater. 2013, 25, 2684−2690. (6) Arsenault, A. C.; Puzzo, D. P.; Manners, I.; Ozin, G. A. PhotonicCrystal Full-Colour Displays. Nat. Photonics 2007, 1, 468−472. (7) Wang, X. Q.; Wang, C. F.; Zhou, Z. F.; Chen, S. Robust Mechanochromic Elastic One Dimensional Photonic Hydrogels for 11356

DOI: 10.1021/acsnano.7b05885 ACS Nano 2017, 11, 11350−11357

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DOI: 10.1021/acsnano.7b05885 ACS Nano 2017, 11, 11350−11357