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Iridescent and Glossy Effect on Polymer Surface using Micro/Nano Hierarchical Structure: Artificial Tulip Queen of the Night Petals Jun Ho Oh, Ju Yeon Woo, Sunghwan Jo, and Chang-Soo Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06376 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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ACS Applied Materials & Interfaces

Iridescent and Glossy Effect on Polymer Surface using Micro/Nano Hierarchical Structure: Artificial Tulip Queen of the Night Petals Jun Ho Oh,† Ju Yeon Woo,‡ Sunghwan Jo,§ Chang-Soo Han†, *

† School

of Mechanical Engineering, Korea University, 145 Anam-Ro, Seongbuk-Gu,

Seoul 02841, Rep. of Korea

‡ Institute

of Advanced Machinery Design Technology, Korea University, 145 Anam-Ro,

Seongbuk-Gu, Seoul 02841, Rep. of Korea

§ Department

of Micro/Nano Systems, Korea University, 145 Anam-Ro, Seongbuk-Gu,

Seoul 02841, Rep. of Korea

* Corresponding author: [email protected] KEYWORDS: Hierarchical Pattern, Scattering, Diffraction, Tulip Queen of the Night, Micro/Nano Structure

ACS Paragon Plus Environment

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ABSTRACT

Many petals in nature have a hierarchical structure that imparts various optical properties. Among these, the petals of the Queen of the Night tulip exhibit an iridescent and glossy color due to the diffraction and scattering of light. Herein, we report a bioinspired micro/nano hierarchical structure that mimics Queen of the Night tulip petal surfaces. Using a method that combined soft lithography and UV-Ozone treatment, we fabricated nanoscale line patterns with a line-width of 600 nm on micro wrinkles of 15 μm width and 3 μm height. Using optical microscopy in dark-field mode and monochromatic light diffraction measurements, we found that these hierarchical structures on a polydimethylsiloxane (PDMS) substrate synergistically improved the scattering and diffraction effects, unlike the pristine, nano-, and micro-structures. In addition, using a dye-colored PDMS material, we fabricated artificial Queen of the Night petals with


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iridescent and glossy effects. They show great potential for a range of applications, such as coloring, smart displays, dynamic gratings, and light-control devices.


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Introduction The structures and functions of many parts of plants exhibit various characteristics. In particular, unique surface structures are found in the petals of many plants.1,2 For example, some petals are made of micro/nano hierarchical patterns that act as hydrophobic surfaces or important tools for pollinator attraction due to their unique structural shapes.3 In addition, the periodic patterns of these micro/nano hybrid lines can act as diffraction gratings to manipulate light, and exhibit iridescent and glossy characteristics through their interaction with light. Interestingly, owing to these optical properties, pollen mediators might be responsible for pollinating plants. In 2009, Whitney et al. reported that these optical characteristics could play a role in attracting bees.3 However, the hypothesis of iridescent signaling of flowers is rather controversial. Therefore, studies on this phenomenon are ongoing.4,5 Furthermore, many studies have been conducted to develop structural colors or to make hydrophobic substrates using hierarchical structures on elastomeric wrinkles.6,7 However, the development of a simple and size-controllable fabrication method for micro/nano hierarchical structures is still challenging.


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With regard to natural hierarchy, Queen of the Night petals fabricated using shapememory polymers and cellulosic films have been studied.8,9 However, their images do not show a glossy effect or iridescent character. Moreover, experimental research to investigate the optical effect of Queen of the Night petals has not been attempted.

Results and Discussion In this study, we fabricated a hierarchical structure of micro/nano line patterns on a polydimethylsiloxane (PDMS) substrate by combining the soft-lithography process10 with surface UV-Ozone (UVO) treatment11-14. In addition, thin film buckling of PDMS could be obtained by attaching a nanosheet.15,16 We controlled the size and angle of the micropattern with respect to a fixed nanopattern. Next, the scattering and diffraction effects due to the hierarchical structure were investigated via optical measurement methods. Finally, we fabricated a hierarchical pattern very similar to that of Queen of the Night tulip petals. Figure 1a shows a photograph of tulip blossoms and the atomic force microscopy (AFM) image for the surface of a natural tulip petal. Based on our measurements, a Queen of the Night petal has an average pattern width of 550 nm and a micropattern with a width of 15 µm and height of ~2-5 µm. Furthermore, the nanopatterns on the tulip surface are aligned parallel on microwrinkles, which leads to unique optical properties due to this hierarchical structure. To fabricate the pattern of the micro/nano hierarchical structure, a PDMS substrate with a nanoscale line pattern was firstly


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prepared using a soft-lithography process, as shown in Figure 1c. In this process, uncured PDMS was poured into a prepared nanoscale line-patterned Polyethylene terephthalate (PET) mold having a width of 200 nm and a pitch of 600 nm. The sample was then degassed for 15 mins and cured in a 60 C oven for 6 h. After curing, the nano-patterned PDMS substrate was removed from the mold. Thus, we obtained a PDMS substrate with nanoscale line patterns of 600 nm width and 200 nm pitch, because this is a replica of the original mold. In the next step, to fabricate microline patterns, both ends of the PDMS substrate with nanoscale line patterns were stretched and fixed. UVO treatment was then applied to the PDMS substrate, and the surface of PDMS was chemically modified. This chemical modification converted the PDMS surface into a silicate-like layer. After UVO treatment for 10 mins, releasing the extended PDMS substrate resulted in microbuckling due to the difference in the Young’s moduli of the stiff upper surface and the soft foundation. After the entire process was completed, we obtained a substrate with a micro/nano hierarchical structure. Figure 1d shows a photograph of the rainbow-colored & glossy surface due to the hierarchical structure on the purple-dyed PDMS substrate. The optical effects of artificial Tulip petals looks very similar with that of the natural one. To analyze the characteristics of the micro- and nanopatterns, we observed the sample surface using optical microscopy and AFM. Figure 2a shows a graph showing the height at which the polymer penetrates the nanograting pattern mold during the soft-lithography process upon adjusting the content ratio of PDMS prepolymer (mixture of PDMS base and curing agent) to silicone oil. The PDMS prepolymer exhibited a viscous and


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elastomeric form. Therefore, it could not perfectly penetrate the mold, which was several hundred nanometers in size. To solve this problem, silicone oil was added in the PDMS prepolymer to increase the height of the nanopattern and to make the surface uniform. By changing the PDMS base:Curing agent:Silicone oil ratio, we estimated the appropriate ratio to lower the viscosity of PDMS and maintain the hardening conditions. At a 20:1:1 ratio, the height of the nanopatterns was more uniform compared with that obtained using the ratios 10:1:none and 20:1:none. The ratio of 6.5:1:2.5 used in the previous study resulted in a tendency of the substrate to break easily after curing.17 This disadvantage shows the weak point of micropattern formation, which occurs subsequent to stretching. Therefore, we chose a ratio of 20:1:1. The ratio of PDMS prepolymer to silicone oil was controlled so that the polymer was relatively well-impregnated and hardened in the mold. Figure 2b-c shows the width and height according to the UVO exposure time and the strain. To compare PDMS wrinkling, we used O2 plasma18 and UVO treatments11-14 after stretching the PDMS substrate; then, the PDMS substrate was released. In the case of O2 plasma treatment, the width of the pattern was about 1 µm and the height was 100 nm (Figure S1). This was too small and shallow to mimic tulip petals. In contrast, the UVO


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treatment produced micropatterns with a width of ~15-20 µm and a height of 3 µm. Therefore, we chose UVO treatment to mimic the width and height of the micropatterns of Queen of the Night petals. When UVO treatment was performed on the surface of the PDMS substrate, some of the elements in PDMS, such as carbon dioxide, water, and organic compounds, escaped from the substrate. Simultaneously, the silicon components in the upper surface changed into SiO2 (silicate layer).11 According to the well-known theoretical model, the wavelength of the wrinkles (𝜆) could be obtained as follows.13,19,20

1

𝐸𝑓

3

(1)

λ = 2πℎ(3𝐸𝑠)

where h is the resulting silicate layer thickness, 𝐸 = 𝐸/(1 ― 𝜈2) is the plane-strain modulus, and 𝜈 is the Poisson ratio. The subscripts f and s denote the stiff thin film and the substrate, respectively. The amplitude of the wrinkle pattern, A, can be expressed as

1

ε0

2

𝐴 = ℎ(ε - 1) c

(2)

Where ε0 is the applied strain and εc is the critical strain.


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2

εc = -

1 3𝐸𝑠 3 ( ) 4 𝐸𝑓

(3)

The microwrinkle formation in this study is shown in Figure 2, and is similar to the results of other studies.11-14 Yang et al. noted that there are two conditions for buckling to occur. Firstly, a compressive stress should occur in the upper surface of the substrate, and the magnitude of the stress should be greater than the buckling point. Secondly, the lower part of the substrate should be flexible enough to allow the deformation of the upper layer.20 Considering these two conditions, we controlled the shape of the microwrinkles by adjusting the exposure time and strain. AFM was used to measure the wrinkled shape. Figure 2b-c shows the width and height of the microwrinkle according to the process conditions of the UVO treatment. The exposure times were 3, 5, 10, 30, and 60 mins, and the strain was fixed at 50%. The width decreased to 15 µm after a 10 mins irradiation, and then sharply increased upon further irradiation. According to Equation 1, a longer exposure time would result in a larger width of the wrinkle (𝜆). Accordingly, the thickness of the silicate layer, h, increased with the processing time. The height only changed in the range of ~3-5 µm. Then, the strain was set as 30%, 40%, and 50% for a 10 mins irradiation.


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When the strain was 50%, we obtained the desired width of 15 µm. We found that the change in height according to the strain was limited in the range of 2-4 µm. Fortunately, the height was similar to that of a real tulip petal surface. The images of the wrinkled substrate according to each condition were obtained via optical microscopy (Figure S2). Figure 2d-f shows the surface characteristics of the micro, nano, and micro/nano hierarchical patterns determined via AFM. The cross-sectional shape can be inferred upon inserting a yellow line profile in each AFM image. In the hierarchical pattern, there were nanopatterns on the microhill.

Queen of the Night tulip petals have a specular hierarchical structure that imparts several optical properties. Such optical properties make the petal surface iridescent and glossy. This is because the structure scatters the light received by the surface. There have been studies about the hierarchical micro/nano structure of rose petals. They focused on the optical effect of micro/nano pattern in terms of the light management and harvesting. 21,22 In our study, we mainly investigate the Tulip petals in terms of scattering and diffraction to understand the effects of the micro and/or nano patterns. For this, we measured the extent of scattering using the dark-field mode (DFM) of optical microscopy.23,24 The DFM is widely used to detect scattering phenomena occurring in nanosized objects in biological and materials fields. Figure 3a-c shows


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the DFM optical microscopy images that were used to analyze the nano-, micro-, and hierarchical micro-/nanopatterns. Additionally, the insets are camera images of each pattern. The different colors was due to the background color in the inset of figure 3a and micro/nano structures in the insets of Figure 3b-c. When DFM irradiates light from the optical microscope to the surface, it only collects scattered light. So stronger intensity of the scattering makes brighter image, because we used the same light intensity in optical microscope for measurement. In Figure 3d, a PDMS substrate with only nano patterns exhibited enhanced brightness compared to the pristine surface (Figure S3). PDMS surface with a micropattern shows the alternating bright and dark lines. Finally, the surface with micro-/nanopattern showed entirely more brightness along with broadening of white regions. Another optical property that we could observe for the hierarchical structure was the diffraction effect. 0th order diffraction could be observed on the micropattern. In a monochromatic laser diffraction experiment, a laser beam having a wavelength of 523 nm was shot at the center of the substrate, and the image was captured using a camera (Figure S4). Figure 3d-f shows the optical patterns of the nanopattern, micropattern, and hierarchical pattern, respectively. Additionally, the AFM images are presented in the insets (yellow squares). In the case of the nanopattern, the diffraction phenomenon did not occur, because there are no microsize wrinkles. The diffraction phenomena occurred on the micropattern and hierarchical pattern. These patterns have the same spacing of the diffraction spots, and diffusive lines spreading from the diffraction spot are vertically formed to the micropattern line. Kim et al. demonstrated that these diffusive lines were due to Mie scattering by the cracks in the micropatterns, one of the


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complex diffraction phenomena.25 For our sample, we observed that these diffusive lines from the diffraction spots in the hierarchical structure came out in several folds, which was due to the influence of the scattering caused by the nanopattern. Another effect of the nanopattern was the adjustment of the angle of the diffusive lines to right angles to the line of the diffraction spot. When the micropattern and nanopattern are aligned, as shown in Figure 3f, the angle between the diffraction spots and the diffusive line is 90°. Different samples were obtained by setting the micropattern and nanopattern angles at 15°, 30°, 45°, and 90°, and then the diffraction was measured. When the micropattern and nanopattern are tilted at a certain angle, the line of the diffraction spot also forms a certain angle with the diffusive line. Each sample is shown exactly tilted at an angle of 75°, 60°, 45°, and 0° (Figure S5). It is noteworthy that the change in angle between the micro- and nanopatterns does not significantly affect the optical effects.

To realize iridescent and glossy colors, we fabricated various colored PDMS substrates using our hierarchical patterns. Firstly, a multicolor hierarchical substrate was fabricated by mixing a small amount of color dye with uncured prepolymer PDMS. This


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process did not cause any problems in the curing of PDMS and did not affect the curing time. As shown in Figure 4a, we produced nine colors of substrates: white, red, yellow, blue, green, black, brown, flesh, and blood. We observed that these substrates showed a glossy effect and iridescent pattern. In addition, we used a purple dye and fabricated substrates in the shape of petals. As shown in Figure 4b-c, these artificial petals showed a glossy and iridescent character. Figure 4b shows the photographs of a tulip-shaped sample taken outdoor during the day and night. During the day, the sample looked iridescent without an additional flash, and during the night, the sample showed a similar optical effect after the flash was turned on. Figure 4c is an image taken indoor. When the flash was turned on toward the sample, the iridescent and glossy characteristics were obvious. The right-side of Figure 4c shows an enlarged image of the part where the iridescent characteristic is clear. When the tulip-shaped samples are curved, as shown in Figure 4b-c, under the bright environment, the optical properties appear only along the line receiving the light. For comparison, Figure 1d shows an artificial tulip Queen of the Night sample with a flattened shape. This substrate exhibited iridescent and glossy effects more clearly when the flash was turned on indoor. Also, in Figure S6, for a color-dyed PDMS substrate without


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pattern, there is no special optical effect. The realistic imitation of tulip petals presents the possibility for application in an industry that needs an aesthetic appearance. Figure 4d shows the image of the substrate color-shifted along the tension. As the microwrinkles expanded according to the tensile, the glossy characteristic disappeared and only the iridescent color remained due to the presence of the nanopattern. These characteristics were maintained even after repeated testing of tension and shrinkage. Figure 4e shows a photograph of the color substrates with hierarchical patterns after being creased, twisted, and bent. Even with these physical changes, the hierarchical pattern of the substrate exhibits unique optical properties. This effect could be used for the substrate with a curved surface. Conclusion In conclusion, we presented a bioinspired structure of Queen of the Night tulip petals having a unique structure that causes iridescent and glossy effects. We developed a fast, inexpensive, and facile fabrication method for such hierarchical structure patterns on PDMS substrates, using a combination of soft lithography and UVO treatment. Our hierarchical structure showed optical characteristics very similar to those of Queen of the

Night petals under DFM optical microscopy. In addition, using 0th order diffraction measurements, it was confirmed that the nanopattern controlled the angle of the diffusive lines generated after light passes through the substrate. This implied that the angle-tuned hierarchical structure polarized the light on one axis. Furthermore, we successfully


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mimicked colored tulip petals by mixing a color dye into the polymer. This approach is applicable to various applications associated with such colors.


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Experimental Methods Fabrication of Hierarchical-Pattern PDMS Substrate: Firstly, soft lithography was performed to fabricate a hierarchical-pattern substrate. In this process, a PET mold with a nanometer line of 200 nm width and 600 nm pitch was used, and PDMS (Sylgard 184, Dow Corning Co.) was mixed with PDMS oil (Element14 PDMS 50-E, Momentive Co.) and poured onto the mold at a ratio of Base PDMS:Curing agent:Oil = 20:1:1. Defoaming was carried out for 15 mins at room temperature in a vacuum oven (OV-11, JEIO Tech Co.), and additional curing was carried out at 60 °C for 6 h. After the curing process, the PDMS substrate was peeled off from the PET mold, and a nanopattern having a width of 600 nm and pitch of 200 nm was obtained. Then, we began the process of making a micropattern. Both ends of the substrate were stretched and fixed, followed by UVO treatment (MT-UV-O23, Minuta Co.) for 10 mins. Microwrinkles were formed upon releasing the expanded PDMS substrate after the treatment. Finally, we obtained a hierarchical pattern in which nanoscale lines were formed on microwrinkles. Fabrication of Color dye-PDMS composite: A color dye (Silc Pig, Smooth-On Inc.) was added to the PDMS when mixing the PDMS prepolymer with the curing agent. We conducted degassing for 10 mins and curing for 6 h, as described above. Since the amount of color dye was about 1/100 that of the PDMS prepolymer, it did not affect the curing time and results. Characterizations: The topologies of the samples were observed via AFM (NX10, Park Systems Co.). All the photographs were taken with a DSLR camera (EOS100D, Canon Co.). The transmittance was measured using UV-Vis spectrophotometry (Cary 5000,


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Varian Co.). Optical DFM data were measured via optical microscopy (LV150n, Nikon Co.).


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Figure 1. a) Tulip Queen of the Night petals: Photo and AFM images(scale bar, 5㎛), b) Optical effects in micro/nano hierachical structure, c) Schematics of micro/nano hierarchical pattern fabrication process, d) Photograph of artificial tulip Queen of the Night petals made of PDMS substrate with hierarchical micro/nano structure (scale bar, 1cm).


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Figure 2. Relation of pattern period and height. a) Height of nano pattern according to oil content(scale bar, 1㎛), b-c) Graph correlating width and height according to UV-Ozone treatment time and strain, and AFM images. d) Nano pattern (PDMS base: Curing agent: Oil=20:1:1), e) Micro pattern (50% strain, UVOzone 10mins treatment), f) Hierarchical pattern.


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Figure 3. Images of DFM (photograph image in inset yellow box; scale bar=1cm). a) Nano pattern substrate, b) Micro pattern substrate, c) Hierarchical pattern substrate, and the diffraction mages of d) Nano pattern, e) Micro pattern, f) Hierarchical pattern and AFM image in inset yellow box.


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Figure 4. a) Fabricated color substrates with hierarchical pattern along with color dyes (White, Red, Yellow, Blue, Green, Black, Brown, Flesh, Blood), and Artificial tulip shape using hierarchical pattern substrate of purple color b) in day, night of outdoor (scale bar=1cm), c) in flash on and off, d) hierarchical pattern substrate stretched from 0 to 100%, and e) Photograph image of creasing, twisting and bending substrate.


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ASSOCIATED CONTENT

Supporting Information. AFM image after O2 plasma treatment to PDMS substrate. Optical microscope image. a) according to exposing time of UVO treatment at 50% strain, b) according to strain change at 10mins UVO treatment. Dark-field mode image of PDMS substrate without pattern. Schematic of monochromatic light diffraction experiment. Optical images at the various angles (15 °, 30 °, 45 ° and 90 °) between the micropattern and nanopattern. Color-dyed PDMS substrate without pattern. (PDF) Tensile testing video (AVI) This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: * E-mail: [email protected] Author Contributions:


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J. H.O., J.Y.W., S.J, C. -S.H. conceived the idea and designed the experiments, J. H.O. performed the experiments and analyzed the data, J. H.O. and C.-S. H. discussed and wrote the manuscript.

ACKNOWLEDGMENT

This work was supported by the Basic Science Research Program (Grant Nos. 2018R1A2A1A05023556 and 2018R1D1A1B07047119) through the National Research Foundation funded by the Ministry of Science and ICT and Korea University Grant, Korea.

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