- Email: [email protected]
Microstructure of butterfly wing scale and simulation of structural color
Optik 127 (2016) 1729–1733
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Microstructure of butterfly wing scale and simulation of structural color Xiaoyan Liu a,b,∗ , Shasha Zhang a , Hongbo Zhang a a b
College of Textiles, Donghua University, Songjiang District, Shanghai 201620, China Key Laboratory of Textile Science & Technology, Ministry of Education, Shanghai 201620, China
a r t i c l e
i n f o
Article history: Received 7 April 2015 Accepted 9 November 2015 Keywords: Microstructure Butterfly Scale Structural color Nanoparticle
a b s t r a c t Structural color has received an increasing amount of attention from researchers. Most hope to realize structural color because of its bright colors, durability and environmental friendliness. In this paper, we observed the butterfly microstructure and then predicted the probable optimal experimental parameters to realize the structural color on the substrate. The SEM images showed mesh and porousness were regularly arranged along the wing scale. The surface model of 2-D wing scale were set up and displayed based on SEM observations. The reflectivity spectrum of wing scale was also measured and analyzed. At the same time, the structural color was deduced based on the colloid nanoparticles ordered arrangement. Supposing the colloidal SiO2 nanoparticles (NPs) assembled on the substrate, the reflective intensity would change with nanoparticle (NP) size and NP film layer number. NP size appeared effecting structural color hue, and layer number effecting color saturation. In addition, structural color also varied observed from the different irradiant angle even if under the same NP size. These observation and deduction results of the structural color could effectively guide the materials design especially for the optical and color development. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction The natural structural color has long attracted scientific interest, and it is a type of coloration originating from microstructure variation at a length scale comparable to the optical wavelength [1–3]. It is found in nature, for example in pearl, jewel beetle, peacock tail and fishes. This coloration is generally accompanied with a brilliant metallic luster [4–6]. In the wings of butterflies, for example, a combination of multilayer interference, optical gratings, photonic crystals and other optical structures gives rise to complex color mixing [7–10]. They speculated that the optical properties of the butterfly wings had a structural origin. The scales are arranged on butterfly wings in arrays of precise and repeated structures. The complex groove shapes display several optical effects, such as interference, scattering and diffraction [11–13]. The structural color has a variety of potential applications, because of its long-term resistance to discoloration due to chemical change. Furthermore, it cannot be reproduced by pigments, and
pigment-free coloration is preferable from ecological viewpoint [14,15]. People observe the butterfly, peacock, beetle bug and so on, to find the mechanism between their brilliant color and their initial structure [16,17]. During the last few decades, much effort has been directed toward the brightest and most vivid structurebased colors in nature that arise from the interaction of light with surfaces on the micro- and nanoscale [18–20]. For textiles, if structural color can be realized on the fabric, then the gorgeous textile effect can be obtained. Especially the waterpollution problem will be controlled, and it will greatly improve the textile processing and environment. In this paper, the butterfly wing scales were observed and modeled based on their surface character. We supposed the colloids were applied on the substrate, the relationship between colloids and structural color were also deduced. These results would promote structural color development and application in materials [21,22].
2. Experimental 2.1. Sample
∗ Corresponding author at: College of Textiles, Donghua University, 2999 North Renmin Rd., Songjiang District, Shanghai 201620, China. Tel.: +86 21 67792411; fax: +86 21 67792229. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.ijleo.2015.11.036 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
In this paper, the wings of butterfly Papiliohelenus and Papilioxuthus were taken as samples, shown in Table 1. They were raised in Guangxi Province in China.
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X. Liu et al. / Optik 127 (2016) 1729–1733
Table 1 Two butterfly pictures. Name
Original sample
Butterfly with opened wing
Papiliohelenus
Papilioxuthus
28 24
1-the surface of the back wing 2-the reverse of the back wing 3-the surface of the front wing 4-the reverse of the front wing
2
4
reflectivity(%)
20
1
16 12
3
8 4
Fig. 1. SEM of Papiliohelenus butterfly scale.
0 300
400
500
600
700
800
wavelength(nm)
2.2. Tester
Fig. 3. Reflectance spectrum of the front and back wing of Papiliohelenus.
To observe the scale microstructure, the SEM pictures were taken by JSM-5600LV. The U-4100 (UV-VIS-NIR Spectrophotometer) was used to test the reflectance spectrum. 2.3. Theoretical calculation method of structural color
Based on the equation of reflectivity (R) and reflection coefficient (r) in our previous transfer matrix calculation [23,24], the color values can then be obtained from the integral function given by CIE standard colorimetric system. Finally the chromaticity coordinate (x,y) can be calculated.
In order to theoretically predict possible color, the structural color from the order arranged nanoparticles (NPs) was deduced in this paper.
70
4
60
reflectivity(%)
50
3
40
1-the surface of the back wing 2-the reverse of the back wing 3-the surface of the front wing 4-the reverse of the front wing
30 20
2
10
1 0 300
400
500
600
700
800
wavelength(nm)
Fig. 2. SEM of Papilioxuthus butterfly scale.
Fig. 4. Reflectance spectrum of the front and back wing of Papilioxuthus.
X. Liu et al. / Optik 127 (2016) 1729–1733
Fig. 5. Surface model of the butterfly wing scale.
3. Results and discussion 3.1. Morphology of butterfly scale The details of the periodic structure on Papiliohelenus and Papilioxuthus butterfly wings were shown by high magnification SEM images in Figs. 1 and 2.
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There were many mesh structures on Papiliohelenus wing scale. In Fig. 1(a), there were four smooth crenates on top of the scale. The length of the scale was about 150 m, and the width was about 60 m. In Fig. 1(b), there existed linear ridges along the long wing direction, and the interdistance was about 2 m. If the net was taken as circles, the maximum and the minimum diameter were 1 m and 0.1 m, respectively. The diameter of most circles was 0.5 m. Two adjacent ribs were bound together by a row of smaller round sub-ribs with diameters of 0.1–1 m. There also were porous structures in the Papilioxuthus butterfly. Fig. 2(a) showed the scales arranged closely, one layer stacked each other, similar to the tile on the roof. There were 2–3 crenates on the top of the scale. The width was about 55 m, and the length was 120 m. In Fig. 2(b), there also were linear ridge departed from the scales, and there was about 2.3 m gap space, and the thickness of the ridge was about 0.8 m. The mesh neared to the ridge had longer size along the transverse, and the middle mesh was nearly round. These butterfly wing scales had rib and ridge structures, and the ribs did not connect two ridges directly. While they had some
Fig. 6. Reflectance spectrum under different NP parameters.
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sub-across between the thinnest ribs, which resulted in some holes of anomalous shape but not rectangle ones. 3.2. Light spectrum of the butterfly wing scale To discover the light reflection spectrum, the two butterfly wings were tested on the spectrometer. Fig. 3 curves illustrated the front and back wings all had the same reflect rule. The reflectivity all increased with the wavelength. Moreover, the reflectance increased largely during 400–500 nm, that meant the responding color would appear at 400–500 nm. When the wavelength was higher than 500 nm, the reflectance changed stably. Compared with these curves, the reflectivity of the reverse wing was higher than that of the surface, which maybe from reverse wing was smoother than the surface wing. The curves 1 and 2 in Fig. 4 showed the reflectance of surface and reverse of the back wing of Papilioxuthus butterfly were approximate, and the reflectivity was nearly to zero. For the front wing, the reflectance suddenly increased at 450 nm wavelength. The reflectivity of the reverse of the front wing was higher than 60%, and it was higher than that of the Papiliohelenus wing. Compared with the two butterfly wing spectrums, the reflectivity of Papilioxuthus, especially for the front wing, were higher than that of Papiliohelenus. Combined with the SEM picture, it can also find the porous structure in Papilioxuthus was more compacted orderly than that in Papiliohelenus. That implied the light can reflect more when it irradiated on the former wing scale, then the reflectivity can increase. The spectrum now also verified the results. 3.3. Surface model of the butterfly microscale Based on the two butterflies SEM picture, the probable models of two butterflies were shown in Fig. 5 using Solidworks software. Here the Papiliohelenus wing was supposed having uniform circle mesh along the scale, and Papilioxuthus having the unit cell, including the round mesh between both of the notches. The structure models were set up to imitate the functional ultrastructure of the butterfly scales. This configuration constructed a 2D array of round squares surrounded by organic nanometer-scale cylinders (the main and sub-ribs) with a periodicity along its length direction and along the main ribs. This long-range ordered organic structure with a very small periodicity can be considered as 2D photonic crystal slab, and the brilliant color was attributed to the existence of the photonic band gap (PBG) in this structure. 3.4. Numerical calculation of the structural color In most cases, the colloidal SiO2 NPs were the most constitute of the natural structural color, and the NP film with good uniformity
Fig. 7. CIE chromaticity diagram of SiO2 NP under different layer number. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
and high density. To clearly direct our further experimental simulation, the experimental parameters of NP size (100–350 nm) and film number (3–33) were determined. Based on the equation of the reflectivity and NP parameter [23,24], the reflectivity could be calculated. The relation of the reflectance between colloidal SiO2 size (r ) and NP film layer number were shown in Fig. 6. In Fig. 6(a) and (b), it can be found the reflectivity was always lower at 100 nm, 150 nm NP size although the layer number increased. There was no peak formed during the wavelength, so it cannot produce the structural color. When NP was 200–350 nm, under the same layer number, the photo band center wavelength gradually increased with NP size. The interference peak occurred in the high frequency area at the right of the main peak. When the layer number increased over 17, the main peak reflectivity almost was stable. Both of the NP size and film number had effects on the reflective peak intensity and width, it resulted in different color. It can be seen the layer number should be up to 17 more with 200–350 nm NPs, and their reflectivity and the photo band gap would be obvious in Fig. 6. The colloidal SiO2 size 250 and 300 nm was then taken as example, the corresponding color based on the CIE Chromaticity diagram was illustrated in Fig. 7. Where A, B, C, D, E were respectively stack layer numbers, A = 3, B = 5, C = 9, D = 17, E = 33. For 250 nm NP size, the hue mainly were almost green. For 300 nm, the hue mainly were about yellow. For the layer number 17 and 33, the structual color are approximate. Under the same diameter, the structural color has the same chroma, but the saturation was adjacent. This meant the NP size influenced the structural color hue, but the stack thickness effected color saturation. When the stack layer increased to a certain number, the structural color were near. 3.5. Irradiant angle effect on the structural color The obvious feature of the structural color was the color varied with irradiated angle. Take the 250 nm and 300 nm as sample, the reflectance spectrum of SiO2 NP was calculated and shown in Fig. 8 under 0◦ , 10◦ , 20◦ , 30◦ , 40◦ , 50◦ and 60◦ .
Fig. 8. Reflectance spectrum of SiO2 NP under different irradiant angle.
X. Liu et al. / Optik 127 (2016) 1729–1733
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References
Fig. 9. CIE Chromaticity diagram of SiO2 NP under different irradiated angle.
From the picture, the wavelength of the highest reflectivity decreased with the irradiant angle increasing. The photonic band gap location also depended on the irradiated angle, and it resulted in different structural color. Fig. 9 showed the different structural color under different irradiated angle. Increasing the irradiated angle, the wavelength of PBG became shorter, but the highest reflectivity was near. It implied the irradiate angle probably effected the structural color hue. 4. Conclusions To have knowledge of the structural color and some applications in polymer science, the two kinds of butterflies were observed and analyzed in this paper. There were some porous and mesh observed in the wing scale. These butterfly wing scales had rib and ridge structures, and they had some sub-across between the thinnest ribs. The surface models were then set up using Solidworks software based on SEM observations. In order to obtain the structural color, the colloidal SiO2 nanoparticles were supposed in preparing for the structural color. Based on the theory model deduction, the reflectivity made from different SiO2 NP size and NP film layer numbers were calculated, and the responding structural color was also displayed. When NP was 200–350 nm and the layer number increased over 17, the main peak reflectivity almost was stable. The NP size appeared effecting on the structural color hue, and the layer number effecting color saturation. The different irradiant angle displayed different structural color for the same NP size, and it influenced more color hue. Whether the wing scale had regular mesh and ordered arrangement, or the colloids NP feature influenced the structural color, these results can give us direction to realize structural color on the substrate or some polymer preparation in our further experimental work. Acknowledgments This project was supported by NSFC (51103021), Research Fund for the Doctoral Program of Higher Education of China (20110075120005), and the Fundamental Research Funds for the Central Universities.
[1] S. Vignolini, P.J. Rudall, A.V. Rowland, A. Reed, E. Moyroud, R.B. Faden, J.J. Baumberg, B.J. Glover, U. Steiner, Pointillist structural color in Pollia fruit, Proc. Natl. Acad. Sci. U. S. A. 109 (2012) 15712–15715. [2] A. Saito, Material design and structural color inspired by biomimetic approach, Sci. Technol. Adv. Mater. 12 (2011) 064709. [3] L.P. Biró, Photonic nanoarchitectures of biologic origin in butterflies and beetles, Mater. Sci. Eng. B 169 (2010) 3–11. [4] P. Vukusic, D.G. Stavenga, Physical methods for investigating structural colours in biological systems, J. R. Soc. Interface 6S (2009) 133–148. [5] O. Sato, S. Kubo, Z.Z. Gu, Structural color films with lotus effects, superhydrophilicity and tunable stop-bands, Acc. Chem. Res. 42 (2009) 1–10. [6] R.O. Prum, T. Quinn, R.H. Torres, Anatomically diverse butterfly scales all produce structural colours by coherent scattering, J. Exp. Biol. 209 (2006) 748–765. [7] Z.W. Han, S.C. Niu, L.F. Zhang, Z.N. Liu, L.Q. Ren, Light trapping effect in wing scales of butterfly Papilio peranthus and its simulations, J. Bionic Eng. 10 (2013) 162–169. [8] K. Chung, S. Yu, C.J. Heo, J.W. Shim, S.M. Yang, M.G. Han, H.S. Lee, Y. Jin, S.Y. Lee, N. Park, J.H. Shin, Angle-independent structural color reflectors inspired by morpho butterfly wings, Adv. Mater. 24 (2012) 2375–2379. [9] M. Kolle, P.M. Salgard-Cunha, M.R.J. Scherer, F.M. Huang, P. Vukusic, S. Mahajan, J.J. Baumberg, U. Steiner, Mimicking the colourful wing scale structure of the Papilio blumei butterfly, Nat. Nanotechnol. 5 (2010) 511–515. [10] P. Kurt, D. Banerjee, R.E. Cohen, M.F. Rubner, Structural color via layer-by-layer deposition: layered nanoparticle arrays with near-UV and visible reflectivity bands, J. Mater. Chem. 19 (2009) 8920–8927. [11] A.R. Parker, H.E. Townley, Biomimetics of photonic nanostructures, Nat. Nanotechnol. 2 (2007) 347–353347. [12] P. Vukusic, J.R. Sambles, Photonic structures in biology, Nature 424 (2003) 852–855. [13] S. Kinoshita, S. Yoshioka, K. Kawagoe, Mechanisms of structural colour in the Morpho butterfly: cooperation of regularity and irregularity in an iridescent scale, Proc. R. Soc. Lond. B 269 (2002) 1417–1421. [14] H. Tada, S.E. Mann, I.N. Miaoulis, P.Y. Wong, Effects of a butterfly scale microstructure on the iridescent color observed at different angles, Opt. Express 37 (1998) 1579–1584. [15] O.L. Pursiainen, J.J. Baumberg, H. Winkler, B. Viel, P. Spahn, T. Ruhl, Nanoparticle-tuned structural color from polymer opals, Opt. Express 15 (2007) 9553–9561. [16] Z.W. Han, S.C. Niu, M. Yang, J.Q. Zhang, W. Yin, L.Q. Ren, An ingenious replica template from the light trapping structure in butterfly wing scales, Nanoscale 5 (2013) 8500–8506. [17] J.Y. Huang, X.D. Wang, Z.L. Wang, Controlled replication of butterfly wings for achieving tunable photonic properties, Nanoletters 6 (2006) 2325–2331. [18] O. Deparis, J.P. Vigneron, Modeling the photonic response of biological nanostructures using the concept of stratified medium: the case of a natural three-dimensional photonic crystal, Mater. Sci. Eng. B 169 (2010) 12–15. [19] Y.Y. Diao, X.Y. Liu, Controlled colloidal assembly: experimental modeling of general crystallization and biomimicking of structural color, Adv. Funct. Mater. 22 (2012) 1354–1375. [20] L.D. Bonifacio, B.V. Lotsh, D.P. Puzzo, F. Scotognella, G.A. Ozin, Stacking the nanochemistry deck: structural and compositional diversity in onedimensional photonic crystals, Adv. Mater. 21 (2009) 1641–1646. [21] Z.H. Shen, Y.Y. Yang, F.Z. Lu, B.F. Bao, B. You, Self-assembly of binary particles and application as structural colors, Polym. Chem. 3 (2012) 2495–2501. [22] H. Yabu, Y. Saito, M. Shimogura, Unique light reflectors that mimic the structural colors of tiger beetles, Polym. J. 46 (2014) 212–215. [23] X.Y. Liu, H.X. Cui, Color evaluation of one-dimensional photonic crystal on the fabric, Fiber. Polym. 15 (2014) 783–787. [24] H.B. Zhang, X.Y. Liu, Optical properties of three-dimensional photonic crystal with face-centered-cubic structure, Adv. Mater. Res. 821 (2013) 622–625.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Microstructure of butterfly wing scale and simulation of structural color Xiaoyan Liu a,b,∗ , Shasha Zhang a , Hongbo Zhang a a b
College of Textiles, Donghua University, Songjiang District, Shanghai 201620, China Key Laboratory of Textile Science & Technology, Ministry of Education, Shanghai 201620, China
a r t i c l e
i n f o
Article history: Received 7 April 2015 Accepted 9 November 2015 Keywords: Microstructure Butterfly Scale Structural color Nanoparticle
a b s t r a c t Structural color has received an increasing amount of attention from researchers. Most hope to realize structural color because of its bright colors, durability and environmental friendliness. In this paper, we observed the butterfly microstructure and then predicted the probable optimal experimental parameters to realize the structural color on the substrate. The SEM images showed mesh and porousness were regularly arranged along the wing scale. The surface model of 2-D wing scale were set up and displayed based on SEM observations. The reflectivity spectrum of wing scale was also measured and analyzed. At the same time, the structural color was deduced based on the colloid nanoparticles ordered arrangement. Supposing the colloidal SiO2 nanoparticles (NPs) assembled on the substrate, the reflective intensity would change with nanoparticle (NP) size and NP film layer number. NP size appeared effecting structural color hue, and layer number effecting color saturation. In addition, structural color also varied observed from the different irradiant angle even if under the same NP size. These observation and deduction results of the structural color could effectively guide the materials design especially for the optical and color development. © 2015 Elsevier GmbH. All rights reserved.
1. Introduction The natural structural color has long attracted scientific interest, and it is a type of coloration originating from microstructure variation at a length scale comparable to the optical wavelength [1–3]. It is found in nature, for example in pearl, jewel beetle, peacock tail and fishes. This coloration is generally accompanied with a brilliant metallic luster [4–6]. In the wings of butterflies, for example, a combination of multilayer interference, optical gratings, photonic crystals and other optical structures gives rise to complex color mixing [7–10]. They speculated that the optical properties of the butterfly wings had a structural origin. The scales are arranged on butterfly wings in arrays of precise and repeated structures. The complex groove shapes display several optical effects, such as interference, scattering and diffraction [11–13]. The structural color has a variety of potential applications, because of its long-term resistance to discoloration due to chemical change. Furthermore, it cannot be reproduced by pigments, and
pigment-free coloration is preferable from ecological viewpoint [14,15]. People observe the butterfly, peacock, beetle bug and so on, to find the mechanism between their brilliant color and their initial structure [16,17]. During the last few decades, much effort has been directed toward the brightest and most vivid structurebased colors in nature that arise from the interaction of light with surfaces on the micro- and nanoscale [18–20]. For textiles, if structural color can be realized on the fabric, then the gorgeous textile effect can be obtained. Especially the waterpollution problem will be controlled, and it will greatly improve the textile processing and environment. In this paper, the butterfly wing scales were observed and modeled based on their surface character. We supposed the colloids were applied on the substrate, the relationship between colloids and structural color were also deduced. These results would promote structural color development and application in materials [21,22].
2. Experimental 2.1. Sample
∗ Corresponding author at: College of Textiles, Donghua University, 2999 North Renmin Rd., Songjiang District, Shanghai 201620, China. Tel.: +86 21 67792411; fax: +86 21 67792229. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.ijleo.2015.11.036 0030-4026/© 2015 Elsevier GmbH. All rights reserved.
In this paper, the wings of butterfly Papiliohelenus and Papilioxuthus were taken as samples, shown in Table 1. They were raised in Guangxi Province in China.
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Table 1 Two butterfly pictures. Name
Original sample
Butterfly with opened wing
Papiliohelenus
Papilioxuthus
28 24
1-the surface of the back wing 2-the reverse of the back wing 3-the surface of the front wing 4-the reverse of the front wing
2
4
reflectivity(%)
20
1
16 12
3
8 4
Fig. 1. SEM of Papiliohelenus butterfly scale.
0 300
400
500
600
700
800
wavelength(nm)
2.2. Tester
Fig. 3. Reflectance spectrum of the front and back wing of Papiliohelenus.
To observe the scale microstructure, the SEM pictures were taken by JSM-5600LV. The U-4100 (UV-VIS-NIR Spectrophotometer) was used to test the reflectance spectrum. 2.3. Theoretical calculation method of structural color
Based on the equation of reflectivity (R) and reflection coefficient (r) in our previous transfer matrix calculation [23,24], the color values can then be obtained from the integral function given by CIE standard colorimetric system. Finally the chromaticity coordinate (x,y) can be calculated.
In order to theoretically predict possible color, the structural color from the order arranged nanoparticles (NPs) was deduced in this paper.
70
4
60
reflectivity(%)
50
3
40
1-the surface of the back wing 2-the reverse of the back wing 3-the surface of the front wing 4-the reverse of the front wing
30 20
2
10
1 0 300
400
500
600
700
800
wavelength(nm)
Fig. 2. SEM of Papilioxuthus butterfly scale.
Fig. 4. Reflectance spectrum of the front and back wing of Papilioxuthus.
X. Liu et al. / Optik 127 (2016) 1729–1733
Fig. 5. Surface model of the butterfly wing scale.
3. Results and discussion 3.1. Morphology of butterfly scale The details of the periodic structure on Papiliohelenus and Papilioxuthus butterfly wings were shown by high magnification SEM images in Figs. 1 and 2.
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There were many mesh structures on Papiliohelenus wing scale. In Fig. 1(a), there were four smooth crenates on top of the scale. The length of the scale was about 150 m, and the width was about 60 m. In Fig. 1(b), there existed linear ridges along the long wing direction, and the interdistance was about 2 m. If the net was taken as circles, the maximum and the minimum diameter were 1 m and 0.1 m, respectively. The diameter of most circles was 0.5 m. Two adjacent ribs were bound together by a row of smaller round sub-ribs with diameters of 0.1–1 m. There also were porous structures in the Papilioxuthus butterfly. Fig. 2(a) showed the scales arranged closely, one layer stacked each other, similar to the tile on the roof. There were 2–3 crenates on the top of the scale. The width was about 55 m, and the length was 120 m. In Fig. 2(b), there also were linear ridge departed from the scales, and there was about 2.3 m gap space, and the thickness of the ridge was about 0.8 m. The mesh neared to the ridge had longer size along the transverse, and the middle mesh was nearly round. These butterfly wing scales had rib and ridge structures, and the ribs did not connect two ridges directly. While they had some
Fig. 6. Reflectance spectrum under different NP parameters.
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X. Liu et al. / Optik 127 (2016) 1729–1733
sub-across between the thinnest ribs, which resulted in some holes of anomalous shape but not rectangle ones. 3.2. Light spectrum of the butterfly wing scale To discover the light reflection spectrum, the two butterfly wings were tested on the spectrometer. Fig. 3 curves illustrated the front and back wings all had the same reflect rule. The reflectivity all increased with the wavelength. Moreover, the reflectance increased largely during 400–500 nm, that meant the responding color would appear at 400–500 nm. When the wavelength was higher than 500 nm, the reflectance changed stably. Compared with these curves, the reflectivity of the reverse wing was higher than that of the surface, which maybe from reverse wing was smoother than the surface wing. The curves 1 and 2 in Fig. 4 showed the reflectance of surface and reverse of the back wing of Papilioxuthus butterfly were approximate, and the reflectivity was nearly to zero. For the front wing, the reflectance suddenly increased at 450 nm wavelength. The reflectivity of the reverse of the front wing was higher than 60%, and it was higher than that of the Papiliohelenus wing. Compared with the two butterfly wing spectrums, the reflectivity of Papilioxuthus, especially for the front wing, were higher than that of Papiliohelenus. Combined with the SEM picture, it can also find the porous structure in Papilioxuthus was more compacted orderly than that in Papiliohelenus. That implied the light can reflect more when it irradiated on the former wing scale, then the reflectivity can increase. The spectrum now also verified the results. 3.3. Surface model of the butterfly microscale Based on the two butterflies SEM picture, the probable models of two butterflies were shown in Fig. 5 using Solidworks software. Here the Papiliohelenus wing was supposed having uniform circle mesh along the scale, and Papilioxuthus having the unit cell, including the round mesh between both of the notches. The structure models were set up to imitate the functional ultrastructure of the butterfly scales. This configuration constructed a 2D array of round squares surrounded by organic nanometer-scale cylinders (the main and sub-ribs) with a periodicity along its length direction and along the main ribs. This long-range ordered organic structure with a very small periodicity can be considered as 2D photonic crystal slab, and the brilliant color was attributed to the existence of the photonic band gap (PBG) in this structure. 3.4. Numerical calculation of the structural color In most cases, the colloidal SiO2 NPs were the most constitute of the natural structural color, and the NP film with good uniformity
Fig. 7. CIE chromaticity diagram of SiO2 NP under different layer number. (For interpretation of the references to color in text, the reader is referred to the web version of this article.)
and high density. To clearly direct our further experimental simulation, the experimental parameters of NP size (100–350 nm) and film number (3–33) were determined. Based on the equation of the reflectivity and NP parameter [23,24], the reflectivity could be calculated. The relation of the reflectance between colloidal SiO2 size (r ) and NP film layer number were shown in Fig. 6. In Fig. 6(a) and (b), it can be found the reflectivity was always lower at 100 nm, 150 nm NP size although the layer number increased. There was no peak formed during the wavelength, so it cannot produce the structural color. When NP was 200–350 nm, under the same layer number, the photo band center wavelength gradually increased with NP size. The interference peak occurred in the high frequency area at the right of the main peak. When the layer number increased over 17, the main peak reflectivity almost was stable. Both of the NP size and film number had effects on the reflective peak intensity and width, it resulted in different color. It can be seen the layer number should be up to 17 more with 200–350 nm NPs, and their reflectivity and the photo band gap would be obvious in Fig. 6. The colloidal SiO2 size 250 and 300 nm was then taken as example, the corresponding color based on the CIE Chromaticity diagram was illustrated in Fig. 7. Where A, B, C, D, E were respectively stack layer numbers, A = 3, B = 5, C = 9, D = 17, E = 33. For 250 nm NP size, the hue mainly were almost green. For 300 nm, the hue mainly were about yellow. For the layer number 17 and 33, the structual color are approximate. Under the same diameter, the structural color has the same chroma, but the saturation was adjacent. This meant the NP size influenced the structural color hue, but the stack thickness effected color saturation. When the stack layer increased to a certain number, the structural color were near. 3.5. Irradiant angle effect on the structural color The obvious feature of the structural color was the color varied with irradiated angle. Take the 250 nm and 300 nm as sample, the reflectance spectrum of SiO2 NP was calculated and shown in Fig. 8 under 0◦ , 10◦ , 20◦ , 30◦ , 40◦ , 50◦ and 60◦ .
Fig. 8. Reflectance spectrum of SiO2 NP under different irradiant angle.
X. Liu et al. / Optik 127 (2016) 1729–1733
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References
Fig. 9. CIE Chromaticity diagram of SiO2 NP under different irradiated angle.
From the picture, the wavelength of the highest reflectivity decreased with the irradiant angle increasing. The photonic band gap location also depended on the irradiated angle, and it resulted in different structural color. Fig. 9 showed the different structural color under different irradiated angle. Increasing the irradiated angle, the wavelength of PBG became shorter, but the highest reflectivity was near. It implied the irradiate angle probably effected the structural color hue. 4. Conclusions To have knowledge of the structural color and some applications in polymer science, the two kinds of butterflies were observed and analyzed in this paper. There were some porous and mesh observed in the wing scale. These butterfly wing scales had rib and ridge structures, and they had some sub-across between the thinnest ribs. The surface models were then set up using Solidworks software based on SEM observations. In order to obtain the structural color, the colloidal SiO2 nanoparticles were supposed in preparing for the structural color. Based on the theory model deduction, the reflectivity made from different SiO2 NP size and NP film layer numbers were calculated, and the responding structural color was also displayed. When NP was 200–350 nm and the layer number increased over 17, the main peak reflectivity almost was stable. The NP size appeared effecting on the structural color hue, and the layer number effecting color saturation. The different irradiant angle displayed different structural color for the same NP size, and it influenced more color hue. Whether the wing scale had regular mesh and ordered arrangement, or the colloids NP feature influenced the structural color, these results can give us direction to realize structural color on the substrate or some polymer preparation in our further experimental work. Acknowledgments This project was supported by NSFC (51103021), Research Fund for the Doctoral Program of Higher Education of China (20110075120005), and the Fundamental Research Funds for the Central Universities.
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