Abstract
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Transcript of Abstract
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
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).