Natural shine: Humidity changes structures

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  • Published: Sep 15, 2015
  • Author: David Bradley
  • Channels: Chemometrics & Informatics
thumbnail image: Natural shine: Humidity changes structures

Wrinkled tulip

Queen of the night tulip

Natural materials, such as the cellulose in plants, can self-assemble into surfaces with visually stunning optical properties, such as iridescence, that change depending on humidity.

Tulips are Eurasian and North African bulbous plants related to lilies and famously have showy, colourful flowers, one strain of which, Queen of the Night, has petals of deep purple, almost black colour. However, these petals also have an intriguing, iridescent shimmer like moonlight dancing on jewellery to put it somewhat poetically. This tulip is not the only iridescent plant though, some rainforest plants of Malaysia, such as the peacock fern (Selaginella willdenowii) have a far more striking colour-shifting property, their iridescent blue leaves turn green when they get wet. The tulip's shimmering colours and the colour-shifting rainforest plant display examples of structural colour, their display is created by optical effects rather than chemical pigments that absorb or reflect specific wavelengths of light.

Now, writing in the Journal of Chemical Physics, chemical engineer Alejandro Rey, of McGill University, Montreal, Canada, and colleagues have shown how plant cellulose can self-assemble into wrinkled surfaces that give rise to such iridescence and colour change effects. Their discovery could offer a new foundation on which to develop our understanding of structural colour in nature, and perhaps create biomimetic materials for a range of decorative and technological applications, such as display technology and humidity sensors.

Sheen

Cellulose is a critical component of the plant cell in which it exists as aligned fibres. However, there is a twist: cellulose exists as a cholesteric phase and as such is of interest to researchers developing photonic devices. Scientists assume that cellulose has this twisting phrase mainly for strength. "The fibres reinforce in the direction they are oriented," explains Rey. "When the orientation rotates you get multi-directional stiffness." He and his colleagues were not particularly interested in the mechanical properties of cellulose, rather they wondered whether this twisted phase might be what gives rise to the structural colours of iridescent petals.

The researchers therefore constructed a computational model to examine the behaviour of cholesteric phase cellulose. In the model, the axis of twisting runs parallel to the surface of the cellulose. The team found that sub-surface helices naturally lead to wrinkling of the surface with nanoscopic ridges forming separated by a few micrometres. The team suggests that the pattern of parallel ridges resembles the microscopic pattern seen on the surface of petals of the Queen of the Night tulip. The ridges split white light by diffraction, rather than refraction, into its many coloured components and create an iridescent sheen.

Striking colours

Adding different amounts of water also affected the optical properties of the surface. More water made the layers twist less tightly, which in turn made the ridges farther apart. A surface with spatially varying pitch (in which some areas were more hydrated than others) was less iridescent and reflected a longer primary wavelength of light than surfaces with a constant pitch. The wavelength shift from around 460 nm (visible blue light) to around 520 nm (visible green light). Rey offers this as a plausible explanation for the blue to green colour shift exhibited by those rainforest plants. More work is still needed to pin down the details of diffraction from nanoscopically textured surfaces, but the model developed by Rey and colleagues does offer a route map that might lead researchers to novel structured materials.

"The results show the optics [of cholesteric cellulose] are just as exciting as the mechanical properties," Rey adds. He said scientists tend to think of the structures as biological armour, because of their reinforcing properties. "We've shown this armour can also have striking colours."

The next step will be to perform mustiscale theory and simulations to unravel the full spectrum of structure-properties relations of biological plywoods in plants and insects and leverage these mechano-optical principles into sensor-actuator devices," Rey told SpectroscopyNOW.

Related Links

J Chem Phys 2015, online: "Nano-wrinkling of Chiral Surfaces: Structure and Diffraction Optics"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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