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Article

Chameleon-Inspired Mechanochromic Photonic FilmsComposed of Nonclose-Packed Colloidal Arrays

Gun Ho Lee, Tae Min Choi, Bomi Kim, Sang Hoon Han, Jung Min Lee, and Shin-Hyun KimACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b05885 • Publication Date (Web): 02 Nov 2017

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1

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

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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 face-

centered 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 − ��. (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 long-

range 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, 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 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 blue-

shift 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 dual-

colored 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 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 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, 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 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-1-

propaneone (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; cross-

sectional 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: kim.sh@kaist.ac.kr.

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)

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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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