Abstract

1. Abstract • Microstructure of butterfly scales are detailed with 3-D structures and thin-films • Iridescent scales reflect bright colors by thin-film effects & other optical phenomena • Balance of radiation is absorbed for thermoregulatory purposes • Numerical and experimental results used to examine function, properties, and structure • Study optical effects in light and cell interaction for microelectronics and optics • Determine optical properties of thin-film biological material • Examine cellular development of thin-film structures for future applications

2. Introduction • Butterfly wings are lined with many wing scales • Complex microstructures in scales can produce structural colors upon interaction with sunlight • Structural colors are not due to pigmentation, but are bright, metallic iridescence or diffractive colors dependent on viewing angle • Radiative properties have multiple functions: display, camouflage, courting, thermoregulation • Model of complex microstructures is of interest to microelectronics industry where unpredictable radiative properties due to the complex circuitry lead to defects and reduced productivity • Understanding the cellular microstructure of butterfly scale and resulting properties can lead to development of innovative organic thin-film materials with unique custom optical qualities

3. Apparent or true color incident sun light reflected light thin films transmitted light Optical Phenomena • Thin-film Interference • strongly affects spectral reflectivity when thin-film thickness are on the order of wavelength of light • incident light is partially reflected and transmitted at each interface between two layers • total spectral reflectivity is the sum of all rays exiting from the surface, taking into account the phase difference between each ray

4. scattered light incident light scattered spectrum white light Optical Phenomena • Scattering • random process • due to surface roughness • incident light is reflected in all directions • Diffraction • due to regularly repeating surface pattern • pattern size ~wavelength of incident light • different wavelengths are scattered in varying but predictable directions • separation of white light into its spectrum

5. Optical Phenomena • Non-planar Specular Reflection • combination of thin-film interference and scattering • thin-film stack curved into patterns much larger than wavelengths of incident light • curvature changes the local angles of incidence, thereby changing the angle of exiting ray • color seen at each angle changes due to angular dependence of specular reflectivity of thin-films • net result is a predictable shift in observed color at different view angles reflected light incident white light curved thin-films local normals

6. Butterfly Microstructure • General butterfly wing scale • made of an organic material called chitin • scales are generally about 100m long • lower lamina is generally smooth • upper lamina has prominent features: • ridges extend up in lines along the length of scale • cross-ribs connect ridges transversely

7. ~100mm cross-ribs ridges ~5mm wing scale laminae scale cross section Papilio blumei • Scale Specialization • series of laminae layers between upper & lower lamina • laminae are separated by thin layers of air & spacers • laminae and air layers make up multilayer structure • structure is curved to form ridges and cross-ribs • separation between ridges is approximately 5mm, too large to cause diffraction • due to curvature of film stack, non-planar specular reflection needs to be considered

8. ~100mm scale cross section ~0.7mm wing scale lamellae ridges lower lamina Morpho menelaus • Scale specialization • tall ridges protrude vertically from scale surface • lamellae films extend from either side of ridge • highly anisotropic, revealing the complex, tree-like pattern only in the transverse cross-section • lamellae layers act as the thin-film stacks • ridges are ~0.7mm apart, suggesting the presence of diffraction when interacting with sunlight

9. Numerical Models • Predicts spectral reflectivity due to thin-film interference • calculation based on model of microstructure • Index of refraction of chitin • optical properties of chitin are limited • n may be wavelength dependent • n(l) found by matching numerical result to experimental data • Coherency considerations • thin-film interference predictable only when light is coherent through its entire optical path • uses reduced number of films to ensure coherency through light’s optical path • Experimental data • modified microscope with monochromatic light • measures spectral reflectivity of small areas • effective for l between 500 nm and 1000 nm

10. lamina layer layer 1 air layer layer 2 . . . layer n d D P. blumei Numerical Model • Alternating layers of lamina and air layers • Air layer has series of spacers made of chitin • average index method: neffective = F nchitin + (1-F) nair • fill factor F = d/D, estimated to be 0.5 • Layer thickness approximated as constants: • lamina layers = 0.095mm • air layers = 0.085mm • Dimensions calculated from SEM picture of scale cross-section

11. 1 unit lamellae lamellae ridge air M. menelaus Numerical Model • Uses the transverse cross section of the scale • Three sections: ridge, air, and lamellae • Spectral reflectivity of lamellae section calculated using thin-film interference model • lamellae layer thickness = 0.054mm • air layer thickness = 0.118mm respectively. • Effect of ridge and air sections • reduce numerical spectral reflectivity by 9% • Dimensions estimated from a SEM picture

12. 0.30 0.25 experimental numerical 0.20 ) l 0.15 R( 0.10 0.05 0.00 400 500 600 700 800 900 1000 l (nm) P. blumei Results • 4 lamina layers used for numerical • Sharp peak in green as observed • n(l) varied linearly from 1.58 to 2.4 in wavelengths 650-980 nm to match experimental results

13. 0.80 experimental 0.60 numerical ) l 0.40 R( 0.20 0.00 400 500 600 700 800 900 1000 l (nm) M. menelaus Results • 3 lamellae layers used • Numerical peaks in violet-blue range as observed • Uses the n(l) found from P.blumei

14. Discussion • R(l) for both species have peaks in visible corresponding to observed iridescent color • Low reflectivity in near-IR allows for efficient solar absorption • Index of refraction of chitin • further study needed to match both P.blumei and M.menelaus results • n(l) may vary for different species • comparison with more accurate experimental data • Partial Coherency effects • more advanced models needed to determine number of films used for modeling • Cellular development of complex microstructures needs further studies

15. Conclusion • Cellular microstructures of iridescent butterfly scales are very complex • Need to study optical phenomena to understand radiative function of the structures • Measuring the optical properties requires combination of numerical simulations and experimental results • Results for M. menelaus and P. blumei show a bright visible color with low infrared reflection • Understanding microscale radiative effects have an impact on improving microelectronics industry • Possible future applications in biomaterials development

16. Acknowledgments & References • Acknowledgments This research is funded by the National Science Foundation under grant numbers CTS-9157278 and DBI-9605833 • References H. Ghiradella, Ann. Entomol. Soc. Am., 77, 637 (1984). H. Tada, S. E. Mann, I. N. Miaoulis, and P. Y. Wong, to be published in Applied Optics. H. Ghiradella, Ann. Entomol. Soc. Am., 78, 254 (1985). P. Y. Wong, I. N. Miaoulis, H. Tada, and S. E. Mann, to be published in ASME Fundamentals of Microscale Biothermal Phenomena. B. D. Heilman, Masters Thesis, Tufts University, 1994. J. B. Hoppert, Mat. Res. Soc. Symp. Proc., 429, 51 (1996).