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
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
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
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
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 lights 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
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
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
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
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
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