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

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

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

incident sun light

reflected light

thin films

transmitted light

Apparent or true color

Optical Phenomena

Scattering

•random process

•due to surface roughness

•incident light is reflected in all directions

incident light scatteredlight

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

white light scatteredspectrum

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

incident white light reflected light

curved thin-films

local normals

Butterfly Microstructure

General butterfly wing scale•made of an organic material called chitin

•scales are generally about 100m 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

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 5m, too large to cause diffraction

•due to curvature of film stack, non-planar specular reflection needs to be considered

~100m

~5m

cross-ribs

laminae

ridges

wing scale

scale cross section

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 ~0.7m apart, suggesting the presence of diffraction when interacting with sunlight

~100m

wing scale

scale cross section

ridges

lamellae

lower lamina

~0.7m

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() 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 between 500 nm and 1000 nm

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 = 0.095m

•air layers = 0.085m

Dimensions calculated from SEM picture of scale cross-section

lamina layerair layer

layer 1layer 2

layer n

.

.

.

d

D

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 = 0.054m

•air layer thickness = 0.118m respectively.

Effect of ridge and air sections

• reduce numerical spectral reflectivity by 9%

Dimensions estimated from a SEM picture

M. menelaus Numerical Model

air

ridge

lam

ella

e

lam

ella

e

1 unit

P. blumei Results

4 lamina layers used for numerical

Sharp peak in green as observed

n() varied linearly from 1.58 to 2.4 in wavelengths 650-980 nm to match experimental results

0.00

0.05

0.10

0.15

0.20

0.25

0.30

400 500 600 700 800 900 1000

(nm)

R(

)

experimental

numerical

M. menelaus Results

3 lamellae layers used

Numerical peaks in violet-blue range as observed

Uses the n() found from P.blumei

0.00

0.20

0.40

0.60

0.80

400 500 600 700 800 900 1000

(nm)

R(

)

experimental

numerical

Discussion

R() 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() 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

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

Acknowledgments & References

Acknowledgments

This research is funded by the National Science Foundation under grant numbers CTS-9157278 and DBI-9605833

References

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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).