Chameleon-Inspired Mechanochromic Photonic Films Composed of

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Chameleon-Inspired Mechanochromic Photonic Films Composed of Nonclose-Packed Colloidal Arrays Gun Ho Lee, Tae Min Choi, Bomi Kim, Sang Hoon Han, Jung Min Lee, and Shin-Hyun Kim ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b05885 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 3, 2017

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

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

ACS Paragon Plus Environment

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ABSTRACT

Chameleon uses a nonclose-packed array of guanine nanocrystals in iridophores to develop and tune skin colors in full visible range. Inspired from the biological process uncovered in panther chameleons, we design photonic films containing a nonclose-packed face-centered cubic (fcc) array of silica particles embedded in an elastomer. The nonclose-packed array is formed by interparticle repulsion exerted by solvation layers on the particle surface, which is rapidly captured in the elastomer by photo-curing of the dispersion medium. The artificial skin exhibits the structural color that shifts from red to blue under stretching or compression. The separation between inelastic particles enables the 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, Nonclose-packed arrays, Structural colors, Mechanochromism


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Several 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 bandgap modeling has revealed that panther chameleons change colors primarily through the active tuning of a lattice of guanine nanocrystals within iridophore cells.3 Theses 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 wavelength of the visible light.4-6 As the period of 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 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 fingerprint 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 stop band position.10 Through a local modulation of elastic modulus, the color pattern can be encrypted; the hidden pattern is disclosed when the films are subjected to extensional stress.11,12 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


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providing a full-color change.18 However, delicate procedures of fabrication and low mechanical stability of inverse opal structures restrict practical uses; small extensional strain is only allowed. Alternatively, colloids composed of an inelastic core and elastic shell are crystallized and fused by 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 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 Nonclose-packed array of colloids can provide a large deformation of the elastic matrix and therefore enables a full-color change in the absence of colloidal rearrangement. To achieve the large interparticle separation, charged colloids have been spontaneously crystallized in an aqueous medium through electrostatic repulsion, which are immobilized by a hydrogel.25-27 Although the nonclose-packed arrays embedded in water-swollen gels provide mechanochromic property, the low mechanical stability of gels against stretching and vaporization of water in the air environment only allows a short-term use in restricted conditions. Thus, a simple and scalable method to create nonclose-packed arrays in 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 nonclose-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 solvation layer on the surface of particles. The repulsive potential leads to spontaneous crystallization of silica particles above threshold volume fraction and the regular


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array is captured by rapid photo-polymerization 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 blue-shift of color on relief regions and red-shift on engraved regions, developing multicolor patterns. This approach based on 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 Nonclose-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 nonclose-packed crystals.3 As the cytoplasm undergoes rapid volume change, a lattice parameter is altered in the range of 240 - 480 nm, while maintaining the lattice structure, thereby leading to a drastic color shift in full visible range (Figure 1a). We mimic the iridophore structure by embedding a nonclose-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 fraction 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


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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. 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 the dispersion into a gap between two slides glasses by a capillary force and irradiating with ultraviolet (UV) (Figure S1 of 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 the color and the low contrast of refractive index between silica (np = 1.45) and pPEGPEA (nm = 1.502) leads to negligible scattering out of 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 stop band (Figure S2). The transmittance is as high as 80% out of stop band and the sum of reflectance and transmittance is close to 95% even for short wavelength, indicating negligible scattering in entire visible range. The photonic film exhibits blue-shift of reflection color as stretched (Movie S1 of Supporting Information). The red film at normal reflection turns to green and blue for the strains of 0.20 and 0.47 respectively (Figure 1f). With a backlight, the film shows complementary color as light at stop band is blocked, while light out of 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 stretched; magenta is originated from the color mixing of red and blue and yellow is of red and green, respectively.


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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 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 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 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 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 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’s diffraction from (111) planes of nonclose-packed facecentered cubic (fcc) lattice (Figure 2d):

= 2 = √ ,

(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 Maxwell-Garnett average of refractive indices of silica particles and pPEGPEA matrix:


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1 = + − .

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(2)

The coincidence between the peak positions of spectra and λmax from eq 1 indicates that silica particles form nonclose-packed fcc structure whose (111) planes are aligned along with 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, ∆λ/λmax, as approximately 0.04 (Figure 2d), indicating an insignificant difference in ordering. By contrast, the peak for ϕ = 0.20 has the value of ∆λ/λmax as large as 0.081, implying colloidal array less ordered. In addition, the peak is located at 543 nm which is smaller than 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 nonclose-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 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 silica particles. The thickness of solvation layer, t, is expected to be independent of particle diameter, d. Therefore, the surface-to-surface separation at the threshold volume fraction for large particles is same to that of small particles. Therefore, larger particles occupy more volume and result in 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 prepare photonic films with various stop band positions with silica particles with different diameters while maintaining ϕ =


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0.33 which is larger than ϕth for all particles (Figure S4). Therefore, a colloidal array with a longrange order fully occupy 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 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 Young’s modulus of 0.46 MPa for the strain smaller than 0.47, which increases to 1.03 MPa for the 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 a contact. As strain increases, the silica particles are brought closer, leading to the increase of modulus. Strain at the fracture slightly decreases as the film contains the 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 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 reduce surface energy. We confirm the reduction of the surface energy from 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 color and reflectance spectrum same to the pristine (Figure S6). The non-sticky property is maintained for several days.


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High reversibility of mechanochromism The elastic photonic films instantly change the 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 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 shifts from red to blue, covering almost entire visible range (Figure 3a and 3b). 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 nonclose-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 hydrogel only allow 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 the 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 the large extensional strain, insufficient particle separation leads to plastic deformation and therefore, results in a residual strain for color tuning in a 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℃ (Figure S7). Moreover, the photonic films maintain resonant wavelength and reflectivity during a 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


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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,

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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 colloidal array from blue-shift of the stop band. The stop band wavelength, λmax(εx), blue shifts as much as εz λmax (εx = 0) as a film is stretched: = 2 1 − = 2 1 − ! ,

(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 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 a contact between two particles for given ranges of εx, εy, and εz. The interparticle separation at strain-free state is sufficient to avoid rearrangement of particles during the stretching, thereby enabling the elastic extension without a residual strain. We confirm no rearrangement of silica particles embedded in the stretched photonic film from cross-sectional SEM image (Figure S9). As expected from the lattice model, the silica particles follow the position of 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 rubbers); that is, the deformation of the composite film is governed by 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 the large


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stain, inelastic particles are brought close, partially losing the property of elastic matrix. That is, the volume is not conserved as silica particles resist the matrix deformation during the stretching. During the blue shift of 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 refractive index contrast between a slice containing (111) plane at the middle and an intermediate slice. The effective refractive indices of two slices are estimated using eq 2 based on model structures (Figure S10a). The volume fraction of matrix increases as the matrix is expanded by the strain for the slice containing (111) plane, whereas the volume fraction of silica particles increases as silica particles invade from (111) planes for the intermediate slice (Figure S10b). Therefore, the index contrast decreases as strain increases. As the strength of 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), implying that the reflectance reduction is caused by the contrast reduction. Color change by compression The elastic photonic films show a color shift in a response to compressive stress. When the film is compressed by stamps, the film under the relief region is contracted, thereby causing a blueshift of color (Figure 4a). At the same time, the film under engraved regions is expanded as the volume of the composite is roughly conserved, thereby causing red-shift.33 To confirm the color shift, the elastic photonic film that exhibits red reflection color is compressed with a stamp composed of relieves of small ‘K’s that form a large ‘K’. The single-colored film turns dualcolored pattern by the compression (Figure 4b). Reflectance spectrum is also changed from a


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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 by reflectance spectra taken at local areas; spectrum at small ‘K’ has a sharp peak at 591 nm, whereas that at background has a sharp peak at 619 nm. The photonic film can develop dual-colored patterns in a high resolution. For example, a stamp of line pattern composed of 100-µm-width relieves and 100-µm-width engraves 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 green dot array on red surrounding (Figure 4e). The development of the dual-colored patterns is reversible (Movie S4). CONCLUSION In summary, we mimic the photonic structure of dermal iridophores of panther chameleons by embedding a nonclose-packed array of silica particles in an elastic polymer matrix.3 The sufficient separation between particles enables the reversible color-shift in the full visible range in the absence of significant particle rearrangement under extensional stress as the iridophores do. Only the 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 refractive-index contrast. The problem is potentially overcome by swelling the photonic structure with nonvolatile organic solvent. As the swelling increases the interparticle separation, a larger blueshift 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 skins can be visualized by attaching the


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photonic film on the surfaces (Figure S11 and Movie S5); elastic modulus of films, approximately 1 MPa, is comparable to that of skin, thereby minimizing mechanical mismatch between two layers during the deformation.34 The mechanochromic films have a great potential as active color reflectors for large-scale wallpaper or signboard displays in the combination with site-addressable pneumatic compressors.35,

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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 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 EXPERIMENTAL METHODS Preparation of photocurable colloidal dispersions. Monodisperse silica particles are synthesized by two steps: Seed particles are synthesized by two-phase method,39 which are then grown to silica particles with target diameter by Stöber method.40 The suspension is fully dried in a convection oven at 70°C for 12 h after washing with ethanol and 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-1propaneone (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; 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 silica-in-


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PEGPEA dispersion; the protocol to prepare concentrated dispersions has been reported in 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 render the film to have 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 microscopy; ×10 lens with 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). ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.


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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; crosssectional SEM image of stretched film; film on the skin (PDF). Color change by stretching; non-sticky, fluorinated film; reversible change of color and spectrum; dual-color patterns formed by compression of photonic film with stamps; color change of photonic film on the skin by folding and unfolding elbow (AVI). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions 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 the characterization; S.-H.K. designed and supervised the research. ACKNOWLEDGMENT 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.


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(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. Photonic-Crystal 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 Touch Sensing and Flexible Displays. Adv. Opt. Mater. 2014, 2, 652–662. (8) Jia, X.; Wang, J.; Wang, K.; Zhu, J. Highly Sensitive Mechanochromic Photonic Hydrogels with Fast Reversibility and Mechanical Stability. Langmuir 2015, 31, 8732–8737. (9) Hong, R.; Shi, Y.; Wang, X.-Q.; Peng, L.; Wu, X.; Cheng, H.; Chen, S. Highly Sensitive Mechanochromic Photonic Gel towards Fast-Responsive Fingerprinting. RSC Adv. 2017, 7, 33258–33262. (10) Lawrence, J. R.; Ying, Y.; Jiang, P.; Foulger, S. H. Dynamic Tuning of Organic Lasers with Colloidal Crystals. Adv. Mater. 2006, 18, 300–303. (11) Ye, S.; Fu, Q.; Ge, J. Invisible Photonic Prints Shown by Deformation. Adv. Funct. Mater. 2014, 24, 6430–6438. (12) Ding, T.; Cao, G.; Schäfer, C. G.; Zhao, Q.; Gallei, M.; Smoukov, S. K.; Baumberg, J. J. Revealing Invisible Photonic Inscriptions: Images from Strain. ACS Appl. Mater. Interfaces 2015, 7, 13497–13502.


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(13) Fudouzi, H.; Sawada, T. Tuning Stop Band of Soft Opal Film by Deformation for Strain Sensing Applications. Proc. SPIE 2006, 6369, 63690D–1. (14) Sun, X.; Zhang, J.; Lu, X.; Fang, X.; Peng, H. Mechanochromic Photonic-Crystal Fibers Based on Continuous Sheets of Aligned Carbon Nanotubes. Angew. Chem. Int. Ed. 2015, 54, 3630–3634. (15) Zhang, J.; He, S.; Guan, G.; Lu, X.; Sun, X.; Peng, H. The Continuous Fabrication of Mechanochromic Fibers. J. Mater. Chem. C 2016, 4, 2127–2133. (16) Fudouzi, H.; Sawada, T. Photonic Rubber Sheets with Tunable Color by Elastic Deformation. Langmuir 2006, 22, 1365–1368. (17) Ito, T.; Katsura, C.; Sugimoto, H.; Nakanishi, E.; Inomata, K. Strain-Responsive Structural Colored Elastomers by Fixing Colloidal Crystal Assembly. Langmuir 2013, 29, 13951–13957. (18) Arsenault, A. C.; Clark, T. J.; Freymann, G. V.; Cademartiri, L.; Sapienza, R.; Bertolotti, J.; Verkis, E.; Wong, S.; Kitaev, V.; Manners, I.; Wang, R. Z.; John, S.; Wiersma, D.; Ozin, G. A. From Colour Fingerprinting to The Control of Photoluminescence in Elastic Photonic Crystals. Nat. Mater. 2006, 5, 179–184. (19) Viel, B.; Ruhl, T.; Hellmann, G. P. Reversible Deformation of Opal Elastomers. Chem. Mater. 2007, 19, 5673–5679. (20) Schäfer, C. G.; Gallei, M.; Zahn, J. T.; Engelhardt, J.; Hellmann, G. P.; Rehahn, M. Reversible Light-, Thermo-, and Mechano-Responsive Elastomeric Polymer Opal Films. Chem. Mater. 2013, 25, 2309–2318. (21) Schäfer, C. G.; Viel, B.; Hellmann, G. P.; Rehahn, M.; Gallei, M. Thermo-cross-Linked Elastomeric Opal Films. ACS Appl. Mater. Interfaces 2013, 5, 10623–10632.


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(22) Scheid, D.; Lederle, C.; Vowinkel, S.; Schäfer, C. G.; Stühn, B.; Gallei, M. Redox-and Mechano-Chromic Response of Metallopolymer-Based Elastomeric Colloidal Crystal Films. J. Mater. Chem. C 2014, 2, 2583–2590. (23) Zhao, Q.; Finlayson, C. E.; Snoswell, D. R. E.; Haines, A.; Schäfer, C. G.; Spahn, P.; Hellmann, G. P.; Petukhov, A. V.; Herrmann, L.; Burdet, P. et al. Large-Scale Ordering of Nanoparticles Using Viscoelastic Shear Processing. Nat. Commun. 2016, 7, 11661. (24) Finlayson, C. E.; Goddard, C.; Papachristodoulou, E.; Snoswell, D. R. E.; Kontogeorgos, A.; Spahn, P.; Hellmann, G. P.; Hess, O.; Baumberg, J. J. Ordering in Stretch-Tunable Polymeric Opal Fibers. Opt. Express 2011, 19, 3144–3154. (25) Asher, S. A.; Holtz, J.; Liu, L.; Wu, Z. Self-Assembly Motif for Creating Submicron Periodic Materials. Polymerized Crystalline Colloidal Arrays. J. Am. Chem. Soc. 1994, 116, 4997–4998. (26) Foulger, S. H.; Jiang, P.; Lattam, A. C.; Smith, D. W.; Ballato, J. Mechanochromic Response of Poly (Ethylene Glycol) Methacrylate Hydrogel Encapsulated Crystalline Colloidal Arrays. Langmuir 2001, 17, 6023–6026. (27) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shinohara, T.; Sawada, T.; Yonese, M.; Optically Tunable Gelled Photonic Crystal Covering Almost the Entire Visible Light Wavelength Region. Langmuir 2003, 19, 977–980. (28) Raghavan, S. R.; Walls, H.; Khan, S. A. Rheology of Silica Dispersions in Organic Liquids: New Evidence for Solvation Forces Dictated by Hydrogen Bonding. Langmuir 2000, 16, 7920– 7930.


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Page 20 of 26

(29) Krisanangkura, P.; Packard, A. M.; Burgher, J.; Blum, F. D. Bound Fractions of Methacrylate Polymers Adsorbed on Silica Using FTIR. J. Polym. Sci. B. Polym. Phys. 2010, 48, 1911–1918. (30) Kim, S.-H.; Park, H. S.; Choi, J. H.; Shim, J. W.; Yang, S.-M. Integration of Colloidal Photonic Crystals toward Miniaturized Spectrometers. Adv. Mater. 2010, 22, 946-950. (31) Yang, D.; Ye, S.; Ge, J. From Metastable Colloidal Crystalline Arrays to Fast Responsive Mechanochromic Photonic Gels: An Organic Gel for Deformation-Based Display Panels. Adv. Funct. Mater. 2014, 24, 3197–3205. (32) Chris E. Finlayson, Haines, A. I.; Snoswell, D. R. E.; Kontogeorgos, A.; Vignolini, S.; Baumberg, J. J.; Spahn, P.; Hellmann, P. Interplay of Index Contrast with Periodicity in Polymer Photonic Crystals. Appl. Phys. Lett. 2011, 99, 261913. (33) Yang, D.; Ye, S.; Ge, J. Old Relief Printing Applied to The Current Preparation of Multicolor and High Resolution Colloidal Photonic Crystal Patterns. Chem. Commun. 2015, 51, 16972–16975. (34) Yu, B.; Kang, S.-Y.; Akthakul, A.; Ramadurai, N.; Pilkenton, M.; Patel, A.; Nashat, A.; Anderson, D. G.; Sakamoto, F. H.; Gilchrest, B. A.; Anderson, R. R.; Langer, R. An Elastic Second Skin. Nat. Mater. 2016, 15, 911–918. (35) Unger, M. A.; Chou, H. P.; Thorsen, T.; Scherer, A.; Quake, S. R. Monolithic Microfabricated Valves and Pumps by Multilayer Soft Lithography. Science 2000, 288, 113– 116. (36) Thorsen, T.; Maerkl, S. J.; Quake, S. R. Microfluidic Large-Scale Integration. Science 2002, 98, 580–584.


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(37) Fuhrmann, D. A.; Thon, S. M.; Kim, H.; Bouwmeester, D.; Petroff, P. M..; Wixforth, A.; Krenner, H. J. Dynamic Modulation of Photonic Crystal Nanocavities Using Gigahertz Acoustic Phonons. Nat. Photonics 2011, 5, 605–609. (38) Tumbleston, J. R.; Shirvanyants, D.; Ermoshkin, N.; Janusziewicz, R.; Johnson, A. R.; Kelly, D.; Chen, K.; Pinschmidt, R.; Rolland, J. P.; Ermoshkin, A.; Samulski, E. T.; DeSimone, J. M. Continuous Liquid Interface Production of 3D Objects. Science 2015, 347, 1349–1352. (39) Yokoi, T.; Sakamoto, Y.; Terasaki, O.; Kubota, Y.; Okubo, T.; Tatsumi, T. Periodic Arrangement Of Silica Nanospheres Assisted by Amino Acids. J. Am. Chem. Soc. 2006, 128, 13664–13665. (40) Hartlen, K. D.; Athanasopoulos, A. P.; Kitaev, V. Facile Preparation of Highly Monodisperse Small Silica Spheres (15 to > 200 nm) Suitable for Colloidal Templating and Formation of Ordered Arrays. Langmuir 2008, 24, 1714–1720. (41) Yang, D.; Ye, S.; Ge, J. Solvent Wrapped Metastable Colloidal Crystals: Highly Mutable Colloidal Assemblies Sensitive to Weak External Disturbance. J. Am. Chem. Soc. 2013, 135, 18370−18376.


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Figure 1. Mechanochromic films containing a nonclose-packed colloidal array. (a) Chameleon relaxed 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 nonclose-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 nonclose-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).


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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 volume fraction below the threshold (ϕ < ϕth), whereas they form nonclose-packed crystal at volume fraction above the threshold (ϕ > ϕth). (b, c) A 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, ∆λ, normalized by λmax (red triangles). The black line indicates Bragg’s equation for staked (111) planes of nonclose-packed fcc structure.


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Figure 3. Reversible color change in full visible by extension. (a) A series of OM images and 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, ∆n, between two neighboring slices with equal thickness (red circles) estimated from model structures: one slice contains (111) plane in the middle and the other contains intermediate plane. Insets show the pairs of two slices with ε = 0, 21.0, 41.2, and 60.8%.


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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 (orange line) and compressed photonic film (green line). Reflectance spectra locally measured from small ‘K’ (blue line) and background (red 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).


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SYNOPSIS Table of Contents Graphic


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Chameleon-Inspired Mechanochromic Photonic Films Composed of

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