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8 Photonic Bionanostructures -- Colors of Butterflies and Beetles

8 Photonic Bionanostructures -- Colors of Butterflies and Beetles

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Biology on the Nanoscale

e.g., moths or butterflies (Fig. 11.60b) to see in low-light conditions. These surfaces of nodules are arranged in a hexagonal array with a periodicity of 240 nm to

introduce a gradual refractive index profile at an interface between chitin (a polysaccharide with a refractive index, n, of 1.54) and air (n = 1), and reduce reflectivity

by a factor of about ten.

After the first photonic crystal in an animal was identified in 2001 [11.171], the

scientific effort in this subject has accelerated, as discussed below.

11.8.1 Structures

Two structures have been identified to explain many of the interference color

phenomena of butterflies and beetles [11.172]:

(1) Two-color phenomena are caused by a multilayer structure with alternating

high and low refraction indices (see the blue Morpho butterfly in Fig. 11.61).

These structures do not change the orientation of polarized light. Chitin is the

dominant component in many of the layers, but proteins, melanin, lipids, etc.,

also contribute [11.172]. The multilayer stacks contain 10–20 bilayers with,

e.g., thicker chitin layers (194 nm, n = 1.56) and thin layers of air (10 nm,

n = 1.0). A simplified description for the dominant wave length λ for a

multilayer reflector with two alternating layers is given by [11.169]


2a n¯ − sin2 θ


and for normal incidence λ = 2a¯n/m, where a is the sum of the thicknesses

of the two different layers, n¯ the average refractive index, θ the angle from the

multilayer normal, and m an integer. This well describes the observation that

thicker layers result in longer reflected wavelengths and that grazing incidence

gives rise to a blue shift of the reflected light. For an ideal multilayer stack with

the refractive indices na and nb and the number p of bilayers, the reflectivity

(ratio of reflected and incident light intensity) is given by (see [11.172])

R =











demonstrating that R increases with the number p of bilayers and the ratio

nb /na >1 of the refractive indices.

In Fig. 11.61 the blue iridescent wings of a Morpho butterfly are shown

together with the structure of the scales that reflect the blue light. This structure

can be mimicked by artificial layered structures (Fig. 11.61c).

(2) The observed color phenomena are explained by a twisted (helicoidal) multilayer chitin-rich structure. The chitin molecules are predominantly arranged


Photonic Bionanostructures – Colors of Butterflies and Beetles





Fig. 11.61 The iridescent blue wings of a Morpho butterfly. (a) The nanoarchitecture gives the

wings a distinctive iridescent blue color. (b) Scanning electron micrograph of the structure of the

scales of the wing. (c) A mimic of the structure in (b), fabricated by focused ion beam chemical

vapor deposition (FIB-CVD). Both structures give a wave length peak around 440 nm at an angle

of about 30◦ , but the replica can cover areas only of micrometers. (Reprinted with permission from

[11.173]. © 2005 Japanese Society of Applied Physics)



Biology on the Nanoscale

Fig. 11.62 Iridescent cuticle of a beetle and replica. (a) The Manuka (scarab) beetle exploits

chiral films with a structure similar to a liquid crystal to make its cuticle iridescent. (b) Biomimetic

replicas (each around 2 cm2 ) made of titania exhibit different colors depending on the pitch of the

film and the incidence angle of light. The circular polarization properties of the replica and of the

beetle are also the same. (c) A scanning electron micrograph of the chiral reflector in the cuticle

of the beetle and (d), the replica. Scale bar, 400 nm [11.174]. (Reprinted with permission from

[11.170]. © 2007 Nature Publishing Group)

helically. Within each layer, the thread-like molecules lie parallel to each other

(Fig. 11.62c), but the molecules of different layers show a twist (chirality) of

thread orientation (director). A total rotation of the director of the layers through

360◦ is called the pitch. These structures – when birefringent – change the

orientation of the polarization of light and are therefore referred to as optically

active systems (see [11.172]).

When white light is incident on a surface of a helicoidal structure (parallel

to the twist), selective reflection takes place. The wavelengths of the reflected

maxima are varying with an angle v to the surface according to (see [11.170])

λ = 2Sn sin v

with S the half-pitch. The highest reflectivity for a given color is obtained for an

optical thickness (thickness times n) of a half-pitch equal to the wavelength. In a


Lotus Leaf Effect – Hydrophobicity and Self-Cleaning


chitinous material with a mean refractive index of about 1.53 and a half-pitch of

165 nm the structure will reflect blue green light with a wavelength of 505 nm

(see [11.172]). The Manuka (scarab) (Fig. 11.62a) exploits chiral films to make

its cuticle iridescent with a circular polarization. Biomimetic replicas made of

TiO2 (Fig. 11.62b, d) have different colors depending on the pitch of the film

and the viewing angle.

11.8.2 Formation Processes of Photonic Bionanostructures

The material for the formation of scales or cuticle is made by epidermal cells with a

size much larger than the nanostructures of the reflective multilayers. This suggests

that the deposition pattern is defined by the presence of small organelles in the

cell, such as mitochondria, microtubules, and microfibrils. For the formation of the

helicoidal reflection layers, such as in the case of the Manuka beetle (Fig. 11.62c),

the chitin molecules, initially in a liquid environment, self-assemble into a liquid

crystal and become closer to each other during drying.

The same few designs for reflective nanomultilayers are found again and again

within highly unrelated species, suggesting that the basic eukaryote cell contains

an array of preexisting nanostructures that can be called upon in the manufacture

of complex photonic nanostructures in any taxon (group of organisms). If the same

molds, scaffolds, templates, or machineries are used each time, it is not surprising

that the photonic devices show similarities. Equally, if similar physical processes

are involved, such as molecular self-assembly, then similar nanoarchitectures could

be expected to reoccur. The goal in the field of optical biomimetics, therefore, could

be to replicate such nanomachinery that will self-assemble optical nanostructures

with precision. DNA machines are already made by self-assembly [11.175], so this

goal is not unrealistic [11.170].

11.9 Lotus Leaf Effect – Hydrophobicity and Self-Cleaning

A lotus leaf (Nelumbo nucifera) has been considered for centuries as a

symbol of purity because of its self-cleaning nature (see Fig. 11.63a). This

is due to a multi-lengthscale surface topology with hydrophobic micro- and

nanostructures (Fig. 11.63b) giving rise to superhydrophobicity or water repellency


Contact angle measurements of a droplet on a substrate are used for the determination of the wetting properties. In fact, two different angles are necessary to

describe the wettability of a surface, the advancing and the receding angles, θ A

and θ R , respectively. These angles can be measured when water is evaporated (θ R )

from or absorbed (θ A ) by a droplet. In the case of perfect hydrophobicity or superhydrophobicity (θA /θR = 180◦ /180◦ ), the spherical drop touches the surface at a

single point [11.179] and moves readily on the surface.



Biology on the Nanoscale

Fig. 11.63 Hydrophobicity as a prerequisite for the lotus effect, the rose petal effect, and the water

strider. (a) Lotus effect. A water droplet collects dirt from the surface of a lotus leaf. (b) Close-up

of a lotus leaf (Nelumbo nucifera), an example of a superhydrophobic plant surface which originates from the multilength-scale structuring by micrometer-sized bumps and nanoscale waxy hairs

[11.178]. (c, d) Scanning electron micrograph (SEM) of the surface of a red rose petal showing

a periodic array of micropapillae and nanofolds on each papilla top [11.180]. (e) Water strider

(Gerris remigis) walking on the surface of a lake (see [11.178]). (Reprinted with permission from

[11.178] (a) (b) (e) and [11.180] (c) (d). © 2008 Materials Research Society (a) (b) (e) and © 2008

American Chemical Society (c) (d))

In the lotus leaf the surface has hydrophobic hierarchical micro- and

nanostructures (Fig. 11.63b) with a coexistence of micrometer-sized bumps and

nanoscale waxy hairs [11.176, 11.178]. Similar hierarchical micro- and nanostructures are found on the surfaces of rose petals (Fig. 11.63c, d). Hydrophobicity

permits water striders (Gerris remigis) with hierarchical micro–nanostructured legs

to stroll effortlessly on water (Fig. 11.63e), where a single leg can hold up to 1.52

mN, which is 15 times the entire water strider’s body weight [11.178].

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