#structural coloration

[theunnature]

Crude video of Papilio palinurus exhibiting structural coloration. The green and blue color comes not from pigmentation but from the way light is refracted by its scales. The effect is presumably even cooler while on drugs. Neat!

[scienceguy22]

The bee that changes color like a mood ring.

I am talking about "sweat bees," not because they sweat but because they drink our sweat for sodium, a mineral flowers don't provide enough of. These bees are extremely gentle, & you're not likely to notice them on you, maybe just a tiny tickle. They are very small, about 0.12-0.4 in (3-10 mm) & are found on every continent except for Antarctica. They are major pollinators & drink the nectar of wildflowers, fruits, vegetables, prairie plants, orchards, & many crops ignored by honeybees & bumblebees. Interestingly, if you look at a sweat bee's exoskeleton, with its nanostructure removed, you'd see its raw chitin.

Chitin is a carbohydrate polymer similar to cellulose plant cell walls or fungal cell walls, only stronger, stiffer & more structured. Its color would be a transparent beige—essentially colorless. Here's the twist: the spacing of the layers of chitin determines which wavelengths we see. In dry conditions, the spacing is thin, so they appear blue, but when it is humid, the spacing swells & they look green. The bee isn't changing its chemistry or physics; it's simply changing the spacing of the chitin, which increases when the air is humid & comes closer together in dry conditions.

Specifically, when humidity increases, the reflected wavelength shifts upward. Blue (450 nm) shifts to blue-green & finally green (520 nm). It cannot shift far enough to reach yellow, orange, or red, which require much larger spacing. Their color is purely optical engineering, not chemistry. Humidity changes geometry, not molecules. It's the same physics as the color change of oil slicks, peacock feathers, beetle shells & holographic stickers. With oil slicks, rainbow sandwiches are made of oil & water. Its spacing never changes & is not affected by humidity. Its color only changes when you move your head. The same principle applies to holographic stickers, which are rainbow sandwiches made of plastic ridges. Its color changes with angle, not moisture.

The same thing holds true for peacock feathers. The feathers are dead tissue & do not change with humidity; they change with viewing angle. Interestingly, the shells of beetles also have chitin layers in multilayered stacks, but the change in color is not because of humidity—their cuticle is too hard to swell; rather, the color change depends on the angle they are viewed from. Blue butterflies use the same chemistry/physics. Sweat bees are unique because their structural color is dynamic, not fixed. The analogy is this: peacock feathers are like a beautiful stained glass window. The pattern is fixed forever. Beetle shells are like a metallic car paint job. Shiny, but unchanging. Sweat bees are like a stack of clear mirrors that puff up when they get damp. When the spacing increases, the color shifts. Only sweat bees have mirrors that move.

Real blue pigment that is not governed by structural changes is very rare in nature, with only 4 examples: 1. The blue-footed booby. Its feet are blue due to biliverdin & carotenoid interactions. 2. Some fish, blue damselfish, surgeonfish, blue-lined snappers & blue-spotted jawfish, all contain cyanophores, which are blue because of cyanins, a class of actual blue biochromes. 3. Blue Pierid butterflies that contain blue pterins. These pigments absorb red/green wavelengths & reflect blue. 4. Finally, there are species of worms that produce true blue biliproteins & tunicates (sea squirts) that are not worms (but close), & many produce blue biliverdin pigments in their tunics.

[iksplohr]

Video here

[sciencetoastudent]

Image: Tatiana Makotra/Shutterstock.com

This is how blue eyes get their colour

Blue eyes don’t get their colour from pigment - it’s actually way more fascinating than that.

FIONA MACDONALD   20 DEC 2014

54.k265

  Your eyes aren’t blue (or green) because they contain pigmented cells. As Paul Van Slembrouck writes for Medium, their colour is actually structural, and it involves some pretty interesting physics.

As he explains, the coloured part of your eye is called the iris, and it’s made up of two layers - the epithelium at the back and the stroma at the front.

The epithelium is only two cells thick and contains black-brown pigments - the dark specks that some people have in their eye is, in fact, the epithelium peaking through.

The stroma, in contrast, is made up of colourless collagen fibres. Sometimes the stroma contains a dark pigment called melanin, and sometimes it contains excess collagen deposits. And, fascinatingly, it’s these two factors that control your eye colour.

Brown eyes, for example, contain a high concentration of melanin in their stroma, which absorbs most of the light entering the eye regardless of collagen deposits, giving them their dark colour.

Green eyes don’t have much melanin in them, but they also have no collagen deposits. This means that while some of the light entering them is absorbed by the pigment, the particles in the stroma also scatter light as a result of something called the Tyndall effect, which creates a blue hue (it’s similar to Rayleigh scattering which makes the sky look blue). Combined with the brown melanin, this results in the eyes appearing green.

Blue eyes are potentially the most fascinating, as their colour is entirely structural. People with blue eyes have a completely colourless stroma with no pigment at all, and it also contains no excess collagen deposits. This means that all the light that enters it is scattered back into the atmosphere and as a result of the Tyndall effect, creates a blue hue. 

Interestingly, this means that blue eyes don’t actually have a set colour - it all depends on the amount of light available when you look at them.

Structural colouration also gives colour to butterflies, beef and berries, as Van Slembrouck points out.

It’s pretty mind-blowing stuff. Van Slembrouck writes for Medium:

“Imagine that you could shrink yourself to a microscopic size and then climb through the mesh of fibres in the stroma. That’s where structural colouration is coming from…

… and in the mesh are also strands of smooth muscle tissue that contract to dilate (expand) the pupil, pulling the inner edge of the iris toward the outer edge. When this happens, the stroma fibres slacken and may become wiggly as tension is released. This makes me wonder, does that slightly alter the colour of your eye as well?”

Check out Van Slembrouck’s great story to find out how hazel and grey eyes get their colour, and also to check out his beautiful diagrams that explain structural colouring.

Source: Medium