Black Beauties- Super Black Butterfly Scales Alison Sutton Fernandes 0225014.

Black Beauties- Super Black Butterfly Scales

Alison Sutton Fernandes0225014

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 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 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 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

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

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

Alternatives: Embedded in Resin TEM

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.

Reflectance (%) of 6 butterf ly types over full w avelength range of CARY

-10

0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500 2000 2500 3000

Wavelength (nm)

Ref

lect

ance

(%

)

P. erlaces xanthias

P. ulysses ulysses

P. lysander

P. gambrisius

P. sesostris

Reflectance (%) of 6 butterfly types over visible wavelength range using CARY

0

0.5

1

1.5

2

2.5

3

3.5

4

350 400 450 500 550 600 650 700

Wavelength (nm)

Re

flect

an

ce (

%)

P. erlaces xanthias

P. ulysses ulysses

P. lysander

P. gambrisius

P. sesostris

Parides sesostris

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

Cary Visible Region Reflectance

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Paridessesostris

Parides lysander Paridesgambrisius

Parides ulyssesulysses

Parides erlacesxanthius

Species

Ref

lect

ance

(%

)

Microspectrophotometer Visible Region Reflectance on Single Scales

0

0.5

1

1.5

2

2.5

3

Parides sesostris Parides lysander Parides gambrisius Parides ulyssesulysses

Parides erlacesxanthius

Species

Ref

lect

ance

(%

)

Microspectrophotometer

Attempted to Average Pixel Intensities

Microspectrophtometer Visible Region Reflectance based on Pixel Intensites

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Paridessesostris

Parideslysander

Paridesgambrisius

Paridesulyssesulysses

Parideserlacesxanthius

Species

Re

fle

cta

nc

e (

%)

Cary Visible Region Reflectance

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Paridessesostris

Parides lysander Paridesgambrisius

Parides ulyssesulysses

Parides erlacesxanthius

Species

Ref

lect

ance

(%

)

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? 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 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 ResearchMoth 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 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 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

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