Transcript butterfly - Tufts University

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
Apparent or true color
incident sun light
reflected light
thin films
transmitted light
Optical Phenomena
Scattering
•random process
incident light
scattered
light
•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
white light
scattered
spectrum
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
curved thin-films
local normals
reflected light
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
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 5m, too
large to cause diffraction
•due to curvature of film stack, non-planar specular
reflection needs to be considered
cross-ribs
~100m
ridges
wing scale
laminae
scale cross section
~5m
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 ~0.7m apart, suggesting the presence of
diffraction when interacting with sunlight
~100m
scale cross section
~0.7m
wing scale
lamellae
ridges
lower lamina
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
P. blumei Numerical Model
Alternating
Air
layers of lamina and air layers
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 = 0.095m
•air layers = 0.085m
Dimensions
calculated from SEM picture of scale
cross-section
lamina layer
air layer
layer 1
layer 2
.
.
.
layer n
d
D
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 = 0.054m
• air layer thickness = 0.118m respectively.
Effect
of ridge and air sections
• reduce numerical spectral reflectivity by 9%
estimated from a SEM picture
lamellae
air
lamellae
1 unit
ridge
Dimensions
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
0.30
0.25
experimental
numerical
R( l )
0.20
0.15
0.10
0.05
0.00
400
500
600
700
l (nm)
800
900
1000
M. menelaus Results
3
lamellae layers used
Numerical
peaks in violet-blue range as
observed
Uses
the n(l) found from P.blumei
0.80
experimental
numerical
R( l )
0.60
0.40
0.20
0.00
400
500
600
700
l (nm)
800
900
1000
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
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).