Priscilla Simonis

Theoretical condition for transparency in mesoporous layered optical media: application to switching of hygrochromic coatings

Mesoporous Bragg stacks are able to change color upon infiltration or displacement of liquid compounds inside their porous structure. Reversible switching from transparency to coloration offers additional functionality. Based on Bruggemans effective medium theory, we derive a transparency master equation, which is valid for bilayers of arbitrary host materials and pore-filling compounds. The transparency condition fixes pore volume fractions such that the effective refractive index is homogenized through the bilayer, hence, through arbitrary layered optical media built from this bilayer. This general concept is applied to the case of switching of hygrochromic coatings made of mesoporous mixed oxide Bragg stacks.

A two-in-one superhydrophobic and anti-reflective nanodevice in the grey cicada Cicada orni (Hemiptera)

Two separated levels of functionality are identified in the nanostructure which covers the wings of the grey cicada Cicada orni (Hemiptera). The upper level is responsible for superhydrophobic character of the wing, while the lower level enhances its anti-reflective behavior. Extensive wetting experiments with various chemical species and optical measurements were performed in order to assess the bi-functionality. Scanning electron microscopy imaging was used to identify the nanostructure morphology. Numerical optical simulations and analytical wetting models were used to prove the roles of both levels of the nanostructure. In addition, the complex refractive index of the chitinous material of the wing was determined from measurements.

Chapter 5 – Structural Colours

Colouration arises when (1) a white-light illumination beam is spectrally filtered by transmission through an optically responsive material and (2) a vision system with specific discrimination capability has analysed the remaining energy. The resulting optical spectrum can either be caused by absorption (the so-called ‘pigmentary’ colouration mechanism) or by light-wave interferences (the so-called ‘structural’ or ‘physical’ colouration mechanism). In this chapter, we will focus on the latter. The structural colouration effects arising from the presence of overlayers and gratings—in general assembled from chitin-based elements—are exposed and illustrated by typical examples arisen in animal colour displays. Photonic-crystal films, with or without long-range disorder will also be described, along with examples of appropriate living organisms that have evolved these structures.

Synthesis of bio-inspired multilayer polarizers and their application to anti-counterfeiting

Some insects, such as Papilio blumei and Suneve coronata, are known for exhibiting polarization effects on light such as color contrast or geometrical polarization rotation by reflection on their wing scales. The photonic structures found on these species that show these properties are multilayered spherical cavities or triangular grooves which polarize the light due to multiple inner reflections. These polarization effects, in addition to the intrinsic color-mixing properties of these photonic structures, are of interest in the anti-counterfeiting field due to their invisibility to the naked eye. In this paper, we report micro-fabrication techniques to produce bio-inspired cylindrical grooves (C-grooves) and triangular grooves (V-grooves) that demonstrate the same properties. Theoretical analyses were carried out by using multi-scale simulation (MS) as well as by finite-difference time-domain (FDTD) in order to compare the polarization capability of both structures. The V-grooves show greater polarization contrast than the C-grooves, but the spectrum is specular. The C-grooves exhibit lower polarization effects but have a dispersive spectrum. In both cases, the structures show additional optical properties, such as diffraction, macroscopic color contrast under a polarizer, and contrast inversion due to geometries which contribute to their uniqueness.

Radiative contribution to thermal conductance in animal furs and other woolly insulators

This paper deals with radiations contribution to thermal insulation. The mechanism by which a stack of absorbers limits radiative heat transfer is examined in detail both for black-body shields and grey-body shields. It shows that radiation energy transfer rates should be much faster than conduction rates. It demonstrates that, for opaque screens, increased reflectivity will dramatically reduce the rate of heat transfer, improving thermal insulation. This simple model is thought to contribute to the understanding of how animal furs, human clothes, rockwool insulators, thermo-protective containers, and many other passive energy-saving devices operate.

How Nature Produces Blue Color

Today, blue is a very fashionable color in European countries. This has not always been the case (Pastoureau, 2000), as cultural perceptions have slowly evolved since prehistoric times. In cave paintings, white, red and black have been the only available tones and these colors remained basic for Greek and Latin cultures, where blue was neglected or even strongly devalued. The word caeruleus, which is often used for brightly blue species, in naming plants and insects, is etymologically related to the word cera, which designates wax (not to the world caelum – sky – as often believed): it meant first white, brown or yellow (Andre, 1949), before being applied to green and black, and much lately, to a range of blues. Latin and Greek philosophers were so diverted from blue that they even did not notice its presence in the rainbow: for Anaximenes (585-528 BC) and later for Lucretius (98-55 BC), the rainbow only displayed red, yellow and violet; Aristotle (384-322 BC) and Epicurus (341-270 BC) described it as red, yellow, green and violet. Seneca (ca. 4 BC 65 AD) only mentioned red, orange, green, violet but, strangely, also added purple, a metameric color not found in the decomposition of white light. Later in the Middle-Ages, Robert Grosseteste (ca. 1175-1253) revisited the rainbow phenomenon in its book “De Iride” and still did not find there any blue color (Boyer, 1954). Blue emerged slowly in minds and art, only after the advent of technological breakthroughs in stained glass fabrication (as introduced in the 12th century rebuild of St Denis Basilica) and after the progressive use of blue dyes, which followed the extension of woad cultivation, all after the 13th century.

Total Internal Reflection Accounts for the Bright Color of the Saharan Silver Ant

The Saharan silver ant Cataglyphis bombycina is one of the terrestrial living organisms best adapted to tolerate high temperatures. It has recently been shown that the hairs covering the ant’s dorsal body part are responsible for its silvery appearance. The hairs have a triangular cross-section with two corrugated surfaces allowing a high optical reflection in the visible and near-infrared (NIR) range of the spectrum while maximizing heat emissivity in the mid-infrared (MIR). Those two effects account for remarkable thermoregulatory properties, enabling the ant to maintain a lower thermal steady state and to cope with the high temperature of its natural habitat. In this paper, we further investigate how geometrical optical and high reflection properties account for the bright silver color of C. bombycina. Using optical ray-tracing models and attenuated total reflection (ATR) experiments, we show that, for a large range of incidence angles, total internal reflection (TIR) conditions are satisfied on the basal face of each hair for light entering and exiting through its upper faces. The reflection properties of the hairs are further enhanced by the presence of the corrugated surface, giving them an almost total specular reflectance for most incidence angles. We also show that hairs provide an almost 10-fold increase in light reflection, and we confirm experimentally that they are responsible for a lower internal body temperature under incident sunlight. Overall, this study improves our understanding of the optical mechanisms responsible for the silver color of C. bombycina and the remarkable thermoregulatory properties of the hair coat covering the ant’s body.

Possible uses of the layered structure found in the scales of Hoplia coerulea (Coleoptera)

The male of the beetle Hoplia coerulea is known for its spectacular blue-violet iridescence. The blue coloration is caused by the presence of an interesting photonic structure inside the scales which cover the dorsal parts of the insects body. This structure can be described as the stacking of chitin plates supporting arrays of parallel rods. The change of colour of this structure with humidity is investigated, as well as its response to some other external conditions, such as mechanical strain.