What does it mean to be blue? The wings of a morpho butterfly are some of the most beautiful
structures in nature, and yet they contain no blue pigment.
Artists use paint, dyes and ink to make their creations. Nature draws on a palette of biological
pigments, also known as biochromes.
But there's another way of producing color. It's called "structural color." And it harnesses
the physics of light at the nanoscale.
Nipam Patel's lab at UC Berkeley is studying how morpho butterflies form the special structures
that cover their wings -- scales -- while still inside the pupae.
Morphos live mostly in the tropics. When resting, they fold their wings up, showing their dark
earth-toned undersides.
The brown, yellow and black colors are generated by pigments.
But the other side is all about structural color. It gives their wings a vibrant, iridescent
blue hue.
Each scale is like a pixel, a tiny tile in a larger mosaic in layers of overlapping rows.
Researchers want to see how structural color takes shape on the wings, but normally this
happens inside the pupae, which is opaque. So they've figured out how to remove the
morpho wings from the pupae and grow them in a Petri dish.
Just like a developing photograph, patterns and colors slowly appear on the ghostly white
wings as each scale's surface transforms over time.
Ridges on the scales' surface are a key component that affect how the wing spreads --
or refracts -- light, similar to a prism.
When light hits these ridges, a phenomena called constructive interference comes into
play.
The spacing within the ridges -- which look like little Christmas trees -- perfectly reinforces
specific wavelengths while canceling out others. This is why your eyes perceive that shimmering
blue.
Scientists aren't sure why, but vertebrates and plants rarely produce blue as a pigment.
For some reason, it's a pigment you don't see much of in nature.
So think of structural color as an evolutionary work-around -- a way of producing brilliant
blues at a nano level. Not just on butterfly wings, but on feathers, beetles --
even our own bodies.