Switchable and Tunable Chemical/Structure Color in a Flexible Hierarchical Surface

Cephalopod skin is capable of fast color changing enabled by tunable skin transparency as well as structure color. Under this inspiration, herein, a flexible surface with unique hierarchical structure that integrates both transparency change in chemical color (optical scattering) and structure coloration (optical interfering) is developed by harnessing wrinkling instability, thanks to the interfacial Au catalysis in soft lithography. As a result, a hierarchical structure in terms of wrinkled film overlaid by nano‐dome array is obtained in the flexile surface. Experiments find that subject to biaxial strains from 0% to 60%, the hierarchical surface first experiences a transition from nontransparent to transparent owing to the flattening of the wrinkles and then exhibits iridescence structure color shifting from blue to red. The switchable and dynamical tunable mechanochromic characteristics are demonstrated in a smart window, offering potentials for developing flexible devices with optical multiple functionality.


Introduction
Vivid color changing in cephalopod skin is an exciting feature for spectacular hiding, communication, and courtship display.Under such inspiration, various artificial soft and active coloration systems are developed utilizing diverse combinations including dielectric elastomer artificial muscles, [1] adaptive infrared-reflecting surfaces, [2] electro-mechano-chromic responsive elastomers, [3][4][5][6] pneumatic microfluidic networks, [7,8] soft photonic crystals in 1D lamellar stacks, [9][10][11] 2D metasurfaces, [12][13][14] 3D opal-shaped arrays, [15][16][17][18][19][20] and plasmasonics. [21]Engineered applications are developed as well, to name a few, living hydrogel microfluidics, [15] visually flexible electronics, [22,23] adhesive patch, [24] and health monitoring. [25]][28] It is also uncovered that iridophores [29][30][31] in the skin cells are layout in terms of close-packed biophotonic crystals, showing strong structure color.It is remarkable that the cephalopod is capable of two optical effects: 1) scattering by dynamic surface wrinkle with biological muscle actuation; and 2) interfering by nanoscale photonic crystal.Under this inspiration, different fabrication strategies for such a hierarchical structure have been proposed by integrating different physical and chemical techniques, i.e., spincoating þ stretch-releasing, [32] nano-pattern transferring þ UVO Cephalopod skin is capable of fast color changing enabled by tunable skin transparency as well as structure color.Under this inspiration, herein, a flexible surface with unique hierarchical structure that integrates both transparency change in chemical color (optical scattering) and structure coloration (optical interfering) is developed by harnessing wrinkling instability, thanks to the interfacial Au catalysis in soft lithography.As a result, a hierarchical structure in terms of wrinkled film overlaid by nano-dome array is obtained in the flexile surface.Experiments find that subject to biaxial strains from 0% to 60%, the hierarchical surface first experiences a transition from nontransparent to transparent owing to the flattening of the wrinkles and then exhibits iridescence structure color shifting from blue to red.The switchable and dynamical tunable mechanochromic characteristics are demonstrated in a smart window, offering potentials for developing flexible devices with optical multiple functionality.radiation þ releasing, [33] and cyclic heating/rinsing treatment. [34]ith such combination, fabrication error will be involved, causing imprecision that is crucial in optical performance.Thus, if a simplified method is developed, it will not only promotes the efficiency in fabrication but also reduces error origins as well, which by far has been rarely reported, to the best knowledge of the authors.
In this work, we propose a novel method to fabricate a flexible hierarchical surface via harnessing morphological instability during soft lithography, which consequently results in a thin wrinkled film with nano-scale dome array.Both of the micro and nanopattern are distributed equal bi-axially and the optical performance is altered when subject to bi-axial mechanical strain.Upon a stretch, the hierarchical surface first experiences a transition from blurring to transparent state, owing to the flattening of wrinkles.It then demonstrates strong iridescence phenomenon as structure color shifts from blue to red.With the capability of switchable and dynamic tunable optical performances, the findings might make an essential contribution to the field of bio-inspired display, visual strain-sensing, and unmanned stealth robots.

Principle for Hierarchical Skin in Fabrication
[37][38] Previous efforts have verified that diverse wrinkle patterns can be triggered by chemical reaction, [39,40] growth, [41,42] swelling, [43,44] and UV treatment, [45][46][47] with several procedures in consequence.In the current research, we developed a soft lithography method on a selected template to induce wrinkle instability via a simplified procedure.
Figure 1a(i) illustrates the fabrication process, and details are described in the method and material section.A piece of template with nanoscale conical holes (dimensions are characterized in Table S1, Figure S1 and S2, Supporting Information) was selected.A layer of Au (%40 nm) was deposited as catalysis.Polydimethylsiloxane (PDMS) was mixed by two components with varying weight ratio and then cured thermally after casting on the template.After that, the PDMS sample was peel off the template, and the surface that replicating the nanostructure of the template immediately self-wrinkled (Figure 1a(ii)).We explain the mechanism of wrinkling instability as follows.Noble metals, Pt or Au, have strong adsorption and can be used to catalyze the cross-linking reaction of PDMS. [48]The electron orbital arrangement of the outermost layer of Au atoms is 5d106s1, and the first ionization energy is larger if compared with that of Pt, so it is difficult for Au to lose electrons or to adsorb reactive molecules.Consequently, Au is not preferred to catalysis PDMS curing.However, it is revealed that nanoscaled Au (with a particle size of less than 5 nm) is an ultrafine particle, which exhibits strong adsorption of reactant molecules and excellent catalytic performance. [49]The high catalytic activity of nano-Au is the result of the quantum size effect together with the gold-support interface. [50]Therefore, in the current methodology, during the cross-linking reaction of the alkenyl-terminated PDMS, the nanoscale Au particles that play an essential role in catalyzing the cross-linking.In the insert of Figure 1a(i), when nano-Au is depoited on the surface of the template, the cross-linking degree of PDMS that close to Au is higher, see that in Equation (1).Therefore, the part of PDMS that close to the nano-Au layer has a larger elastic modulus than the rest of PDMS, forming a film-soft substrate mechanical bilayer system. (1) After curing, the overall PDMS elastomer shrunk its volume by 1.4%, providing an internal stress but confined by the template.With the removal of the template, the internal stress compressed the bilayer system, causing the surface wrinkles in microscale (Figure 1b(i)).Meanwhile, the thin film replicates the pattern of template of nanoholes, presenting an array of nanoscaled domes in Figure 1b(ii).This hierarchical structure can be identified in Figure 1c by zooming-in.As a comparison, curing without Au layer was conducted and no film was obtained, which supported the mechanism (Figure S2, Supporting Information).
We also investigated the curing on templates with different geometries and arrays (Figure 1d) and only the conical hole yields the film/substrate bilayer system, wrinkling pattern while preserving the nanostructure (Figure S3, Supporting Information).The shape of hole on the template affects the formation of bilayer system.Template with cylindrical holes processes a greater surface tension and restricts the PDMS liquid from fully occupying the space in the nano hole, so that the nanostructure was hardly replicated, as compared in the Table S1, Supporting Information.While the conical hole can withhold more PDMS liquid during the filling to facilitate the Au catalysis.
Thus, we select template with conical holes in the following study.The depth of the template will affect the degassing and the filling of the PDMS.For example, with the depth of h = 125 nm, local surface tension restricts the PDMS from fully filling into the hole, so that the nanostructure cannot be successfully replicated.While with a deeper hole, h = 1500 nm, the  (40 nm).Under the Au catalytic activity, a thin film of PDMS that close to the template cross-linked and cured prior to its bulky substrate.Thus, a film/substrate bilayer system is attained.(ii) In the micro scale, the PDMS shrinks in volume after the curing, so that the film/ substrate bilayer system wrinkles upon the removal of template.b) The SEM images of the cross section of hierarchical surface including (i) wrinkled film in microscale with (ii) nanostructures that replicate the template.c) The SEM results of the hierarchical surface by zooming-in.d) The effect of template depth h on the self-wrinkling instability.Wrinkled surface is attainable only at h = 400 and 900 nm.
nanostructure through can be replicated by PDMS, but is easily broken during the peeling off process.Only templates with depth of h = 400 and 900 nm yield the hierarchical structure.However, nanostructure is not obvious in the case of h = 900 nm owing to the difficulty of sufficient degassing.Thus, in the following study, we focus on samples fabricated with the AAO template with h = 400 nm to probe the material property and optical performance.

Multimodal Surface Instability
To evaluate the skin morphology, we investigate the effect of the material property on the wrinkle patterns.Different PDMS were prepared, with weight ratio of 5:1, 6:1, 8:1, 10:1, and 20:1 between the two ingredients (prepolymer and casting agent) and casted on the same template (period 450 nm, diameter 450 nm, and height 400 nm).It has been suggested that this ratio, especially on the curing agent, will affect the mechanical performance of PDMS, [51,52] including the modulus and mechanical strength.Thus, concerning the wrinkle on bilayer system, we probe the effect of the weight ratio on the Young's modulus of PDMS, where different wrinkle patterns were identified, e.g., crater, peanut, peanut wrinkle, wrinkle, and laminate (Figure 2a(i)), illustrating diversified morphology in instability.With the increase of the weight ratio of PDMS, the Young's modulus of the film is decreased.A maximum Young's modulus of the film peaks at a PDMS ratio of 6:1 with a level of 2.01 MPa (Figure S4, Supporting Information).That is to say, at the same level of compressible stress, the surface morphology evolved into different patterns as the Young's modulus varied.This behavior was observed in the gel-elastomer bilayer system as well. [53]igure 2a(ii)-(iv) illustrates the hierarchical surface in different scales, with correspondence to PDMS component ratio.By zooming-in, we characterized the local pattern from the microto nanoscale.The nanostructure of the template was successfully replicated regardless of component ratios.To characterize the wavelength of the wrinkle, we used fast Fourier transform (FFT) on images of the wrinkle pattern.A series of symmetric 2D patterns is identified after FFT (Figure S5, Supporting Information), thus the wavelengths can be evaluated along one dimension by a laser sensor (LJ-X8080, Keyence).A monotonic increase in the wrinkle's wavelength (from 11.8 to 18.3 μm) and amplitude (from 2.8 to 7.7 μm) is identified in Figure 2b,c, which coincides with the theory of morphological instability. [36]In linear buckling theory, the wavelength and amplitude of the wrinkles are determined as [46] (deduction in Figure S6, Supporting Information).While in nonlinear buckling theory, they are expressed as [54] λ In our experiment, the in-plane strain shall not exceed 30%, so the linear buckling theory can explain the variation that both the wavelength and amplitude are increased with the Young's modulus of the PDMS film.
The optical transparency is presented by measuring transmittance (R1, Ideaoptics) (see the definition of haze level in Figure S7, Supporting Information), where the wrinkled surface was blurred with a transmittance between 4% and 9.5%.A wavelength of 500 nm was selected to evaluate the effect of wrinkles on transmittance (Figure 2d).With the increase of wrinkle wavelength, the surface becomes opaque (from 25.1% to 3.3%).For comparison, we fabricated other surfaces with wrinkle pattern only by UVO treatment (Figure S8, Supporting Information), nano-pattern only by direct lithography and conventional PDMS as well.These transmittances show that the hierarchical surface and wrinkle-only surface were both nontransparent (4.6% and 6.9%), while the nano-only surface failed to scatter the light, illustrating high transparency (82.8%) (Figure S8, Supporting Information).This verifies the conclusion that the nontransparent character in the hierarchical surface is contributed by the wrinkles, which scatters the light in micro scale.

Dynamic Tunable Optical Transparency
A biaxial stretch was applied on the hierarchical surface to investigate strain-tunable transparency, which is similar to the muscle actuation in cephalopod.At each strain level, the diffraction area of a laser beam was recorded, whose radius R was defined and measured (Figure S9 and 10, Supporting Information).In the hierarchical surface of PDMS component ratio of 10:1, R varied from 55 to 5 mm during the stretch (Figure 3a).The surface morphologies were also recorded by microscopy.Stretching would gradually flattens the wrinkles, resulting in an incremental transparency in all PDMS ratios (Figure 3a).The uniaxial strain was also applied as comparison (Figure S11, Supporting Information).The transmittance spectrums of sample with component 10:1 were measured at different strain states.Upon a strain of 30%, wrinkles were completely released and the hierarchical surface exhibits a transmittance (79.8% @500 nm) close to a wrinkle-free sample (90.3% @500 nm), as displayed in Figure 3b.However, uniaxial stretch hardly adjusts the  transparency (Figure S11, Supporting Information), since the wrinkles were distributed in two dimensions.Dynamic mechanical stretch-release test was performed to evaluate the optical transparency recovery in either single or repeated cycles.The samples with varying component ratios were all capable of recovering its original pattern in a cyclic biaxial loading-unloading process (Figure 3c,d).While the uniaxial experiment only partly released the wrinkle, resulting a small increase in transmittance (Figure S11 and Video S1, Supporting Information).We next characterize the reversibility by a fatigue study.2000 cycles of stretch release were applied to a hierarchical surface with a component ratio of 10:1 (Figure 3e).The hierarchical surface can withstand repeated mechanical actuation, showing a consistent performance of transparent state (21.7%AE 1.15%) at 30% strain and blurring state (4.0%AE 0.18%) at 0% strain.Therefore, in the following study, we will focus to hierarchical surface fabricated by the PDMS with a component ratio of 10:1.

Switching to Structure Color
To demonstrate the dual and switchable optical performance, we recorded the coloration of the hierarchical surface from both a top view and an inclined side view throughout a continuous biaxial stretch increased at a rate of 1% s À1 (Figure S12 and Video S2, Supporting Information).From the top view, hierarchical surface experienced transition from blurring to transparent at the strain level of 30% (Figure 4a).A QR code under the surface became visible gradually and was ready to be read-out.At the strain level of 15%, the structure color was uncovered, as the wrinkles were completely released.From the side view, the hierarchical skin has a significant iridescent effect and its color shifted in sequence of blue, green, yellow, and red, covering the visible range, as the strain reaching at 60%.The stretching enlarges the space between each nano-dome, which is equivalent to the bandgap of photonic crystals, and enables the structure color a redshift.According to Bragg's diffraction equation, the www.advancedsciencenews.com www.advintellsyst.comexpression of structural colors by two-dimensional grating patterns is dðsinθ þ sinγÞ ¼ mλ w , where d is the grating of the nanostructures, θ or γ is the angle between the incident light or the observer and the normal of the surface, respectively.λ w is the center wavelength of the observed light, and m is the order of diffraction.Therefore, by adjusting the grating, the structure color is altered as well.A visible spectrums recorded the switchable optical performance from the transparency tuning to structural coloration, as a result, a broadened spectrum to a narrow-peaked spectrum were obtained, as the light transmitted from scattering to interfering (Figure 4b).Even the wrinkles were not fully released, at the strain level of 15%, the structure color was identified in the green range.Through successive stretching, the hierarchical surface experienced a redshift as its peak shifted over 188 nm.To evaluate the structure coloration, we characterized its view angle dependence by the combination of incident angle α and reflect angle β.A selected view angle dependence is shown in Figure 4c.For example, subject to an incident angle α = 80°, the hierarchical skin displayed different color, covering the entire visible light range, with respect to its reflect angle, a view angle dependence.The overall view angle dependence was summarized as well (Figure 4d).

A Representative Demonstration in a Smart Window
We demonstrate an application as a smart window utilizing single optical performance prior to the structural color.[57] Subject to a voltage, SMAS are electrically heated, and it contract by shrinking the gap of the coiled spring.This window can be actuated in the daytime when environmental temperature increases, and the SMAS elongate.In contrast, it can also be tuned electrically when the SMAS is powered manually (Figure 5a(i)).Figure 5a(ii) compares the two states (transparent and nontransparent) of the window.When the hierarchical surface is stretched, the window is fully transparent for observation, and when the stretch is released, the hierarchical surface recovered to original pattern, and the window is not transparent to protect privacy.Figure 5b illustrates the actuation in the window by electrical heating on the SMAS.When an electrical current of 1.6 A is applied, the springs are heated and shrink after 10 s.When the time reaches 20 s, the hierarchical surface gradually becomes transparent, and the pattern behind is uncovered.At 25 s, the hierarchical surface is completely transparent.When the power is off, the temperature of the spring gradually decreases, and SMAS recovers.Finally, at 15 s after the power off, it turns into an opaque state again.Figure 5c measures the transmittance of the smart window in repeated actuation.At a wavelength of 500 nm, the initial transmittance is about 7%.When the power-on time reaches 15 s, the transmittance of the smart window starts to increase to 10%, and it increases to 85% in the next 10 s.After the power off, due to the continuous deformation of the SMAS, the smart window is stretched as well, so the transmittance increases slightly.As the SMAS cool down, after 45 s, the transmittance of the window declines to 10%. Figure 5d illustrates the regulation at different temperature in the SMAS heating.During the entire process, the transmittance of the smart window is stable in the initial and final state, and the error is relatively large in the intermediate change process.

Conclusion
Inspired by the dual functions of coloration in biology, a hierarchical surface with nano-scale dome array on a micro-scale wrinkled film was fabricated, featuring a switchable optical performance between scattering and interfering.A simplified fabrication method is proposed by harnessing surface instability via Au catalysis during soft lithography.Under a biaxial mechanical stretching, the hierarchical surface exhibits two sequential mechanochromic performances: changing transparency by flattening the wrinkles, then changing its structure color by tuning the period in photonic scale.With the two capability that switchable and tunable, the hierarchical surface can facilitate new soft display, camouflage, and smart window.

Experimental Section
Fabrication and Characterization of the Hierarchical Surface: Anodic aluminum oxide (AAO) template (VS450-100-450, TOPMEMBRANES) was used as received (Supporting information Table1).Gold film was sputtered (DISCOVERY635) with a thickness of 40 nm on the surface of the template.The PDMS prepolymer and curing agent (184, Dow Corning) were mixed at different ratio and stirred on a mixer (DS20-PRO, DLAB) at a speed of 800r min À1 for 3 min.Then they were vacuumed for 20 min under the condition of 0.098 MPa to completely remove the bubbles in the mixture.The mixture was uniformly flowed on the surface of the template, and vacuum was applied to enable fully contact between the PDMS and the template for 20 min.Then the glass substrate was placed on the heating plate (WH2000-2K, WIGGENS), when the temperature was maintained at 65 °C for 4 h.When the PDMS was completely cured, the template was slowly and carefully peeled off to obtain the hierarchical sample.A 3D color microscope (VK9700K, Keyence), a microscope (BS53M, Olympus), and a scanning electron microscope (500, Gemini SEM) characterized the wrinkle patterns of the samples.
Shape Memory Alloy: Springs shape memory alloy springs (ZNLBM-2KG) is used as purchased.When the current was 1 A, the spring were stretched 40, 60, 80, and 100 mm, respectively, The tensile force all reached maximum within 50 s under the four elongations at about 5-7 N. At the stretch length of 40 mm, the change of the stretch force with the time was relatively stable.
SEM Characterization: Images of SEM were obtained in State key Laboratory for Manufacturing Engineering System by GeminiSEM (ZEISS Field Emission SEM) instrument of model 500.
Measuring the Transmittance: The sample was stretched to a prescribed strain and fixed at a frame; then it was placed inside the spectrometer (R1, Ideaoptics).Visible light was incident vertically to the sample for measuring the transmission spectrum.
Measuring the Spectrum: Each sample was fixed to the frame and stretched to a prescribed strain, after which it was tested inside a spectrometer (R1, Ideaoptics).The sample was exposed to a visible light beam at different angles of incidence, and a probe was placed at different angles, denoted by α, to measure the reflective spectrum for structural color.

Figure 1 .
Figure 1.Fabrication and characterization of hierarchical surface.a) (i) The proposed fabrication procedure.A template with conical hole is deposited with a layer of Au in nanoscale(40 nm).Under the Au catalytic activity, a thin film of PDMS that close to the template cross-linked and cured prior to its bulky substrate.Thus, a film/substrate bilayer system is attained.(ii) In the micro scale, the PDMS shrinks in volume after the curing, so that the film/ substrate bilayer system wrinkles upon the removal of template.b) The SEM images of the cross section of hierarchical surface including (i) wrinkled film in microscale with (ii) nanostructures that replicate the template.c) The SEM results of the hierarchical surface by zooming-in.d) The effect of template depth h on the self-wrinkling instability.Wrinkled surface is attainable only at h = 400 and 900 nm.

Figure 2 .
Figure 2. The multimodal wrinkle patterns on the hierarchical surfaces, including crater, peanut, wrinkle, and laminate, at different PDMS component ratios.a) (i)-(iv) illustrate the hierarchical surfaces where the nano-domes were replicated.b,c) The wrinkle wavelength and amplitude of the surfaces vs. PDMS component ratio.d) The value of transmittance at 500 nm versus PDMS ratio, which are all most nontransparent owing to the disordered wrinkles and nano-domes.

Figure 3 .
Figure 3. Strain-tunable transparency.a) The diffraction area and wrinkle releasing in PDMS sample (ratio 10:1) under a biaxial strain up to 30%.b) The transparency in all PDMS component ratios is tuned by the biaxial stretch to highly transparent state.c) In PDMS sample of 10:1, when the wrinkles are fully released, the surface shows a transparency close to an original PDMS sample.d) In a mechanical loading-unloading cycle, all the samples showed a reversible optical performance.e) A cyclic loading of 2000 cycles for fatigue test.

Figure 4 .
Figure 4.The hierarchical surface with structure color after stretched.a) Tunable and sequential optical performance by biaxial stretch.The wrinkle is released by transparency change then the structure color is unveiled and regulated.The dual optical performance of the hierarchical surface was recorded from the top view and the incline view concurrently under equal biaxial stretch.The scale bar is 10 mm.b) The change of visible spectrum from a narrowed range and to widened wavelength domain.c) When the incident light is 80°, the sample changes its view angle dependence.The inset is the observation of structure color.d) The reflective light in spectrum versus the incident light angle is summarized.

Figure 5 .
Figure 5. Illustration of the dynamic optical performance.a) (i) A smart window enabled by the hierarchical surface that can protect the house privacy.(ii) The tunability of the window transparency in demonstration.b) The window is actuated by a set of shape memory alloy (SMA) springs, when the hierarchical surface is stretched electrically.c) The transmittance of the hierarchical surface when the voltage is loaded in 5 cycles.d) A series of snapshots of a dynamic actuation cycle, showing the voltage/temperature regulation.