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Black Beauties- Super Black Butterfly Scales Alison Sutton Fernandes 0225014 Why Butterflies? 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%)

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black beauties super black butterfly scales

Black Beauties- Super Black Butterfly Scales

Alison Sutton Fernandes

0225014

why butterflies
Why Butterflies?
  • 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
Surface Reflections
  • 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%

  • In butterfly scales, you get values as low as 0.4%.
the role of the butterfly wing scale
The Role of the Butterfly Wing Scale
  • 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.
typical scale structure
Typical Scale Structure
  • 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.
parides hecuba
Parides Hecuba
  • 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.
other methods
Other Methods

SEM:

  • Upper limit to resolution
  • Difficulty seeing inner structure
  • Hard to establish exact size of features

Alternatives:

  • Embedded in Resin
  • TEM
cary se spectrophotometer
Cary SE Spectrophotometer
  • 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.
microspectrophotometer
Microspectrophotometer

Spectral information from single scales.

Problems:

  • Drifting dark current
  • Limited integration time
  • Very small area
  • Surrounding reflections and extraneous light
  • Lambertian assumption
  • Equipment failure
microspectrophotometer18
Microspectrophotometer
  • Attempted to Average Pixel Intensities
conclusion
Conclusion
  • 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
Why Colour and Black?
  • 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
Thermoregulation
  • 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
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
Application
  • 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
References
  • 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
Acknowledgments
  • 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