Transcript Black Beauties- Super Black Butterfly Scales

Black Beauties- Super Black
Butterfly Scales
Alison Sutton Fernandes
0225014
Why Butterflies?
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Butterflies have irridescent
colours formed by photonic
crystals.
But what about the intense
black areas on the wings?
Wing scales with very low
reflectance (>0.5%)
Possibilities of emulating them
with other materials.
http://www.thaishop4you.com/buttrfly_big_view/bf163.htm
Surface Reflections
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Any interface that involves a change in refractive
index gives rise to surface reflections. Surfaces like
black cardboard and paint, even though they appear
black still reflect about 4%.
To a simple approximation, these surface reflections
are governed by Fresnel equations. For air (ni) and
chittin (nt):
R= ((nt-ni)/(nt+ni))2 = ((1-1.56)/(1+1.56))2
= 4.8%
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In butterfly scales, you get values as low as 0.4%.
The Role of the Butterfly Wing Scale
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The material the butterfly wing is made from, chitin,
is effectively transparent. Yet when it adopts certain
structures it can cause interference and diffraction of
light rays to produce a range of colours.
In the case of black scales the main role of the upper
part of the wing scale appears to be to collimate the
light- to transmit it to an absorbent membrane
beneath, and minimise surface reflections. It is this
part of the Scale I hoped to investigate.
Begun investigations with 17 samples and a range of
methods to see what different solutions there were
and which were most effective.
High Resolution Optical Microscope
Typical Scale Structure
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The arrangement of scales
on the wing resembles that
of shingles on a roof. In
most species two distinct
layers are present- ground
and cover scales.
Typical scale dimensions are
of the order 75micm by 200
micm. (scales come off as a
fine dust). Underside tends
to be plain and featureless,
while interior and external
visible top surface exhibit
interesting microstructure.
Honeycomb Structure
Cross Ribs
Parides Hecuba
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Two butterflies of the Parides family (Hecuba and
Rotuse) instead of honeycomb structure had
microribbing extending across between the ridges,
effectively blocking the inner layers below.
Resulted in some of the lowest reflectances recorded.
Fractured Scales
Other Methods
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SEM:
Upper limit to resolution
Difficulty seeing inner structure
Hard to establish exact size of features
Alternatives:
Embedded in Resin
TEM
Cary SE Spectrophotometer
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Measures the reflectance of a sample over a
range of wavelengths using an integrating
sphere.
Zero calibrated using a light trap– extremely
absorbing.
Samples must be of sufficient size (limited to
5 species).
Beam must be carefully positioned.
Scales easily lost.
Reflectance (%) of 6 butterfly types over full w avelength range of CARY
100
90
80
P. erlaces xanthias
Reflectance (%)
70
60
P. ulysses ulysses
50
P. lysander
40
P. gambrisius
30
P. sesostris
20
10
0
0
500
1000
1500
-10
Wavelength (nm)
2000
2500
3000
Reflectance (%) of 6 butterfly types over visible wavelength range using
CARY
P. erlaces xanthias
4
3.5
P. ulysses ulysses
Reflectance (%)
3
P. lysander
2.5
P. gambrisius
2
P. sesostris
1.5
1
0.5
0
350
400
450
500
550
Wavelength (nm)
600
650
700
Parides sesostris
Microspectrophotometer
Cary Visible Region Reflectance
1.8
1.6
Reflectance (%)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Parides
sesostris
Parides lysander
Parides
gambrisius
Parides ulysses Parides erlaces
ulysses
xanthius
Species
Microspectrophotometer Visible Region Reflectance on Single Scales
3
2.5
Reflectance (%)
Spectral information from
single scales.
Problems:
 Drifting dark current
 Limited integration time
 Very small area
 Surrounding reflections
and extraneous light
 Lambertian assumption
 Equipment failure
2
1.5
1
0.5
0
Parides sesostris
Parides lysander
Parides gambrisius
Species
Parides ulysses
ulysses
Parides erlaces
xanthius
Microspectrophotometer
Microspectrophtometer Visible Region Reflectance
based on Pixel Intensites
0.7
Cary Visible Region Reflectance
Reflectance (%)
0.6
0.5
0.4
0.3
0.2
1.8
0.1
1.6
Reflectance (%)
1.4
1.2
0
1
Parides
sesostris
0.8
0.6
Parides
lysander
Parides
gambrisius
0.4
0.2
0
Parides
sesostris
Parides lysander
Parides
gambrisius
Parides ulysses Parides erlaces
ulysses
xanthius
Parides
ulysses
ulysses
Species
Species
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Attempted to Average Pixel Intensities
Parides
erlaces
xanthius
Conclusion
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Scales with the honeycomb structure were on
average significantly less reflective than those with
crossribbing.
Suggests honeycomb more effective in minimising
surface reflections and collimating light.
The microribbing appeared even more effective.
All scales exhibited extremely low reflectances
Why Colour and Black?
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Camouflage.
Sex Attractant.
An absorber, attenuator, or deflector for ultrasonics to
defeat echolocations by bats.
Signalling
Identification- seen from a large distance,
distinguishable from background.
Eyespots- scare away predators.
Effective use of light. When ample light is available to
species, pigments are generally found. When light
becomes scarce, more structural colour used (light is
not lost and absorbed, but a lot reflected back).
Thermoregulation
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Butterflies bask to gain sufficient body temperatures for flight
activity. (Berwaerts, 2001)
Butterflies with fully spread wings did warm more efficiently.
(Heinrich, 1986)
Descaled wings reached lower temperatures. (Berwaerts, 2001)
Butterflies can develop different scales colours depending on the
season they are born in.
Behavioural factors, such as wing orientation seem more
important. (Polycyn, 1986)
The changes in reflectance are not great.
Reflective in the infra-red region
In some cases difficult to tell if behaviour adapts to wing colour
or wing colour adapts to behaviour.
Other Research
Moth eyes (Hutley et al):
 Minimise Surface Reflections
 Triangle like projections on surface
 Gradually decreasing diffractive index
A similar type of structure is used to absorb sound wave
in recording rooms without creating interference
through reflections.
Thin films also attempt this method, by layering films of
slightly decreased refractive index to lower surface
reflections.
Application
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Structures could be scaled for specific applications.
You would create selective surfaces (since reflection
in infra-red region is v. high).
Basic computer modelling has already confirmed a
peak below 1% for a simple honeycomb structure.
Important to use nature as inspiration, not as
blueprints.
Needs of an individual organism likely to be very
different form our own.
References
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Berwaerts, K., Van Dyck, H. & Matthysen, E., (2001), Effect of
manipulated wing characteristics and basking posture on thermal
properties of the butterfly Pararge aegeria, Journal of Zoology, 255(2),
pp. 261-267
Ghiradella, H., (1994), Structure of Butterfly Scales- Patterning in an
Insect Cuticle, Microscopy Research and Technique, Apr 1 1994, 27 (5),
pp. 429-438
Heinrich, B., (1986), Comparitive thermoregulation of four montane
butterflies of different mass, Physiological Zoology, 59(6), pp. 616-626.
Lawrence, C. & Large, M. C. J., (), Optical Biomimetics, ,
Lewis, H. L., (1973), Butterflies of the World, Harrap, London
Leo, B., (1999), Mysteries of a Butterfly Wing, Microscope, 47 (2), pp.
79-92.
Polycyn, D. & Chappell, M. A., (1986), Analysis of Heat Transfer in
Vanessa Butterflies: Effects of Wing Position and Orientation to Wind
and Light, Physiological Zoology, 59(6), pp. 706-716
Acknowledgments
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OFTC: Dr. Maryanne Large, Dr. Leon
Poladian, Shelly Wickham
Applied Physics: Professor David
McKenzie, Dr. Stephen Bosi
EMU: Tony Romeo, Dr. Ian Kaplin, Anne
Simpson-Gomes
Tamar Ziv, James Griffin