Today’s post is a guest post coming at you from the fantastic Vivienne Raper, a freelance journalist who also blogs at the blog Outdoor Science! This is also why all the spellings of things appear to have extraneous “u”s.
Imagine owning a swimsuit that makes you look like a tropical fish, shimmering and changing colour as you flash through the water. Not a fan of swimming? How about an opalescent party dress that dazzles like peacock feathers or butterflies’ wings. It might sound like science fiction, but it could be as little as three years away.
The video below shows a sample of what could be one of the world’s most fashionable fabrics. It’s lycra coated with an iridescent plastic film that – in big chunks – looks like metallic chewing gum (see Figure 1). When you turn or twist it, the so-called ‘polymer opal’ changes colour.
But before fashionistas storm the University of Cambridge physics department (where I first saw the fabric), the material only exists in little strips in the laboratory. The scientists who are bringing fashion and physics together hope to produce pieces 10s of metres long and about 20cm wide later this year. There are still challenges to overcome before the material could be used for clothing.
(Figure 1: metallic chewing gum! Source)
Your next question: why does this fabric look so cool? Well, here comes the science bit. Polymer opals are a so-called ‘structural colour’ material. The colour is built into the plastic film’s physical structure unlike, for example, a pink woolly jumper, which gets its colour from dye (What? You thought there were pink sheep?)
Structural colour fabrics are funkier than painted or dyed materials. The colours are more intense, don’t fade over time and look metallic, although the material contains no metal. Polymer opals are non-toxic, unlike a lot of dyes, and change colour as you move your head.
But why, you might ask, can’t I strut my stuff in a technicolour dreamcoat already? After all, we first created synthetic opals in 1974 and opals are structural colour materials. The trouble is opals are brittle. Making a wearable opal is a science challenge. Polymer opal, however, is joyfully rubbery. As one of the scientists told me, “you can jump up and down on it and that’s ok”.
The flickering, shimmering colours inside opals are down to their regular, internal structure. Inside an opal are perfectly round, tiny glass spheres stacked up like billiard balls (see Figure 2). These minute crystals bend light by a process called diffraction. Sound waves can also be bent by diffraction, which is why you can hear someone calling when you’re hiding behind a tree.
(Figure 2: (from Wikipedia entry on opals): Precious opal consists of spheres of silica of fairly regular size, packed into close-packed planes that are stacked together with characteristic dimensions of several hundred nm.)
To replicate an opal’s diffraction effect, you need two translucent materials with different refractive indexes. Refractive index sets the speed of light, and when the light changes speed by going into a higher or lower refractive index you often get a reflection… like the reflections we see in glass windows. If multiple objects in a neatly-ordered pattern reflect, reflections from adjacent objects can interfere (add up) to either suppress or enhance the reflection. This effect is called diffraction and is the cause of “structural colours” found in peacock feathers and gemstone opals. These materials are easy to spot as they change colour as you look from different angles.
The engineering challenge is alternating these materials in a regular pattern through the synthetic opal. All the tiny crystals must be as close as the wavelength of light, around 500 times smaller than a human hair and too small to see with a microscope. Researchers have tried etching the regular patterns onto materials in the past, but the result was a dull, milky opal.
Polymer opals get around this engineering problem because they partly self-assemble. Hard polystyrene spheres are grown to 200 nanometres diameter, hardened with heat or light and coated with a chewing gum-like soft, outer shell of polyethylene acrylate. The result is spheres like Dime Bars (or inverted armadillos) – crunchy on the inside, soft on the outside.
The spheres are pushed into a regular, repeating structure. The soft shells push together into a flexible mass while the central cores form a regular structure that bends light. The size of the spheres controls the colour of the polymer opals. Small, medium and large spheres produce blue, green and red opals, respectively.
The Silly Putty-like outer shells of the spheres explain why polymer opal is flexible and rubbery. The soft shells act a bit like a viscous liquid under pressure because they can slide past each other. This means the opal can be rolled, moulded or squeezed into a thin, plastic coating you can stick to lycra or – ideally – knit into fabric.
What’s more, the material changes colour when it’s bent or stretched. In lay terms, your sci-fi swimsuit will shimmer when you wave your arms over your head or launch into languid breaststroke. As you glide through the water, you’d be pulling the hard inner cores apart or pushing them together. The space between the cores affects the way they bend light and the colours of the material.
All this sounds great, but there’s one problem. Will polymer opals survive the rigors of a swimming pool or beach holiday – chlorine, cold showers, sand and salt water? At the moment, the scientists don’t know. But one of them tells me that polymer opals can survive a spin cycle. He’d left a sample in one of his pockets and his wife put it in the wash. It survived unscathed.
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Disclaimer: This blogpost is by an independent blogger and is not endorsed by the universities or scientists involved in the project.
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Further reading: http://www.np.phy.cam.ac.uk/wp-content/uploads/2010/01/Stretching-the-Imagination-with-Ref.pdf
